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Swift interview questions for freshers
1. What's the big idea behind Swift being a 'type-safe' language? Can you give a simple example?
2. Imagine you're explaining optionals to someone who's never coded before. How would you do it?
3. Can you explain the difference between a struct and a class in Swift, like I'm five? What are the real-world implications of these differences?
4. What's the deal with 'mutating' functions in structs? Why do we need them?
5. Tell me about 'enums' in Swift. When would you use them instead of just using strings or numbers?
6. What are protocols in Swift? How can they help you write better code?
7. What is protocol-oriented programming? Why is it important in Swift?
8. Explain the difference between 'let' and 'var'. Why would you choose one over the other?
9. What's the point of using 'guard let'? How does it make your code safer?
10. What are closures in Swift? Can you give a simple example of when you might use one?
11. How do you handle errors in Swift? What's the 'do-try-catch' thing all about?
12. What are generics in Swift? Why should we care about them?
13. What is the difference between 'nil' and an empty string in Swift?
14. Can you explain how memory management works in Swift? What is ARC?
15. What are extensions in Swift? How can they be helpful?
16. What are the different access control levels in Swift (e.g., private, public, internal)? Why are they important?
17. Explain the concept of a computed property in Swift. How is it different from a stored property?
18. What are the benefits of using Swift's collection types (arrays, dictionaries, sets)? How do they differ?
19. What's the deal with 'defer' in Swift? When would you use it?
20. How do you perform asynchronous operations in Swift? What are some common techniques?
21. Explain the difference between '==' and '===' in Swift (if there is one, and if not, why?).
22. What are higher-order functions in Swift? Can you name a few common ones?
23. Describe a situation where you might use a delegate in Swift.
24. What are Key-Value Observing (KVO) and Key-Value Coding (KVC) in Swift? When might you use them?
25. How would you typically structure a Swift project? What are some common architectural patterns?
26. What is the purpose of the 'Codable' protocol in Swift?
27. What is a framework? Have you used any frameworks and how was the experience?
28. Can you explain the concept of immutability in Swift and why it's important?
29. What is method swizzling and why should you avoid it?
30. What is dynamic dispatch and static dispatch? How do they relate to Swift?
Swift interview questions for juniors
1. What is the difference between `let` and `var`? Can you explain with an example?
2. Imagine you're explaining what an optional is to a friend. How would you describe it and why it's useful?
3. What are the basic data types in Swift (like Int, String, Bool)? Can you give an example of when you might use each?
4. What is a function in Swift? Can you write a simple function that adds two numbers together?
5. What is an Array, and how do you add or remove elements from it?
6. What is a Dictionary, and how is it different from an Array?
7. Explain the concept of a loop in Swift. Give an example of using a 'for' loop.
8. What is the purpose of an 'if' statement? Can you give an example of using one?
9. What is a class in Swift? How does it relate to creating objects?
10. What is the difference between a struct and a class in Swift? When might you choose one over the other?
11. What is inheritance in Swift, and why is it useful?
12. Can you describe what a protocol is and how it is used in Swift?
13. What is the point of using guard statements in Swift?
14. Explain the difference between '==' and '===' in Swift, if any.
15. How do you handle errors in Swift using 'try', 'catch'?
16. What are closures in Swift? Have you used them before?
17. What is a Swift Package? Have you used any?
18. How do you use storyboards to build a user interface in iOS?
19. What is Auto Layout and how does it help create user interfaces that work on different screen sizes?
20. Explain the Model-View-Controller (MVC) design pattern.
21. What is the purpose of using enums in Swift? Can you show me an example?
22. Have you used any debugging tools in Xcode? Which ones do you find the most useful?
Swift intermediate interview questions
1. What are higher-order functions in Swift, and can you provide an example of how you've used one?
2. Explain the difference between `class` and `static` keywords when defining type members.
3. Describe the use cases for `defer` in Swift, and how it affects the control flow.
4. What are Swift's KeyPaths, and how can they be used with KVO?
5. How do you handle errors in Swift, and what are the advantages of using `Result` type?
6. Explain the concept of property observers (`willSet`, `didSet`) and their use cases.
7. Describe the differences between `Any` and `AnyObject` in Swift.
8. What are generics in Swift, and why are they useful? Provide an example.
9. Explain the use of `inout` parameters in Swift functions.
10. How does Swift's memory management work, and what is ARC?
11. What are protocols in Swift, and how do they enable polymorphism?
12. What are extensions in Swift, and what are their limitations?
13. How do you handle concurrency in Swift using Grand Central Dispatch (GCD)?
14. What are closures in Swift, and how do they capture values?
15. Explain the concept of optionals in Swift and how to unwrap them safely.
16. Describe the use of `guard` statements in Swift.
17. What are the benefits of using Swift's `Codable` protocol for serialization and deserialization?
18. How do you create and use custom operators in Swift?
19. What are Swift's access control levels (e.g., `private`, `fileprivate`, `internal`, `public`, `open`) and when would you use each?
20. Explain the difference between a `struct` and a `class` in Swift, and discuss their respective use cases.
Expert Swift interview questions
1. Explain the concept of 'copy-on-write' optimization in Swift and how it affects the performance of value types, using an example with arrays.
2. How does Swift's memory management handle retain cycles in closures, and what techniques can be used to prevent them, such as weak and unowned references?
3. Describe the differences between 'inout' parameters and returning a new value from a function. When would you choose one over the other, especially considering performance?
4. What are the advantages and disadvantages of using Swift's 'Codable' protocol for serialization and deserialization, and how does it compare to manual parsing?
5. Explain the use cases for Swift's property wrappers and provide an example of how you might create a custom property wrapper to enforce data validation.
6. Describe the role of Swift's 'Result' type and how it simplifies error handling compared to traditional try-catch blocks, especially in asynchronous operations.
7. How can you leverage Swift's generics to write more reusable and type-safe code, and what are the limitations of generics in certain scenarios?
8. Explain the concept of Protocol-Oriented Programming (POP) in Swift and how it promotes code reusability and testability compared to Object-Oriented Programming (OOP).
9. Describe the differences between 'Any' and 'AnyObject' in Swift and provide examples of when you might use each, considering type safety.
10. How does Swift handle concurrency using Grand Central Dispatch (GCD), and what are some common pitfalls to avoid when working with concurrent code?
11. Explain the use of Key-Value Observing (KVO) in Swift and how it can be used to observe changes in object properties, considering its performance implications.
12. Describe the purpose of Swift's 'defer' statement and how it ensures that code is executed regardless of how a function exits, including error handling.
13. How does Swift's optional chaining work, and what are some scenarios where it can simplify code while avoiding potential runtime crashes?
14. Explain the concept of associated types in protocols and how they allow you to define protocols that work with generic types, ensuring type safety.
15. Describe the use cases for Swift's 'dynamic' keyword and how it affects the behavior of method dispatch at runtime, considering its impact on performance.
16. How can you use Swift's Combine framework to handle asynchronous events and data streams, and what are the advantages of using Combine over traditional delegation patterns?
17. Explain the concept of method swizzling in Swift and how it can be used to modify the behavior of existing methods at runtime, considering its potential risks.
18. Describe the role of Swift's 'autoreleasepool' and how it manages memory for Objective-C objects in Swift code, especially when interoperating with legacy code.
19. How does Swift handle bridging between Swift types and Objective-C types, and what are some common challenges to consider when working with mixed codebases?
20. Explain the concept of SwiftUI's 'Environment' and how it allows you to pass data and dependencies down the view hierarchy, considering its impact on testability.
21. Describe the differences between 'ObservableObject' and '@StateObject' in SwiftUI, and when you would use each for managing state in your views.
22. How can you use Swift's actors to safely manage concurrent access to shared mutable state, preventing data races and ensuring thread safety?
23. Explain the purpose of Swift's opaque return types (using 'some View' in SwiftUI) and how they allow you to hide implementation details while still ensuring type safety.
24. Describe the use of Swift's new concurrency features like 'async' and 'await' and how they simplify asynchronous programming compared to GCD and closures.
25. How can you leverage Swift's macro system to generate code at compile time, and what are the benefits of using macros for code reduction and performance optimization?
26. Explain the concept of existential types (e.g., `any Protocol`) in Swift and how they differ from generics and concrete types, particularly in the context of protocol conformance.
27. Describe the advantages and disadvantages of using Swift's Core Data framework for data persistence, and how it compares to other options like Realm or SQLite.

99 Swift interview questions to hire top developers


Siddhartha Gunti Siddhartha Gunti

September 09, 2024


Hiring Swift developers can be challenging without the right resources. Interviewers need a reliable set of questions to assess candidates across different experience levels efficiently, from freshers to seasoned experts like how to hire mobile developers.

This blog post provides a curated list of Swift interview questions, categorized by experience level: freshers, juniors, intermediate, and experts; we've also included some MCQs. It's designed to equip recruiters and hiring managers with the tools needed to identify top Swift talent.

By using these questions, you can effectively gauge a candidate's Swift knowledge and skills, ensuring you make informed hiring decisions; for a more objective assessment, consider using Adaface's Swift online test before the interview to filter candidates.

Table of contents

Swift interview questions for freshers
Swift interview questions for juniors
Swift intermediate interview questions
Expert Swift interview questions
Swift MCQ
Which Swift skills should you evaluate during the interview phase?
3 Tips for Using Swift Interview Questions
Hire Top Swift Developers with Confidence
Download Swift interview questions template in multiple formats

Swift interview questions for freshers

1. What's the big idea behind Swift being a 'type-safe' language? Can you give a simple example?

Swift's type safety means the language strongly encourages you to be clear about the type of data you're working with. It prevents you from accidentally passing a value of one type where another type is expected. This helps catch errors early in development, preventing unexpected behavior or crashes at runtime. Essentially, the compiler checks that your code uses data in a consistent and intended way.

For instance, consider this example:

let myString: String = "Hello"
let myInt: Int = 42

// myString = myInt  // This would cause a compile-time error because you can't assign an Int to a String.
print(myString)
print(myInt)

In this case, the compiler would flag an error if you tried to assign myInt to myString because they are declared with different types. This prevents a potential runtime issue. You would need to explicitly convert the Int to a String to perform such assignment: myString = String(myInt).

2. Imagine you're explaining optionals to someone who's never coded before. How would you do it?

Imagine you have a box. This box might contain something, like a toy. Or, it might be empty. An optional is like that box in programming. It might hold a value (the toy), or it might hold nothing (be empty).

Think of it this way: if you ask a program for someone's middle name, they might have one, or they might not. An optional lets you represent both possibilities: either there is a middle name (a string), or there isn't (nothing, which we represent with null or nil). If the box (optional) is empty, trying to get the "toy" out would be like trying to use a value that doesn't exist, and the program will crash. Optionals provide a way to safely check if there is a value inside before you try to use it.

3. Can you explain the difference between a struct and a class in Swift, like I'm five? What are the real-world implications of these differences?

Imagine you have building blocks. A struct is like a set of LEGOs. If you give your friend a copy of your LEGO spaceship, they can build their own, and changing theirs doesn't change yours. They are independent. A class is like sharing a single whiteboard drawing. If your friend changes the drawing on the whiteboard, you see the changes too. Both of you are working on the same drawing.

In Swift, this means structs are value types (copied when passed around), so changes to a struct instance don't affect the original. Classes are reference types (a reference to the same object is passed around), so changes to a class instance do affect the original if multiple references exist. Think about modeling data where copies are important (coordinates, colors) - use a struct. For things that are unique and shared (like a game's player or a network connection), a class might be better. Also, classes have inheritance, which lets you build upon existing classes.

4. What's the deal with 'mutating' functions in structs? Why do we need them?

In many programming languages (like Swift, but the concept exists elsewhere too), structs are value types. This means when you pass a struct around, you're actually passing a copy of it. If you want a function to modify the original struct instance, you need to mark it as mutating. Without the mutating keyword, the function would operate on a copy, and any changes would be lost when the function ends.

The mutating keyword essentially grants the function permission to directly modify the struct's underlying memory. This is crucial for methods that need to update the struct's internal state. For example, consider a struct that represents a point in 2D space. A method to move the point would need to be mutating in order to change the point's x and y coordinates.

5. Tell me about 'enums' in Swift. When would you use them instead of just using strings or numbers?

Enums in Swift (enumerations) are a powerful way to define a type consisting of a set of named values, called cases. They provide a way to group related values in a type-safe manner. For example,

enum Direction {
    case north, south, east, west
}

Instead of strings or numbers, enums offer type safety and readability. Using strings or numbers can introduce errors due to typos or invalid values. Enums prevent these errors by enforcing a predefined set of possible values. They also support associated values (allowing you to attach additional information to each case) and can conform to protocols, making them more versatile than simple strings or numbers. Enums are preferred when you have a fixed set of related options or states, improving code clarity and preventing logical errors.

6. What are protocols in Swift? How can they help you write better code?

Protocols in Swift define a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. Classes, structs, and enums can then adopt these protocols, providing concrete implementations for the requirements. Think of them as interfaces in other languages.

Protocols help write better code in several ways:

  • Abstraction: They enable you to abstract away concrete types, working with generic code that operates on anything conforming to a specific protocol. This promotes loose coupling and makes your code more flexible.
  • Code Reusability: You can write extensions to protocols that provide default implementations of methods. This avoids code duplication among conforming types.
  • Polymorphism: Protocols allow different types to respond to the same method call in their own way, leading to more dynamic and adaptable systems.
  • Testing: Easy mocking by using protocols rather than concrete implementations.

7. What is protocol-oriented programming? Why is it important in Swift?

Protocol-oriented programming (POP) is a programming paradigm that emphasizes the use of protocols (interfaces) to define and structure code. Instead of focusing on inheritance from classes, POP focuses on defining what functionality a type should have through protocols, and then implementing that functionality via protocol conformance. This approach promotes code reusability and flexibility.

In Swift, POP is particularly important because it complements value types (structs and enums) very well. Swift's protocols can be adopted by classes, structs, and enums, offering a unified way to define behavior. Protocol extensions allow you to provide default implementations for protocol methods, promoting code reuse and avoiding code duplication, addressing some of the limitations of traditional object-oriented inheritance. This allows for a more flexible and composable system. Swift's standard library and frameworks like SwiftUI heavily leverage protocol-oriented programming.

8. Explain the difference between 'let' and 'var'. Why would you choose one over the other?

var and let are both used for variable declaration in JavaScript, but they have different scoping rules. var has function scope, meaning it's accessible throughout the entire function in which it's declared (or globally if declared outside any function). let, on the other hand, has block scope, limiting its accessibility to the block (e.g., inside an if statement or a for loop) where it's defined. Also, var allows redeclaration within its scope, while let doesn't. var variables are hoisted to the top of their scope and initialized with undefined, while let variables are also hoisted but not initialized, leading to a ReferenceError if accessed before declaration.

I would choose let over var in modern JavaScript development because let's block scoping prevents unintended variable hoisting and accidental variable re-declarations, which can lead to bugs. Using let generally results in more predictable and maintainable code. const is another alternative that is usually preferred for values that should not be reassigned.

9. What's the point of using 'guard let'? How does it make your code safer?

The primary purpose of guard let is to improve code readability and safety by handling optional unwrapping in a clear and concise manner, especially for early exits from a function or scope when a required value is nil. It ensures that the code only proceeds when the optional value contains a concrete value, thus preventing unexpected nil pointer exceptions or crashes. By unwrapping and binding the optional value to a new constant, we can safely use that constant within the scope where guard let is declared.

guard let makes code safer in the following ways:

  • Early Exit: If the optional is nil, the else block is executed, forcing an exit (usually via return, throw, break, or continue). This reduces nesting and makes control flow easier to follow.
  • Scope Safety: The unwrapped constant is available for the rest of the scope after the guard statement. This prevents accidental use of potentially nil values and promotes cleaner, more reliable code, reducing opportunities for runtime errors.

10. What are closures in Swift? Can you give a simple example of when you might use one?

Closures in Swift are self-contained blocks of functionality that can be passed around and used in your code. They can capture and store references to any constants and variables from the context in which they are defined. This allows closures to remember and operate on these values even when the original scope is no longer available.

A simple example would be using a closure for a callback. For instance, when performing an asynchronous network request, you can pass a closure that will be executed when the request completes. This closure can access and update UI elements or perform other actions based on the response data:

func fetchData(completion: @escaping (String?) -> Void) {
    // Simulate fetching data asynchronously
    DispatchQueue.global().async {
        let data = "Some data"
        DispatchQueue.main.async {
            completion(data)
        }
    }
}

fetchData { (result) in
    if let data = result {
        print("Data received: \(data)")
    }
}

11. How do you handle errors in Swift? What's the 'do-try-catch' thing all about?

Swift provides robust error handling using the Error protocol and do-try-catch blocks. Functions that can throw errors are marked with the throws keyword. To handle these potential errors, you enclose the code that might throw an error within a do block. Inside the do block, you use the try keyword before calling the throwing function. If the function throws an error, control is immediately transferred to the catch clauses.

Each catch clause specifies a pattern to match against the thrown error. If a catch clause's pattern matches the error, that clause's code block is executed. You can have multiple catch clauses to handle different types of errors. For instance:

enum MyError: Error {
 case invalidInput
 case networkError
}

func riskyFunction(input: Int) throws {
 if input < 0 { throw MyError.invalidInput }
 // ... some code that might throw networkError
}

do {
 try riskyFunction(input: -1)
 // ... code to execute if no error is thrown
} catch MyError.invalidInput {
 print("Invalid input provided.")
} catch MyError.networkError {
 print("A network error occurred.")
} catch {
 print("An unexpected error occurred: \(error)")
}

12. What are generics in Swift? Why should we care about them?

Generics in Swift enable you to write flexible, reusable functions and types that can work with any type. Instead of writing separate functions for Int, String, etc., you can write one generic function that works for all types. This is achieved through type parameters. For example, func myFunc<T>(value: T) -> T { return value } defines a generic function where T is the type parameter.

We should care about generics because they promote code reusability and type safety. Using generics reduces code duplication, making the codebase cleaner and easier to maintain. Generics enforce type safety at compile time, preventing runtime errors that might occur with Any type.

13. What is the difference between 'nil' and an empty string in Swift?

In Swift, nil and an empty string ("") represent different concepts. nil signifies the absence of a value; it indicates that a variable of an optional type does not currently hold any value. An empty string, on the other hand, is a valid string value that simply contains no characters. It's a string that exists but is empty.

Think of it this way: nil means there is no string, whereas "" means there is a string, but it's just empty. You can perform string operations on an empty string, but trying to perform string operations on a nil string will usually lead to a runtime error (unless you've handled the optional unwrapping safely, for example with optional chaining).

14. Can you explain how memory management works in Swift? What is ARC?

Swift uses Automatic Reference Counting (ARC) for memory management. ARC automatically manages the memory lifecycle of class instances, freeing up memory when instances are no longer needed. It works by keeping track of how many references there are to each class instance. When the reference count drops to zero, meaning no other part of the application is using that instance, ARC deallocates the memory used by that instance. This prevents memory leaks.

Essentially, ARC inserts retain and release calls in your code at compile time. Retain increments the reference count, indicating that an instance is still in use. Release decrements the reference count. When the count reaches zero, the instance is deallocated. However, strong reference cycles (where two instances hold strong references to each other, preventing deallocation) can occur, so Swift provides mechanisms like weak and unowned references to break these cycles. Weak references do not increase the reference count, and become nil when the object they refer to is deallocated. Unowned references also don't increase the reference count but are assumed to always have a value (should be used carefully).

15. What are extensions in Swift? How can they be helpful?

Extensions in Swift allow you to add new functionality to existing types, such as classes, structures, enumerations, or protocols. This is done without modifying the original source code of the type. They are similar to categories in Objective-C, but extensions in Swift don't have names. Extensions can add new computed properties, define new methods, provide new initializers, define subscripts, define and use new nested types, and make an existing type conform to a protocol.

Extensions are helpful for several reasons:

  • Adding functionality to types you don't own: You can extend types from frameworks or libraries without subclassing.
  • Organizing code: You can group related methods or properties into extensions to improve code readability and maintainability. For example, you might use extensions to separate protocol conformance from the main type definition.
  • Retroactive modeling: Enable existing type to conform to new protocols.
  • Code reusability: Extending functionality without creating a new object.

16. What are the different access control levels in Swift (e.g., private, public, internal)? Why are they important?

Swift defines five access control levels: open, public, internal, fileprivate, and private. open and public enable entities to be used within any source file from their defining module, and also from another module that imports the defining module, with open being more permissive allowing subclassing and overriding outside the defining module. internal allows use within any source file from the defining module, but not from outside. fileprivate restricts use to the defining source file. private restricts use to the enclosing declaration, and extensions of that declaration in the same file.

Access control is crucial for encapsulation and modularity. It hides implementation details, preventing unintended dependencies and modifications from external code. This improves code maintainability, reduces the risk of errors, and allows developers to change internal implementations without breaking external code. It also aids in designing clear APIs and promoting code reuse by exposing only the necessary functionality.

17. Explain the concept of a computed property in Swift. How is it different from a stored property?

A computed property in Swift doesn't actually store a value. Instead, it provides a getter and an optional setter to indirectly access and compute other properties' values. Think of it as a calculated value on demand. It's declared using var (even though it might not seem to store a direct value) followed by a type, a getter block (get {}), and optionally a setter block (set(newValue) {}).

In contrast, a stored property directly holds a value in memory. It's declared using either var (for mutable properties) or let (for immutable properties). Stored properties are the fundamental way to store data within a Swift type, while computed properties provide a way to dynamically calculate or derive values from stored properties or other sources. Example:

struct Circle {
    var radius: Double
    var area: Double {
        get {
            return Double.pi * radius * radius
        }
        set(newArea) {
            radius = sqrt(newArea / Double.pi)
        }
    }
}

18. What are the benefits of using Swift's collection types (arrays, dictionaries, sets)? How do they differ?

Swift's collection types (Arrays, Dictionaries, Sets) offer several benefits, including type safety, performance optimizations, and ease of use. They are generic, ensuring that you can only store a specific type of element within each collection, preventing runtime errors. Swift's collections are also value types, which promotes predictable behavior and simplifies memory management through copy-on-write semantics.

Arrays are ordered collections of elements, accessed by their index (starting from 0). Dictionaries store key-value pairs, where each key must be unique and conform to the Hashable protocol; they provide fast lookups by key. Sets are unordered collections of unique elements; they are useful for checking membership efficiently and performing set operations like union or intersection. Arrays guarantee element order, Dictionaries guarantee key uniqueness, and Sets guarantee element uniqueness.

19. What's the deal with 'defer' in Swift? When would you use it?

In Swift, defer is used to execute a block of code just before the current scope exits. This applies whether the scope exits normally or due to an error (e.g., throw). It essentially postpones the execution of the code block until the very end.

Use cases for defer include resource cleanup (e.g., closing files, releasing locks), ensuring paired actions are always performed (e.g., beginEditing/endEditing or acquireLock/releaseLock), and generally guaranteeing certain operations happen regardless of the exit path. Consider this example:

func processFile(filename: String) throws {
 let file = FileHandle(forReadingAtPath: filename)
 defer { 
 file?.closeFile()
 print("File closed.")
 }

 // ... file processing logic here, possibly throwing errors ...
}

In this example, file?.closeFile() will be executed whether processFile completes normally or throws an error during file processing.

20. How do you perform asynchronous operations in Swift? What are some common techniques?

Swift offers several ways to perform asynchronous operations. Some common techniques include:

  • DispatchQueues: Using DispatchQueue.global(qos: .background).async or custom queues lets you execute code concurrently, preventing blocking the main thread. For example:

    DispatchQueue.global(qos: .background).async {
        // Perform a long-running task here
        DispatchQueue.main.async {
            // Update the UI on the main thread
        }
    }
    
  • Grand Central Dispatch (GCD): GCD is a low-level C-based API providing queues to manage concurrency. DispatchWorkItem can be used to encapsulate a task and manage its execution.

  • async/await: Introduced in Swift 5.5, async/await provides a more readable and structured way to handle asynchronous code. You define asynchronous functions using the async keyword and use await to suspend execution until the asynchronous operation completes. For example:

    func fetchData() async throws -> Data {
        let (data, _) = try await URLSession.shared.data(from: url)
        return data
    }
    
    // Calling the asynchronous function
    Task {
        do {
            let data = try await fetchData()
            // Process the data
        } catch {
            // Handle the error
        }
    }
    
  • Operation and OperationQueue: This provides a more object-oriented approach to managing concurrent operations, allowing you to define dependencies and control the execution order of tasks.

21. Explain the difference between '==' and '===' in Swift (if there is one, and if not, why?).

In Swift, == is the equality operator, used to compare the values of two instances. The types being compared must conform to the Equatable protocol. Swift provides default implementations of Equatable for many common types (e.g., Int, String, Bool). You can define your own Equatable conformance for custom types, specifying how two instances of that type should be compared for equality.

=== is the identity operator. It checks if two variables refer to the exact same instance in memory. It’s specifically used for comparing reference types (classes). Comparing value types (structs, enums) with === will result in a compile-time error because value types are copied when assigned, so two variables of value type can never refer to the exact same memory location. For example:

class MyClass {}

let instance1 = MyClass()
let instance2 = instance1

instance1 === instance2 // true, same instance
instance1 == instance2 // may or may not be true, depending on Equatable implementation.

22. What are higher-order functions in Swift? Can you name a few common ones?

Higher-order functions in Swift are functions that can take other functions as arguments, return functions as their results, or both. This allows for powerful abstractions and code reuse. They treat functions as first-class citizens.

Some common higher-order functions in Swift include:

  • map: Transforms each element in a collection.
  • filter: Creates a new collection containing only elements that satisfy a condition.
  • reduce: Combines the elements of a collection into a single value.
  • flatMap: Transforms each element into a sequence, then flattens the resulting sequences into a single collection.
  • compactMap: Transforms each element while removing nil values.

23. Describe a situation where you might use a delegate in Swift.

A delegate can be useful when you want one object to communicate with another object without tightly coupling them. For example, imagine you have a CustomTextField class and you want to notify another object (like a ViewController) when the text field's content changes or when the user presses the return key.

In this case, you could define a protocol (e.g., CustomTextFieldDelegate) with methods like textFieldDidUpdate(_ textField: CustomTextField, text: String) and textFieldShouldReturn(_ textField: CustomTextField) -> Bool. The CustomTextField would have a delegate property conforming to this protocol. The ViewController can then adopt this protocol and become the delegate of the CustomTextField, allowing it to receive these notifications and react accordingly. This decoupling allows you to reuse CustomTextField in different contexts with varying responses to its events.

24. What are Key-Value Observing (KVO) and Key-Value Coding (KVC) in Swift? When might you use them?

Key-Value Observing (KVO) and Key-Value Coding (KVC) are mechanisms in Swift (inherited from Objective-C) for accessing and observing object properties indirectly using strings to identify the properties. KVC allows you to access and manipulate object properties by name as strings, rather than using direct property accessors. KVO allows objects to be notified when a specific property of another object has changed.

When to use them:

  • Observing Model Changes: KVO is useful for keeping UI elements in sync with the underlying data model (e.g., automatically updating a label when a property of your data model changes).
  • Decoupling Components: KVO allows you to observe changes in an object without needing to know its internal implementation details, promoting loose coupling.
  • Dynamic Property Access: KVC allows you to access properties whose names might not be known at compile time, such as when dealing with data from external sources or implementing generic data access patterns.
  • Example KVO:
class MyObject: NSObject {
 @objc dynamic var myProperty: String = "Initial Value"
}

let myObject = MyObject()

myObject.observe(\.myProperty, options: [.new]) { (object, change) in
 print("Property changed to: \(change.newValue as? String ?? "")")
}

myObject.myProperty = "New Value"

25. How would you typically structure a Swift project? What are some common architectural patterns?

A typical Swift project structure separates concerns through modularity. You'd have folders for Models (data structures), Views (UI elements), ViewModels (logic for the views), Controllers (handling user input and coordinating between Models and Views), and Services (networking, database interactions). Other folders may include Utilities (helper functions), Extensions, and Resources (images, etc.).

Common architectural patterns include:

  • MVC (Model-View-Controller): Classic pattern, good for simpler apps.
  • MVVM (Model-View-ViewModel): Improves testability and separation of concerns, especially beneficial for complex UIs. ViewModel acts as intermediary between View and Model.
  • VIPER (View-Interactor-Presenter-Entity-Router): More complex, promoting high modularity and testability. Suitable for large projects. Each component has a specific responsibility.
  • Coordinator Pattern: Handles navigation logic, decoupling view controllers. This helps keep view controllers lean.

26. What is the purpose of the 'Codable' protocol in Swift?

The Codable protocol in Swift (which is actually a type alias for Decodable & Encodable) enables the conversion between Swift data types and an external representation like JSON or Property List (plist). In essence, it automates the serialization and deserialization process.

By conforming a type to Codable, you're telling the compiler that this type can be easily converted to and from a storable or transferable format. This eliminates the need to write boilerplate code for parsing JSON responses from APIs or saving data to disk. The Swift standard library provides JSONEncoder and JSONDecoder to handle the encoding and decoding of Codable types to and from JSON, respectively.

27. What is a framework? Have you used any frameworks and how was the experience?

A framework is a reusable, semi-complete software application that provides a foundation to develop other applications. It dictates the structure of the application, providing common functionalities and libraries, so developers can focus on the specific features they need to build. Think of it as a pre-built house structure; you don't need to lay the foundation, but you customize the interior and add furniture.

I've used several frameworks, including React for frontend development. My experience with React was positive. It provided a component-based architecture which made UI development more modular and reusable. Features like the virtual DOM and JSX improved performance and developer experience. I also used Spring Boot for building Java-based backend services. Spring Boot's auto-configuration and dependency injection significantly simplified the development and deployment process. Overall, frameworks like these significantly improve development speed, code quality and maintainability.

28. Can you explain the concept of immutability in Swift and why it's important?

Immutability in Swift means that once a variable or data structure is created, its value cannot be changed. This is achieved using keywords like let for constants. When something is immutable, you're guaranteed that its value will remain the same throughout its lifetime.

Immutability is important because it promotes safer and more predictable code. It simplifies reasoning about program state, reduces the risk of unintended side effects and data corruption, and makes concurrent programming easier. Immutability leads to code that is easier to test, debug, and maintain, as you don't have to worry about hidden state changes.

29. What is method swizzling and why should you avoid it?

Method swizzling is a technique in Objective-C (and Swift using Objective-C runtime) that allows you to change the implementation of an existing method at runtime. Essentially, you're swapping the pointers of two methods, so when one method is called, the other's implementation is executed.

While powerful, method swizzling should generally be avoided due to its potential for creating unexpected behavior, making debugging significantly harder. It introduces global side effects, meaning a swizzle in one part of the code can affect the behavior of the entire application, potentially leading to conflicts with other libraries or frameworks that might also be using swizzling. It can also violate the principle of least astonishment, as the code no longer behaves as expected, making it difficult for other developers (or even yourself later) to understand the program's flow.

30. What is dynamic dispatch and static dispatch? How do they relate to Swift?

Dynamic dispatch determines which method implementation to call at runtime. This allows for polymorphism, where different objects can respond differently to the same method call. In Swift, dynamic dispatch is achieved using class types, protocol requirements without Self or associated types, and the @objc attribute, which enables Objective-C runtime features. This allows for greater flexibility but comes with a performance cost due to the runtime lookup.

Static dispatch, also known as compile-time dispatch, resolves method calls at compile time. This is faster because the specific implementation to be called is known beforehand. In Swift, struct and enum types use static dispatch by default. The final keyword and whole module optimization can also enable static dispatch for class methods. Static dispatch provides better performance but limits flexibility and polymorphism.

Swift interview questions for juniors

1. What is the difference between `let` and `var`? Can you explain with an example?

let and var are both used for variable declaration in JavaScript, but they differ in scope.

var has function scope, meaning it's accessible throughout the entire function it's declared in (or globally if declared outside any function). let has block scope, meaning it's only accessible within the block (e.g., an if statement, a for loop) where it's defined.

function example() {
  var x = 10;
  if (true) {
    let y = 20;
    var x = 30; // modifies the outer x
    console.log(y); // Output: 20
  }
  console.log(x); // Output: 30 (because var x was redeclared inside the if block and modified the function-scoped x)
  //console.log(y); // Error: y is not defined outside the if block
}
example();

2. Imagine you're explaining what an optional is to a friend. How would you describe it and why it's useful?

Imagine you have a box. An optional is like that box. It might contain something (a value), or it might be empty (nothing). It's a way to handle the possibility that a variable might not have a value.

Why is it useful? It helps prevent errors, specifically NullPointerExceptions or similar issues in other languages. Instead of directly accessing a variable that could be null, you can check if the optional contains a value before trying to use it. For example, in Java you might see something like Optional<String> name = ...; and then you'd use name.isPresent() or name.orElse("default name") to handle the potential absence of a name, which forces you to think about the case where there is nothing there.

3. What are the basic data types in Swift (like Int, String, Bool)? Can you give an example of when you might use each?

Swift has several basic data types. Int represents whole numbers (e.g., let age: Int = 30). Double and Float represent floating-point numbers with different precisions; Double offers more precision (e.g., let price: Double = 99.99). String represents textual data (e.g., let name: String = "Alice"). Bool represents a boolean value, either true or false (e.g., let isLoggedIn: Bool = true).

I might use Int for storing ages, counts, or indices. Double would be suitable for representing prices, measurements, or calculations that require decimal precision. String would be used for storing names, addresses, or any other textual data. Bool is perfect for flags, status indicators, or conditions in control flow statements.

4. What is a function in Swift? Can you write a simple function that adds two numbers together?

In Swift, a function is a self-contained block of code that performs a specific task. You give it a name, and you can call it multiple times from different parts of your program to reuse the code. Functions can take inputs, called parameters, and can return a value after they're finished.

Here's a simple Swift function that adds two numbers together:

func addTwoNumbers(number1: Int, number2: Int) -> Int {
    return number1 + number2
}

// Example usage
let sum = addTwoNumbers(number1: 5, number2: 3) // sum will be 8
print(sum)

5. What is an Array, and how do you add or remove elements from it?

An array is a data structure that stores a collection of elements of the same data type in contiguous memory locations. This allows for efficient access to elements using their index (position) within the array. Arrays have a fixed size in many languages, but dynamically sized arrays (like lists in Python or ArrayLists in Java) exist.

Adding and removing elements depends on the language and whether the array is fixed-size or dynamic. For fixed-size arrays, you typically cannot directly add or remove elements. You would need to create a new array with the desired size and copy the elements. For dynamic arrays, you can use methods like:

  • push() (add to the end)
  • pop() (remove from the end)
  • insert(index, element) (add at a specific index)
  • remove(index) or remove(element) (remove at a specific index or element, respectively)

6. What is a Dictionary, and how is it different from an Array?

A Dictionary (also known as a Map or Associative Array) is a data structure that stores data in key-value pairs. Each key is unique within the dictionary, and it is used to access its corresponding value. Think of it like a real-world dictionary where you use a word (the key) to look up its definition (the value).

Arrays, on the other hand, store data in a sequential, ordered manner. Elements in an array are accessed using their index (position), which is typically a number starting from 0. The key differences are:

  • Access Method: Dictionaries use keys, while arrays use indices.
  • Ordering: Arrays maintain a specific order of elements, while dictionaries generally do not guarantee any specific order (although some implementations might).
  • Key Uniqueness: Dictionary keys must be unique, array elements can be duplicated.

7. Explain the concept of a loop in Swift. Give an example of using a 'for' loop.

In Swift, a loop is a control flow statement that allows code to be executed repeatedly. It's used to automate repetitive tasks, saving time and effort. There are several types of loops in Swift, including for loops, while loops, and repeat-while loops.

Here's an example of using a for loop to iterate through an array:

let numbers = [1, 2, 3, 4, 5]

for number in numbers {
    print(number)
}

In this example, the code inside the curly braces {} is executed once for each number in the numbers array. The output would be the numbers 1 through 5, each printed on a new line.

8. What is the purpose of an 'if' statement? Can you give an example of using one?

The purpose of an 'if' statement is to conditionally execute a block of code based on whether a certain condition is true or false. It allows programs to make decisions and follow different execution paths depending on the input or current state.

Here's a simple example in Python:

x = 10
if x > 5:
    print("x is greater than 5")

In this example, the code inside the if block (the print statement) will only be executed if the condition x > 5 is true.

9. What is a class in Swift? How does it relate to creating objects?

In Swift, a class is a blueprint for creating objects. It defines the properties (data) and methods (behavior) that objects of that class will have. Think of it as a template.

The relationship is this: a class describes what an object will be, and an object is an instance of that class. You use the class definition to create actual objects in memory, each having its own set of values for the properties defined in the class. For example:

class Dog {
 var name: String
 init(name: String) {
 self.name = name
 }
 func bark() {
 print("Woof!")
 }
}

let myDog = Dog(name: "Buddy") // myDog is an object, an instance of the Dog class
myDog.bark()

10. What is the difference between a struct and a class in Swift? When might you choose one over the other?

In Swift, both structs and classes are used to create custom data types, but they have key differences. The most significant is that structs are value types, while classes are reference types. When a struct is copied, a completely new instance is created, independent of the original. When a class instance is copied, a new reference to the same underlying object is created. Structs also have a compiler-generated memberwise initializer, which classes don't have by default. Additionally, structs in Swift do not support inheritance.

Choose structs when you want to ensure data is copied and independent, preventing unintended side effects (e.g., for representing simple values like coordinates or sizes). Classes are preferred when you need shared mutable state and identity, or when you need inheritance (e.g., for UI components or managing complex object relationships). Also, consider using structs for types that are small and frequently copied, as they can offer performance benefits due to avoiding heap allocation.

11. What is inheritance in Swift, and why is it useful?

Inheritance in Swift is a mechanism where a new class (subclass or derived class) can inherit properties and methods from an existing class (superclass or base class). This allows you to create a hierarchy of classes, where subclasses inherit characteristics from their superclasses and can add their own unique functionality.

Inheritance is useful for code reuse, reducing redundancy, and establishing a clear relationship between classes. By inheriting from a common superclass, subclasses automatically gain access to its properties and methods, promoting a more organized and maintainable codebase. This also facilitates polymorphism, enabling you to treat objects of different classes in a uniform way through their shared superclass interface.

12. Can you describe what a protocol is and how it is used in Swift?

In Swift, a protocol is a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. Protocols themselves don't implement any functionality; instead, they define what a conforming type (class, struct, or enum) must implement. Think of it as an interface or contract.

Protocols are used extensively in Swift to achieve polymorphism, define interfaces, and enable features like protocol-oriented programming. For example, Equatable protocol requires conforming types to implement the == operator, allowing instances of that type to be compared for equality. Similarly, Collection is a protocol that defines the basic requirements for a collection type, such as arrays and dictionaries. Conformance is indicated by adding the protocol name after the type's name, like so: struct MyStruct: MyProtocol { ... }

13. What is the point of using guard statements in Swift?

Guard statements in Swift enhance code readability and maintainability by providing a concise way to exit a scope if certain conditions are not met. They improve clarity by handling exceptional cases early, preventing deeply nested if statements. This results in cleaner code flow, making it easier to understand the intended logic.

Guard statements are especially useful for:

  • Input Validation: Checking for valid function arguments.

  • Optional Binding: Safely unwrapping optional values.

  • Early Exits: Returning from a function or loop when preconditions are not satisfied. For example:

    func processValue(value: Int?) {
        guard let value = value else {
            print("Value is nil, exiting.")
            return
        }
        // Use the unwrapped 'value' here
        print("Processing value: \(value)")
    }
    

14. Explain the difference between '==' and '===' in Swift, if any.

In Swift, == is used for value equality while === is used for identity equality. == checks if two instances have the same value based on their content; this often involves comparing properties defined within the type. The type can define its own implementation for the == operator.

=== on the other hand, checks if two variables/constants refer to the exact same instance in memory. It verifies if they are the same object, not just if they have the same value. === can only be used with reference types (classes), as value types (structs, enums) always create copies when assigned, so identity comparison wouldn't be meaningful. If two variables point to the same memory address, only then === will return true.

15. How do you handle errors in Swift using 'try', 'catch'?

In Swift, error handling with try, catch involves first defining errors using enums that conform to the Error protocol. When a function can throw an error, it's marked with the throws keyword. To handle potential errors, you use a do-catch block. Inside the do block, you call the throwing function with try. If the function throws an error, control is immediately transferred to the catch block.

For example:

enum MyError: Error {
    case invalidInput
    case notFound
}

func process(value: Int) throws -> Int {
    guard value > 0 else { throw MyError.invalidInput }
    // ... some processing
    return value * 2
}

do {
    let result = try process(value: -1)
    print("Result: \(result)")
} catch MyError.invalidInput {
    print("Invalid input error!")
} catch MyError.notFound {
    print("Not found error!")
} catch {
    print("Some other error occurred: \(error)")
}

16. What are closures in Swift? Have you used them before?

Closures in Swift are self-contained blocks of functionality that can be passed around and used in your code. They can capture and store references to any constants and variables from the context in which they are defined. This is known as closing over those variables. Closures are similar to blocks in C and Objective-C and to lambdas in other programming languages.

Yes, I have used closures extensively. For example:

  • As completion handlers: When performing asynchronous tasks, like network requests.
  • For array manipulations: Using functions like map, filter, and reduce.
  • To create callbacks: To respond to events or actions in a decoupled way.
// Example using a closure as a completion handler
func performRequest(completion: @escaping (String?) -> Void) {
    // Simulate an asynchronous task
    DispatchQueue.global().async {
        sleep(1)
        let result = "Request completed"
        DispatchQueue.main.async {
            completion(result)
        }
    }
}

performRequest { (result) in
    if let result = result {
        print(result)
    }
}

17. What is a Swift Package? Have you used any?

A Swift Package is a reusable collection of Swift, C, C++, and Objective-C code, along with resources like images and data, that can be easily distributed and integrated into other projects. It's a way to modularize code and share it between different applications or developers. The Swift Package Manager (SPM) is the tool used to manage these packages and their dependencies.

Yes, I have used Swift Packages. Some examples include:

  • Alamofire: For making HTTP requests.
  • Kingfisher: For downloading and caching images from the web.
  • SwiftLint: To enforce Swift style and conventions.

18. How do you use storyboards to build a user interface in iOS?

Storyboards provide a visual interface for designing and laying out an iOS app's UI. You drag and drop UI elements (labels, buttons, text fields, etc.) from the object library onto the storyboard canvas. Then you use constraints to define the size and position of these elements, ensuring they adapt to different screen sizes and orientations.

To connect UI elements to your code, you create outlets (using IBOutlet) and actions (using IBAction) in your view controller classes. You then control-drag from the UI element in the storyboard to the corresponding view controller file to establish these connections. Transitions between different views are handled using segues, which can be configured directly within the storyboard to define how one view controller leads to another.

19. What is Auto Layout and how does it help create user interfaces that work on different screen sizes?

Auto Layout is a constraint-based layout system in UIKit (iOS) and AppKit (macOS) that allows developers to create user interfaces that dynamically adapt to different screen sizes and orientations. Instead of specifying fixed positions and sizes for UI elements, Auto Layout defines relationships (constraints) between them.

By using constraints, Auto Layout ensures that UI elements maintain their relative positions and sizes regardless of the screen size. For example, you can specify that a button should always be centered horizontally and vertically, or that a text field should always fill the width of the screen minus a certain margin. This makes it possible to create user interfaces that look good and function correctly on a variety of devices without requiring separate layouts for each one.

20. Explain the Model-View-Controller (MVC) design pattern.

MVC is a software design pattern that divides an application into three interconnected parts: the Model, the View, and the Controller. The Model manages the application's data and business logic. It notifies the View and Controller when its data changes. The View is responsible for presenting the data to the user. It retrieves data from the Model and displays it. The Controller acts as an intermediary between the Model and the View. It receives user input from the View, processes it, and updates the Model accordingly.

Essentially, the Controller handles user requests, the Model manages data, and the View displays the data to the user. This separation of concerns makes the application more maintainable, testable, and scalable. For example, consider a web application:

  • Model: Database tables that store users and their posts.
  • View: HTML templates that display the list of posts by a user.
  • Controller: Code that handles user requests to create, update, or delete posts. PHP, Python, Java, Javascript are common languages to code controllers in.

21. What is the purpose of using enums in Swift? Can you show me an example?

Enums in Swift serve to define a type that represents a set of related values. They enhance code readability and safety by providing a way to work with a fixed list of options. Using enums reduces the risk of errors caused by using incorrect or misspelled raw values.

Here's a simple example:

enum Direction {
    case north
    case south
    case east
    case west
}

let myDirection: Direction = .north

switch myDirection {
case .north:
    print("Going north")
case .south:
    print("Going south")
case .east:
    print("Going east")
case .west:
    print("Going west")
}

22. Have you used any debugging tools in Xcode? Which ones do you find the most useful?

Yes, I've used Xcode's debugging tools extensively. I find several of them particularly useful. The breakpoint navigator is essential for pausing execution at specific lines of code to inspect variables and program state. I also frequently use stepping controls like Step Over, Step Into, and Step Out to navigate through the code execution flow. The variables view is invaluable for examining the values of variables and objects at runtime, and I often use the console (lldb) to execute commands and print variable values, especially when needing more complex data formatting or manipulation.

Another helpful tool is Xcode's memory graph debugger. This is great for identifying memory leaks and retain cycles, which is crucial for building stable and performant iOS applications. I also occasionally use Instruments for more in-depth performance profiling, but for day-to-day debugging, the features directly within Xcode are usually sufficient. For instance, I often use the expression evaluator (within lldb) to quickly test assumptions and modify program state during debugging, like po someObject.someProperty or expr someVariable = newValue.

Swift intermediate interview questions

1. What are higher-order functions in Swift, and can you provide an example of how you've used one?

Higher-order functions in Swift are functions that can take other functions as arguments, return functions as their results, or both. They enable powerful abstractions and functional programming techniques. Common examples include map, filter, reduce, sorted, and flatMap.

I've used map extensively. For instance, if I have an array of integers and want to create a new array containing the square of each number, I could use map like this:

let numbers = [1, 2, 3, 4, 5]
let squaredNumbers = numbers.map { $0 * $0 } // squaredNumbers will be [1, 4, 9, 16, 25]

This avoids explicit loops and makes the code more concise and readable.

2. Explain the difference between `class` and `static` keywords when defining type members.

The class keyword (used with class variables and methods in some languages like Kotlin) indicates that the member belongs to the class itself, but is not tied to any specific instance of that class. It's similar to static but allows for inheritance and polymorphism, meaning a subclass can override the class member.

In contrast, static members are also associated with the class rather than an instance, but they are typically not inheritable or overridable. Think of static as a single, shared copy for all instances (or no instances at all), while class members (where applicable) allow subclasses to have their own version of the member. Example (java): static int count; is shared and static void myMethod() {} cannot be overridden. class modifier allows overriding of the method by any class inheriting from that base class.

3. Describe the use cases for `defer` in Swift, and how it affects the control flow.

In Swift, defer is used to execute a block of code just before the current scope exits. This ensures that the code within the defer block runs regardless of how the scope is exited, whether it's through normal execution, a return statement, or an error being thrown. Common use cases include releasing resources (like closing files or database connections), cleaning up temporary data, or undoing state changes.

The defer statement affects control flow by delaying the execution of its code block until the very end of the current scope. If multiple defer statements are present in the same scope, they are executed in the reverse order of their appearance. For example:

func example() {
  defer { print("First") }
  defer { print("Second") }
  print("Third")
}
example() // Prints: Third, Second, First

4. What are Swift's KeyPaths, and how can they be used with KVO?

KeyPaths in Swift provide a type-safe way to refer to properties of a type. They are represented using the \Root.Value syntax, where Root is the type containing the property and Value is the type of the property itself. They essentially encapsulate a reference to a property, allowing you to get or set the property's value in a type-safe manner at runtime. For instance, \Person.name refers to the name property of the Person struct or class.

Key-Value Observing (KVO) is a mechanism that allows objects to be notified when properties of other objects change. KeyPaths play a crucial role in KVO because they provide a type-safe way to specify which property you want to observe. Using a KeyPath with KVO involves observing the changes on the specified property identified by the KeyPath. For example, to observe the name property of a Person object, you would use the KeyPath \Person.name when registering as an observer. When the name property changes, the observer receives a notification. object.observe(\.propertyName, options: [.new]) { change in ...} is a code block demonstrating the idea.

5. How do you handle errors in Swift, and what are the advantages of using `Result` type?

In Swift, error handling is primarily done using the Error protocol and the do-catch mechanism. Functions that can throw errors are marked with the throws keyword. We use do blocks to wrap code that might throw, and catch blocks to handle specific error types. Alternatively, we can use optional chaining or the try? keyword to handle errors gracefully, especially when the error isn't critical.

The Result type, introduced in recent Swift versions, offers a more explicit and type-safe way to represent operations that can succeed or fail. Instead of throwing errors, a function can return a Result<Success, Failure> enum. Advantages include:

  • Explicitness: It makes it clear that a function can fail, as it's part of the return type.
  • Type Safety: The compiler enforces that you handle both success and failure cases.
  • Clarity: It avoids the potential issues of implicitly unwrapped optionals arising from try?.
  • Composition: Result is easily composable with other functional programming techniques.

6. Explain the concept of property observers (`willSet`, `didSet`) and their use cases.

Property observers in Swift allow you to execute code before and after a property's value is set. willSet is called just before the value is stored, and didSet is called immediately after the new value is stored. Inside willSet, newValue is available to access the upcoming new value. Inside didSet, oldValue gives access to the previous value of the property.

Use cases include:

  • Validating new values: willSet can check if a new value is valid and prevent it from being assigned if it isn't.
  • Updating UI: didSet can update UI elements based on the new property value.
  • Triggering side effects: didSet can trigger other functions or computations that depend on the property's value. For example:
var age: Int = 0 {
    willSet { print("About to set age to \(newValue)") }
    didSet { print("Age changed from \(oldValue) to \(age)") }
}

7. Describe the differences between `Any` and `AnyObject` in Swift.

Any and AnyObject are both used for type erasure in Swift, but they have distinct purposes.

  • Any: Represents an instance of any type, including class types, struct types, enum types, and primitive types (like Int, String, Bool). You can use Any when you need to work with values of unknown or mixed types.
  • AnyObject: Represents an instance of any class type. It's specifically for class instances and doesn't include structs, enums, or primitive types. AnyObject was more relevant in bridging with Objective-C code, as it directly corresponds to id in Objective-C. Note that AnyObject only conforms to class types.

8. What are generics in Swift, and why are they useful? Provide an example.

Generics in Swift enable you to write flexible, reusable functions and types that can work with any type. They allow you to write code that is not specific to a particular type, avoiding code duplication and improving type safety. Instead of writing separate functions for Int, String, or other types, you can write a single generic function that works with all of them.

For example, consider a function to swap two values. Using generics, you can write:

func swapValues<T>(_ a: inout T, _ b: inout T) {
    let temp = a
    a = b
    b = temp
}

var num1 = 5
var num2 = 10
swapValues(&num1, &num2) // num1 is now 10, num2 is 5

var str1 = "hello"
var str2 = "world"
swapValues(&str1, &str2) // str1 is now "world", str2 is "hello"

Without generics, you would need to write separate swapValues functions for each data type like Int, String etc., making code more verbose and harder to maintain. Generics promote code reusability and type safety by ensuring that the types used are consistent at compile time.

9. Explain the use of `inout` parameters in Swift functions.

In Swift, inout parameters allow a function to directly modify the original variable passed as an argument. Normally, function parameters are constants, meaning their values cannot be changed within the function's scope. inout overrides this behavior.

When declaring a function, you precede the parameter's type with the inout keyword (e.g., func modifyValue(value: inout Int)). When calling the function, you must precede the variable you're passing with an ampersand (&) to indicate that it can be modified. For example:

func doubleValue(value: inout Int) {
 value *= 2
}

var myNumber = 5
doubleValue(value: &myNumber) // myNumber is now 10

10. How does Swift's memory management work, and what is ARC?

Swift uses Automatic Reference Counting (ARC) for memory management. ARC automatically frees up memory used by class instances when they are no longer needed. It works by keeping track of how many references (strong references, by default) there are to each class instance. When the reference count drops to zero, meaning no other part of the code is using that instance, ARC deallocates the memory.

ARC eliminates many of the complexities of manual memory management (like in C or Objective-C without ARC), but it's still important to avoid strong reference cycles (retain cycles) where two instances hold strong references to each other, preventing either from being deallocated. weak and unowned references can be used to break these cycles.

11. What are protocols in Swift, and how do they enable polymorphism?

Protocols in Swift define a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. Classes, structs, and enums can then adopt these protocols, promising to implement the requirements. This is similar to interfaces in other languages. Protocols do not provide concrete implementations, instead they declare what functionality should be present, not how it should be implemented.

Protocols enable polymorphism by allowing you to treat different types that conform to the same protocol in a uniform way. You can write code that operates on protocol types rather than concrete types. This means your code can work with any type that conforms to the protocol, regardless of its underlying implementation. For example, you could have an array of type [MyProtocol] and it can hold different instances that conform to MyProtocol. This promotes code reusability and flexibility, as you can easily add new types that conform to the protocol without modifying the code that uses it. Protocol extensions in Swift can provide default implementations, offering a form of "mixin" polymorphism, where conforming types inherit functionality from the extension.

12. What are extensions in Swift, and what are their limitations?

Extensions in Swift add new functionality to an existing class, structure, enumeration, or protocol type. They're similar to categories in Objective-C but don't require access to the original source code. You can use extensions to add computed properties, define instance methods and type methods, provide new initializers, define subscripts, define and use new nested types, and make an existing type conform to a protocol.

Limitations of Swift extensions include that they cannot override existing functionality. Also, you can't add stored properties to existing types via extensions; they can only add computed properties. Initializers added via extensions are designated initializers if the type is a class; however, they must call one of the designated initializers in the original class implementation. If you are adding a new initializer to a value type (struct or enum) and that value type provides default values for all of its stored properties and does not define any custom initializers, you can write an extension initializer without calling another initializer. For classes, you can add a convenience initializer, but it must call a designated initializer from the class. Extensions cannot add stored instance properties. You also cannot add property observers.

13. How do you handle concurrency in Swift using Grand Central Dispatch (GCD)?

Grand Central Dispatch (GCD) is Swift's primary way to manage concurrency. It allows you to execute tasks concurrently, taking advantage of multiple cores, without manually managing threads.

To use GCD, you dispatch tasks to dispatch queues. These queues execute tasks either serially (one after another) or concurrently (multiple tasks at the same time). Common dispatch queues include:

  • Main queue: Executes tasks serially on the main thread (for UI updates).
  • Global queues: Concurrent queues with different priorities (e.g., .userInteractive, .default, .background).
  • Custom queues: Can be serial or concurrent.

Example:

DispatchQueue.global(qos: .background).async {
 // Perform a long-running task in the background
 DispatchQueue.main.async {
  // Update the UI on the main thread
 }
}

GCD manages a thread pool and efficiently schedules tasks, simplifying concurrent programming. You can also use DispatchWorkItem for more control, and mechanisms like semaphores (DispatchSemaphore) for managing access to shared resources and preventing race conditions.

14. What are closures in Swift, and how do they capture values?

Closures in Swift are self-contained blocks of functionality that can be passed around and used in your code. They are similar to blocks in C and Objective-C and to lambdas in other programming languages. Closures can capture and store references to any variables and constants from the surrounding context in which they are defined. This is known as closing over those variables.

When a closure captures a value, it essentially creates a copy of that value (or a reference, depending on the type) to be used inside the closure's body. If the captured value is a reference type, the closure captures a reference to the same instance, so changes made inside the closure will be reflected outside, and vice versa. If the captured value is a value type, a copy is captured, so changes inside the closure do not affect the original value.

15. Explain the concept of optionals in Swift and how to unwrap them safely.

Optionals in Swift are a way to indicate that a variable might not have a value. They essentially wrap a type, signaling that it can either hold a value of that type or be nil. This helps avoid unexpected nil values which can cause crashes.

To safely unwrap optionals, you can use several techniques:

  • Optional Binding (if let): This is the most common way. if let unwrappedValue = optionalValue { /* code to execute with unwrappedValue */ }
  • Guard Let: Similar to if let, but useful for exiting a scope early if the optional is nil. guard let unwrappedValue = optionalValue else { return }
  • Nil-Coalescing Operator (??): Provides a default value if the optional is nil. let value = optionalValue ?? defaultValue
  • Optional Chaining (?): Allows you to access properties and methods of an optional, returning nil if the optional is nil at any point in the chain. optionalValue?.someProperty?.someMethod()
  • Forced Unwrapping (!): Should be used with caution, as it will crash your program if the optional is nil. let unwrappedValue = optionalValue!

16. Describe the use of `guard` statements in Swift.

Guard statements in Swift are used for early exits from a scope (like a function, loop, or if statement) if certain conditions are not met. They provide a concise way to handle required conditions and improve code readability by moving error handling or unmet requirements to the top of the scope, rather than nesting it deeply within conditional blocks.

Specifically, a guard statement requires a condition that must be true for the code to continue. If the condition is false, the else block of the guard statement is executed. This else block must transfer control out of the current scope using statements like return, throw, break, continue, or fatalError(). Here's a simple example:

func processValue(value: Int?) {
 guard let unwrappedValue = value else {
 print("Value is nil.")
 return // Early exit if value is nil
 }
 print("Value is: \(unwrappedValue)")
}

17. What are the benefits of using Swift's `Codable` protocol for serialization and deserialization?

Swift's Codable protocol offers several benefits for serialization and deserialization. It simplifies the process of converting data between Swift objects and common formats like JSON or Property List. By conforming a type to Codable (which is a type alias for Encodable & Decodable), the compiler can often automatically generate the necessary code to handle encoding and decoding, reducing boilerplate.

Specifically, using Codable promotes code readability and maintainability by removing the need for manual parsing and creation of objects from external data. It provides a type-safe approach, minimizing runtime errors that can occur when manually handling data conversion. Errors are caught at compile time rather than runtime, which provides more reliability. Codable also integrates well with Swift's error handling, allowing for proper error propagation during encoding or decoding failures.

18. How do you create and use custom operators in Swift?

In Swift, you can define custom operators, both unary and binary. You first need to declare the operator's precedence and associativity using precedencegroup. Then, you declare the operator itself using the operator keyword, specifying whether it's prefix, postfix, or infix. Finally, you implement the operator as a global function.

Here's an example:

precedencegroup ExponentiationPrecedence {
 associativity: right
 higherThan: MultiplicationPrecedence
}

infix operator ^^ : ExponentiationPrecedence

func ^^(base: Int, exponent: Int) -> Int {
 var result = 1
 for _ in 0..<exponent {
 result *= base
 }
 return result
}

let result = 2 ^^ 3 // result is 8

In this example, ^^ is a custom infix operator for exponentiation. We define its precedence and associativity. Then we define the function which implements that custom operator.

19. What are Swift's access control levels (e.g., `private`, `fileprivate`, `internal`, `public`, `open`) and when would you use each?

Swift's access control levels restrict code usage from other modules and source files. They are, in order of least to most restrictive:

  • private: Accessible only within the declaring type or extension in the same file. Use this to hide implementation details within a single type, especially for properties or methods that should never be used outside the class.
  • fileprivate: Accessible only within the defining source file. This is useful for hiding implementation details that are used by multiple types within the same file, but not outside.
  • internal: Accessible from any source file within the same module (e.g., an app or framework). This is the default access level if you don't specify one. Use this for functionality that you want to expose within your app or framework, but not to external users.
  • public: Accessible from any source file in the same module, and also from any module that imports the defining module. You'd use this for elements in a framework's public API that you want to be usable by anyone.
  • open: Accessible from any source file in the same module, and also from any module that imports the defining module. In classes, it allows subclassing both within the defining module and in any module that imports it. In classes and class members, open is more permissive than public; it implies that the class is designed for inheritance. Use this when you explicitly want other modules to be able to subclass your classes.

20. Explain the difference between a `struct` and a `class` in Swift, and discuss their respective use cases.

In Swift, both struct and class are used to create custom data types. The key difference lies in their nature: struct is a value type, while class is a reference type. When a struct is copied, a new independent instance is created with its own copy of the data. Changes to one copy don't affect the other. Conversely, when a class instance is copied (more accurately, assigned), a new reference to the same underlying instance is created. Modifying the data through one reference will affect all other references pointing to the same instance.

Use cases differ accordingly. structs are ideal for representing simple data structures where value semantics are desired, like geometrical points, sizes, or colors. They promote data immutability and thread safety. classes, on the other hand, are suitable for creating complex objects with identity and shared state, such as UI elements, network managers, or game entities. Inheritance is supported only with classes.

Expert Swift interview questions

1. Explain the concept of 'copy-on-write' optimization in Swift and how it affects the performance of value types, using an example with arrays.

Copy-on-write (COW) is an optimization technique used in Swift to improve the performance of value types, particularly collections like arrays. Instead of creating a completely new copy of a value type when it's modified after being assigned or passed around, Swift shares the same underlying memory buffer between the original and the copies. Only when one of the copies is actually modified does Swift then create a separate, private copy of the data. This avoids unnecessary copying and improves performance, especially for large data structures.

For example, consider two arrays array1 and array2. Initially, array2 = array1 only creates a reference. Both arrays point to the same memory location. If we then modify array2 by appending an element (e.g., array2.append(10)), only at that point is a new copy of the array's data created for array2. array1 remains unchanged and still points to the original data. This delayed copying saves processing time and memory when the copy isn't modified.

2. How does Swift's memory management handle retain cycles in closures, and what techniques can be used to prevent them, such as weak and unowned references?

Swift uses Automatic Reference Counting (ARC) for memory management. Retain cycles in closures occur when a closure captures a strong reference to an instance, and the instance also holds a strong reference to the closure, creating a circular dependency. This prevents ARC from deallocating the memory, leading to memory leaks. To prevent retain cycles, use weak or unowned references when capturing self (or other instances) within a closure.

  • Weak: A weak reference does not increment the reference count. If the referenced instance is deallocated elsewhere, the weak reference automatically becomes nil. Use weak when the captured instance can become nil during the closure's lifetime. For example: [weak self] in self?.doSomething()
  • Unowned: An unowned reference also doesn't increment the reference count, but unlike weak, it's assumed to always have a value. Using it after the instance has been deallocated will result in a runtime crash. Use unowned when you are certain the captured instance will outlive the closure. For example: [unowned self] in self.doSomething()

3. Describe the differences between 'inout' parameters and returning a new value from a function. When would you choose one over the other, especially considering performance?

Both inout parameters and returning a new value allow a function to communicate data back to the caller. However, they differ in how the data is managed. inout modifies the original variable passed into the function directly (pass by reference), while returning a new value creates a copy of the data.

Choosing between them depends on the situation. inout can be more performant when dealing with large structures because it avoids creating a copy. However, it also has the side effect of modifying the original variable which might not always be desirable. Returning a new value is generally safer, as it doesn't alter the original variable, thus preventing unexpected side effects. Here is a brief summary:

  • inout parameters:
    • Modifies the original variable.
    • Potentially more efficient for large data structures.
    • Can lead to unexpected side effects if not used carefully.
  • Returning a new value:
    • Creates a copy of the data.
    • Safer, as it doesn't modify the original variable.
    • May be less efficient for large data structures due to copying.

4. What are the advantages and disadvantages of using Swift's 'Codable' protocol for serialization and deserialization, and how does it compare to manual parsing?

Swift's Codable protocol offers several advantages. It simplifies serialization and deserialization by automatically generating code based on the data structure's properties. This reduces boilerplate and the potential for errors that come with manual parsing. Codable also provides a standardized way to handle JSON, Plist or any supported format encoding and decoding, leading to more maintainable and consistent code across projects. It's generally more type-safe than manual parsing, leveraging Swift's strong typing to prevent runtime errors. However, it can be less flexible for handling complex or non-standard data formats.

Disadvantages include potential limitations when dealing with unconventional JSON structures, requiring custom coding keys or container implementations. Manual parsing, while more verbose, grants complete control over the process, allowing developers to handle discrepancies, perform data transformations, and optimize performance in specific scenarios. When the json/data structure is complex or if fine-grained control is needed, manual parsing offers much more power. Manual parsing can lead to less maintainable code and potential bugs if not handled carefully. Consider also the overhead of performance; for complex objects Codable may take a longer time than if carefully hand-optimized.

5. Explain the use cases for Swift's property wrappers and provide an example of how you might create a custom property wrapper to enforce data validation.

Property wrappers in Swift provide a way to add a layer of abstraction between the code that manages how a property is stored and the code that defines the property. This enables you to reuse the same property-related logic across multiple properties, such as for data validation, thread safety, or lazy initialization. Common use cases include providing default values, enforcing data constraints (like ensuring a string is always uppercase or a number is within a specific range), managing concurrency, and simplifying common property access patterns.

Here's a simple example of a custom property wrapper to enforce data validation, ensuring a string is never empty:

@propertyWrapper
struct NonEmptyString {
    private var value: String = ""

    var wrappedValue: String {
        get { return value }
        set { value = newValue.isEmpty ? "default" : newValue }
    }

    init(wrappedValue initialValue: String) {
        self.wrappedValue = initialValue
    }
}

struct MyStruct {
    @NonEmptyString var name: String
}

var myInstance = MyStruct(name: "")
print(myInstance.name) // Prints "default"

myInstance = MyStruct(name: "Alice")
print(myInstance.name) // Prints "Alice"

6. Describe the role of Swift's 'Result' type and how it simplifies error handling compared to traditional try-catch blocks, especially in asynchronous operations.

Swift's Result type is an enum designed to represent the outcome of an operation that can either succeed with a value or fail with an error. It simplifies error handling by providing a clear and structured way to propagate errors, especially in asynchronous code. Instead of relying on nested try-catch blocks, which can become cumbersome and difficult to manage, Result encapsulates the success or failure state within a single type. For example, an asynchronous network request might return a Result<Data, NetworkError>, clearly indicating the possible outcomes.

Compared to traditional try-catch, Result offers several advantages:

  • Explicit error handling: Forces developers to handle both success and failure cases.
  • Improved readability: Reduces nesting and makes code easier to understand.
  • Type safety: Ensures that errors are of a specific, defined type (e.g., NetworkError).
  • Simplified asynchronous handling: Avoids the complexities of managing errors in asynchronous callbacks.

Using Result promotes cleaner and more robust error management, leading to more maintainable and less error-prone code. Example: func fetchData(completion: (Result<Data, NetworkError>) -> Void)

7. How can you leverage Swift's generics to write more reusable and type-safe code, and what are the limitations of generics in certain scenarios?

Generics in Swift allow you to write code that can work with any type, ensuring type safety without sacrificing reusability. For example, you can create a generic Stack struct that can hold elements of any type: struct Stack<T> { ... }. This avoids writing separate stack implementations for Int, String, etc. Using generics ensures that the compiler can enforce type constraints at compile time, preventing runtime errors related to incorrect type usage. Generics are beneficial when you need to create data structures or algorithms that should operate on different data types without requiring explicit type casting.

Limitations include the inability to directly perform operations specific to certain types without type constraints or specialized implementations. For instance, directly multiplying elements within a generic function without knowing they are numbers requires type constraints like where T: Numeric. Also, Swift generics, especially associated types in protocols, can become complex and difficult to reason about in advanced scenarios involving multiple protocol conformances or conditional conformances. Furthermore, the lack of runtime type information (type erasure) in Swift generics can sometimes limit their usefulness in scenarios where runtime type introspection is required.

8. Explain the concept of Protocol-Oriented Programming (POP) in Swift and how it promotes code reusability and testability compared to Object-Oriented Programming (OOP).

Protocol-Oriented Programming (POP) in Swift emphasizes defining behavior through protocols, which are blueprints of methods, properties, and other requirements that types can conform to. This promotes code reusability by allowing multiple unrelated types to adopt the same protocol and share implementations through protocol extensions. For instance:

protocol Flyable {
    var speed: Double { get }
    func fly()
}

extension Flyable {
    func fly() {
        print("Flying at \(speed) mph")
    }
}

struct Bird: Flyable {
    var speed: Double
}

struct Airplane: Flyable {
    var speed: Double
}

Compared to Object-Oriented Programming (OOP) which often relies on inheritance, POP offers greater flexibility and avoids issues like the fragile base class problem. Testability is improved because you can easily create mock objects conforming to protocols for unit testing, isolating components and verifying interactions based on protocol conformance, rather than class hierarchies. This contrasts with OOP, where mocking often requires subclassing, leading to more complex and tightly coupled tests. POP enables a more decoupled and modular architecture, boosting both reusability and testability.

9. Describe the differences between 'Any' and 'AnyObject' in Swift and provide examples of when you might use each, considering type safety.

Any and AnyObject are both used for type erasure in Swift, but they have key differences.

Any can represent an instance of any type at all, including class types, struct types, enum types, and primitive types (like Int, String, Bool). You might use Any when you need to store values of different types in the same collection, though it sacrifices type safety, potentially requiring downcasting. For example, var mixedArray: [Any] = [1, "hello", true].

AnyObject, on the other hand, is limited to instances of class types. It does not include structs, enums, or primitives. It bridges to id in Objective-C. Use cases include working with legacy Objective-C APIs or when you specifically need to store a collection of class instances of varying types. For example, you might see it with UI elements like let views: [AnyObject] = [UIView(), UIButton()].

10. How does Swift handle concurrency using Grand Central Dispatch (GCD), and what are some common pitfalls to avoid when working with concurrent code?

Swift leverages Grand Central Dispatch (GCD) for concurrency management. GCD provides a way to execute tasks concurrently or serially using dispatch queues. Developers define blocks of code (closures) and dispatch them to queues, allowing the system to manage thread creation and scheduling efficiently. Common dispatch queues include the main queue (for UI updates), global queues (for background tasks with varying priorities), and custom queues (serial or concurrent, for specific needs).

Some common pitfalls include race conditions (when multiple threads access and modify shared resources simultaneously), deadlocks (when threads are blocked indefinitely, waiting for each other), priority inversion (when a high-priority task is blocked by a low-priority task), and improper use of synchronization primitives. To mitigate these, use synchronization mechanisms like locks, semaphores, and dispatch groups carefully. Always be mindful of which queue you're executing code on, particularly when updating the UI (which should always be done on the main queue using DispatchQueue.main.async { /* UI code */ }).

11. Explain the use of Key-Value Observing (KVO) in Swift and how it can be used to observe changes in object properties, considering its performance implications.

Key-Value Observing (KVO) in Swift allows objects to observe changes to specific properties of other objects. You register as an observer for a particular key path on an object, and your observer is notified whenever the value at that key path changes. To use KVO, an object must inherit from NSObject. You add an observer using addObserver(_:forKeyPath:options:context:) and implement observeValue(forKeyPath:of:change:context:) to handle notifications.

KVO can impact performance. Since it relies on runtime magic, it can be slower than other observation mechanisms like Combine or delegation. Overuse or improper usage (e.g., failing to remove observers) can lead to memory leaks and performance bottlenecks. KVO introduces overhead due to dynamic method dispatch and maintaining observer lists. It's important to weigh these performance implications against the benefits of KVO when choosing an observation pattern.

12. Describe the purpose of Swift's 'defer' statement and how it ensures that code is executed regardless of how a function exits, including error handling.

The defer statement in Swift is used to execute a block of code just before the function or scope it's defined in exits, regardless of how the exit occurs. This includes normal completion, return statements, or even if an error is thrown. The purpose is primarily for cleanup tasks, resource management, or ensuring certain actions are always performed.

defer ensures execution even with error handling by placing the deferred code outside the normal control flow. If an error is thrown, Swift unwinds the stack, and as it does, any defer blocks encountered are executed. This guarantees, for example, that a file is closed or a lock is released, even if an unexpected error causes the function to terminate prematurely. For example:

func myFunc() throws {
    let file = // Open a file
    defer { 
        // Close the file
        //file.close()
    }

    // ... potentially throwing operations...
}

13. How does Swift's optional chaining work, and what are some scenarios where it can simplify code while avoiding potential runtime crashes?

Optional chaining in Swift provides a way to access properties, methods, and subscripts of an optional that might currently be nil. If the optional contains a value, the call succeeds; if the optional is nil, the call gracefully returns nil without crashing. The entire chain evaluates to an optional value of the same type as the type of the property, method, or subscript you're trying to access.

Optional chaining simplifies code in scenarios where you need to access nested properties that could be nil at any level. For example, accessing person?.address?.street?.name safely accesses the street name if the person, address, and street optionals all have values, otherwise it will return nil immediately. This avoids multiple if let or guard let statements for each level, leading to more concise and readable code. It helps prevent runtime crashes by handling the nil case gracefully, without attempting to force unwrap a nil value.

14. Explain the concept of associated types in protocols and how they allow you to define protocols that work with generic types, ensuring type safety.

Associated types in protocols provide a placeholder (or alias) for a type that is not specified until the protocol is adopted by a concrete type. They allow protocols to be generic without explicitly declaring generic type parameters in the protocol definition itself. This is particularly useful when a protocol needs to work with types that are related to each other, but the specific types are only known at the time of adoption.

Using associated types ensures type safety. When a type conforms to a protocol with associated types, it must specify the concrete types that the associated types represent. The compiler then enforces that these types are used consistently throughout the conforming type's implementation, guaranteeing that operations involving those types are type-safe. For example:

protocol Container {
 associatedtype Item
 mutating func append(_ item: Item)
 var count: Int { get }
 subscript(i: Int) -> Item { get }
}

Here, Item is the associated type. A concrete type conforming to Container must specify what Item is (e.g., String, Int, etc.), and the compiler will then ensure that the append function only accepts that type and the subscript returns that type.

15. Describe the use cases for Swift's 'dynamic' keyword and how it affects the behavior of method dispatch at runtime, considering its impact on performance.

The dynamic keyword in Swift disables several compiler optimizations and forces method dispatch to occur at runtime instead of compile time. This is primarily used for interoperability with Objective-C, where methods can be added or changed at runtime. Some key use cases include:

  • Objective-C bridging: Allows Swift code to interact with Objective-C code that relies on runtime features like key-value observing (KVO) or method swizzling.
  • Dynamic dispatch: When a method or property is marked with dynamic, Swift uses Objective-C runtime for method calls. This is needed when dealing with frameworks that rely on these mechanisms.

The use of dynamic has a performance cost because it bypasses Swift's static dispatch and utilizes the slower Objective-C runtime dispatch. This can make method calls noticeably slower, especially if used extensively. Therefore, it should only be used when interacting with Objective-C runtime features.

16. How can you use Swift's Combine framework to handle asynchronous events and data streams, and what are the advantages of using Combine over traditional delegation patterns?

Combine provides a declarative way to handle asynchronous events and data streams. You define a pipeline of operators that transform, filter, and react to data emitted by publishers. A simple example is:

import Combine

let publisher = PassthroughSubject<Int, Never>()

let subscriber = publisher
    .filter { $0 > 10 }
    .map { $0 * 2 }
    .sink {
        print("Received value: \($0)")
    }

publisher.send(5)
publisher.send(15)
publisher.send(20)

Combine offers several advantages over delegation: Centralized Error Handling, allowing errors to be propagated through the pipeline, and ensuring proper handling. Type Safety, ensuring that data types are consistent throughout the pipeline, reducing runtime errors. Composition and Reusability where operators can be easily combined to create complex data transformations, and publishers and subscribers can be reused across different parts of the application. Backpressure Management gives subscribers the ability to control the rate at which they receive data, preventing them from being overwhelmed. In short, Combine improves code readability, maintainability, and reduces boilerplate compared to delegation.

17. Explain the concept of method swizzling in Swift and how it can be used to modify the behavior of existing methods at runtime, considering its potential risks.

Method swizzling in Swift is a technique that allows you to change the implementation of an existing method at runtime. Essentially, you're swapping the implementations of two methods. This is achieved by exchanging the IMP (implementation) of two SEL (selector) associated with the methods.

It's useful for tasks like adding logging to existing methods or modifying system behaviors. However, swizzling can be risky. It affects the entire application, not just a specific instance, potentially leading to unexpected side effects or conflicts with other libraries or future updates. Debugging swizzled code can also be quite challenging.

18. Describe the role of Swift's 'autoreleasepool' and how it manages memory for Objective-C objects in Swift code, especially when interoperating with legacy code.

Swift's autoreleasepool is crucial for managing the lifecycle of Objective-C objects within Swift code, particularly when interacting with legacy Objective-C code or frameworks. Objective-C uses manual reference counting (MRC) and automatic reference counting (ARC). autoreleasepool provides a mechanism for delaying the release of Objective-C objects, effectively grouping them for later deallocation. When an Objective-C object is sent an autorelease message, it is added to the current autoreleasepool. The pool is then drained, meaning all objects in the pool are sent a release message at the end of the pool's scope.

In Swift, when you work with Objective-C APIs, ARC automatically handles the memory management of Objective-C objects. However, there are scenarios (like tight loops creating many temporary Objective-C objects) where explicitly using autoreleasepool can prevent memory spikes and improve performance. The autoreleasepool block ensures temporary objects are deallocated more frequently within the loop, rather than waiting for the next ARC cycle. Using it involves wrapping the code that creates the temporary Objective-C objects within an autoreleasepool block:

autoreleasepool {
 // Code that creates Objective-C objects
}

19. How does Swift handle bridging between Swift types and Objective-C types, and what are some common challenges to consider when working with mixed codebases?

Swift provides seamless interoperability with Objective-C through bridging. Swift types can often be used directly in Objective-C code, and vice versa, thanks to automatic bridging. For example, Swift's String is bridged to Objective-C's NSString, and Swift's Array is bridged to Objective-C's NSArray. However, certain Swift-specific features, like structs or enums with associated values, may not have direct equivalents in Objective-C, requiring manual handling or wrapper classes.

Some common challenges when working with mixed codebases include: dealing with optionals (as Objective-C doesn't have a direct equivalent), managing memory (particularly with manual retain-release in Objective-C), and handling nil values differently between the two languages. It's important to be mindful of nullability annotations (_Nullable, _Nonnull) in Objective-C headers to ensure proper bridging and avoid unexpected behavior in Swift. Type conversions, especially with collections, should be handled carefully to prevent runtime errors. When a Swift type is unavailable in Objective-C code you might need to create a wrapper class or function.

20. Explain the concept of SwiftUI's 'Environment' and how it allows you to pass data and dependencies down the view hierarchy, considering its impact on testability.

SwiftUI's Environment is a property wrapper that provides a way to access values shared across your application's view hierarchy. It allows you to inject dependencies and data into views without explicitly passing them down through each level. Views can declare the dependencies they need using @Environment, and SwiftUI will automatically provide the corresponding values from the environment.

Using Environment improves testability because you can easily override environment values during testing. Instead of having to mock and pass data through a complex view tree, you can simply set the desired values in the environment for the test. This makes tests more focused and less brittle, as they are not tightly coupled to the specific implementation details of the view hierarchy. For example:

@Environment(\EnvironmentValues.managedObjectContext) var managedObjectContext

In testing, you can override with something like:

let testView = MyView().environment(\EnvironmentValues.managedObjectContext, mockContext)

21. Describe the differences between 'ObservableObject' and '@StateObject' in SwiftUI, and when you would use each for managing state in your views.

ObservableObject is a protocol that allows a class to publish changes to its properties, which SwiftUI views can then observe and react to. It's a general mechanism for data binding. @StateObject is a property wrapper specifically designed to manage the lifecycle of an ObservableObject within a SwiftUI view. It ensures that the object is created only once when the view appears and persists across view updates. Without it, the ObservableObject would be re-initialized every time the view is redrawn, leading to data loss and unexpected behavior. Use @StateObject to create and manage your ObservableObject only in the view where it is first initialized. In subsequent views, use @ObservedObject or @EnvironmentObject to observe and react to changes in the ObservableObject without re-initializing it.

22. How can you use Swift's actors to safely manage concurrent access to shared mutable state, preventing data races and ensuring thread safety?

Swift actors isolate their state, ensuring only one thread can access it at a time, thus preventing data races. To use them, define an actor and encapsulate the shared mutable state within it. Access to the actor's properties and methods is implicitly serialized.

Here's an example:

actor Counter {
 private var value = 0

 func increment() async {
 value += 1
 }

 func getValue() async -> Int {
 return value
 }
}

let counter = Counter()

// Usage (must be in an async context)
Task { 
 await counter.increment()
 let currentValue = await counter.getValue()
 print(currentValue)
}

Accessing counter's increment or getValue methods from different threads will be safely managed by the actor's isolation, preventing concurrent access and data corruption. The await keyword is used because calls to the actor are potentially suspending operations.

23. Explain the purpose of Swift's opaque return types (using 'some View' in SwiftUI) and how they allow you to hide implementation details while still ensuring type safety.

Opaque return types, like some View in SwiftUI, let you return a specific type from a function or computed property without revealing its underlying concrete type. This is crucial for information hiding and abstraction. The compiler knows the exact type at compile time, ensuring type safety, but the caller only sees the promise of a View (or whatever protocol you specify). This allows you to change the implementation details – say, the specific combination of UI elements – without breaking client code, as long as the returned value conforms to the declared protocol.

For example, you can return some View representing a complex view hierarchy, but the caller only knows they're getting a View. If you later change the implementation to use a different set of UI elements, the calling code doesn't need to be recompiled as long as it still gets back something that conforms to the View protocol. This promotes modularity and reduces coupling between components.

24. Describe the use of Swift's new concurrency features like 'async' and 'await' and how they simplify asynchronous programming compared to GCD and closures.

Swift's async and await keywords drastically simplify asynchronous programming compared to GCD and closures. Instead of managing dispatch queues and nesting completion handlers, async allows a function to be marked as asynchronous, enabling it to pause execution until an await call completes. await suspends the current function until the asynchronous task finishes, resuming execution with the result, all while freeing up the underlying thread to handle other tasks.

This approach offers several benefits. First, code becomes much more readable and resembles synchronous code, reducing complexity and making it easier to reason about. Second, error handling is simplified; traditional try-catch blocks can be used directly with await calls, instead of having to propagate errors through closures. Finally, debugging is significantly easier because the call stack remains intact across await points, unlike with GCD where the stack can be fragmented.

25. How can you leverage Swift's macro system to generate code at compile time, and what are the benefits of using macros for code reduction and performance optimization?

Swift's macro system allows you to generate code at compile time, reducing boilerplate and potentially improving performance. You define macros that transform code based on patterns. For example, you could use a macro to automatically generate implementations for protocols or create optimized versions of functions based on input types.

Benefits include reduced code duplication, improved code readability, and compile-time optimizations. By generating code tailored to specific use cases, macros can eliminate runtime overhead. For instance, a macro could unroll a loop or specialize a function based on known parameters, resulting in faster execution. Macros can also enforce coding standards or generate code based on data models.

26. Explain the concept of existential types (e.g., `any Protocol`) in Swift and how they differ from generics and concrete types, particularly in the context of protocol conformance.

Existential types, like any Protocol in Swift, represent a value whose underlying type conforms to a specific protocol, but the exact type is not known at compile time. This is different from generics, where the compiler knows the concrete type and can generate specialized code for it. With existential types, the compiler only knows that the value conforms to the protocol, enabling dynamic polymorphism. The actual type is determined at runtime.

Unlike concrete types (e.g., String, Int), which are fully known at compile time, existential types introduce a level of indirection. When you use any Protocol, you're essentially working with a 'box' that contains a value conforming to Protocol. This box incurs a performance cost (dynamic dispatch) but provides flexibility in working with heterogeneous collections of conforming types. Essentially, any Protocol lets you treat different concrete types that all conform to Protocol in a uniform manner, even if those types have completely different underlying structures.

27. Describe the advantages and disadvantages of using Swift's Core Data framework for data persistence, and how it compares to other options like Realm or SQLite.

Core Data offers several advantages, including a managed object context for easy data manipulation, automatic undo/redo support, and powerful querying capabilities using predicates. It also integrates well with other Apple frameworks and supports data migration. However, Core Data can have a steeper learning curve compared to simpler solutions. Performance can also be a concern if not used correctly, and it's not a true relational database, which might be a disadvantage for certain complex data models.

Alternatives like Realm are often faster and easier to use, with a more modern API. SQLite provides direct SQL access and greater control but requires more manual coding. Realm isn't based on SQL, which could be a drawback for some. Core Data is object-graph management framework not a Database; therefore, SQLite is typically what CoreData uses under the hood. For simpler apps with straightforward data needs, Realm or even UserDefaults might suffice. For more complex relational data, SQLite offers maximum flexibility. Core Data is a good middle ground, especially when tight integration with the Apple ecosystem and features like data migration are important.

Swift MCQ

Question 1.

Consider the following Swift code:

class Address {
    var street: String?
}

class Person {
    var address: Address?
}

let person = Person()

let streetName = person.address?.street ?? "Unknown Street"

What will be the value of streetName?

Options:
Question 2.

What is the primary purpose of the defer keyword in Swift?

Options:
Question 3.

What will be the output of the following Swift code snippet?

let numbers: [Int?] = [1, nil, 3, nil, 5]
let mappedNumbers = numbers.map { $0.map { $0 * 2 } }
print(mappedNumbers)

options:

Options:
Question 4.

What is the primary purpose of the guard statement in Swift?

Options:
Question 5.

Consider the following Swift code:

enum CustomError: Error {
    case invalidInput
    case networkError
}

func processData(input: String) -> Result<Int, CustomError> {
    guard let number = Int(input) else {
        return .failure(.invalidInput)
    }
    // Simulate network operation that might fail
    if number > 100 {
        return .failure(.networkError)
    }
    return .success(number * 2)
}

let result = processData(input: "150")

switch result {
case .success(let value):
    print("Success: \(value)")
case .failure(let error):
    print("Failure: \(error)")
}

What will be printed to the console?

Options:
Question 6.

What is the result of the following Swift code snippet?

let numbers: [String?] = ["1", nil, "3", "4", nil, "6"]
let mappedNumbers = numbers.compactMap { $0 }.flatMap { Int($0) }
print(mappedNumbers)

options:

Options:
Question 7.

What is the primary purpose of the compactMap function in Swift when used with an array of optional values?

options:

Options:
Question 8.

What is the primary risk associated with using implicitly unwrapped optionals (IUOs) in Swift?

Options:

Options:
Question 9.

Consider the following Swift code:

class MyClass {
    var myProperty: Int = 0 {
        willSet(newValue) {
            print("About to set myProperty to \(newValue)")
        }
        didSet(oldValue) {
            print("myProperty changed from \(oldValue) to \(myProperty)")
        }
    }
}

let myObject = MyClass()
myObject.myProperty = 10
myObject.myProperty = 10

What will be printed to the console?

Options:
Question 10.

Consider the following Swift code:

protocol Animal {
    func makeSound() -> String
}

struct Dog: Animal {
    func makeSound() -> String { return "Woof!" }
}

struct Cat: Animal {
    func makeSound() -> String { return "Meow!" }
}

// Question: What is the key difference between using `any Animal` and `some Animal` as a return type for a function?

Which of the following statements best describes the core difference?

Options:
Question 11.

Which of the following is the primary purpose of conforming to the Codable protocol in Swift?

Options:
Question 12.

What is the purpose of the inout keyword when used as a parameter modifier in a Swift function?

Options:
Question 13.

What is the result of using try? when calling a function that throws an error in Swift?

Options:
Question 14.

Consider the following Swift code:

class DataManager {
    lazy var data: [Int] = {
        print("Data initialized")
        return Array(1...1000)
    }()

    init() {
        print("DataManager initialized")
    }
}

let manager = DataManager()
// some code here

When will the print("Data initialized") statement be executed?

Options:
Question 15.

Which of the following statements best describes the primary purpose of extensions in Swift?

Options:

Options:
Question 16.

Consider the following Swift code:

class Person {
    let name: String
    var apartment: Apartment?
    init(name: String) {
        self.name = name
    }
    deinit { print("\(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    weak var tenant: Person?
    init(unit: String) {
        self.unit = unit
    }
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person? = Person(name: "John")
var unit4A: Apartment? = Apartment(unit: "4A")

john?.apartment = unit4A
unit4A?.tenant = john

john = nil
// Point A

print("End of scope")

What will be printed when the execution reaches // Point A?

Options:
Question 17.

Consider the following Swift struct:

struct Person {
    var name: String
    var age: Int
}

Which of the following demonstrates the correct way to create a KeyPath to access the age property of a Person instance?

Options:
Question 18.

In Swift, what is the key distinction between using the open keyword versus the public keyword when defining a class?

Options:
Question 19.

Consider the following Swift code:

func processValues<T, U>(array1: [T], array2: [U]) where T: Equatable, U: Equatable {
    // Some processing logic here
}

Which of the following best describes the purpose of the where T: Equatable, U: Equatable clause in this generic function?

Options:
Question 20.

Consider the following Swift code snippet:

class MyClass {
    var name: String
    lazy var myClosure: () -> String = {
        return "Hello, \(self.name)"
    }

    init(name: String) {
        self.name = name
    }

    deinit {
        print("MyClass instance deallocated")
    }
}

var instance: MyClass? = MyClass(name: "Example")
print(instance!.myClosure())
instance = nil

Without using a capture list, what potential issue does this code have, and how can capture lists resolve it? Which of the following options explains this the best?

options:

Options:
Question 21.

In older versions of Swift (prior to Swift 4), how would you typically determine the actual runtime type of an instance, and what is a key consideration when comparing types directly?

options:

Options:
Question 22.

Consider the following Swift code:

protocol Vehicle {
    var numberOfWheels: Int { get }
    func startEngine()
}

protocol Car: Vehicle {
    var modelName: String { get }
}

struct MyCar: Car {
    var modelName: String
    var numberOfWheels: Int

    func startEngine() {
        print("Engine started")
    }
}

Which of the following statements is true regarding protocol inheritance in this scenario?

Options:
Question 23.

Which keyword is used to allow a method within a struct to modify the struct's properties?

Options:
Question 24.

Consider the following protocol definition in Swift:

protocol DataProcessor {
 associatedtype Input
 associatedtype Output

 func process(data: Input) -> Output
}

Which statement best describes the role of associatedtype in this context?

Options:
Question 25.

Consider the following Swift code:

class Animal {}
class Dog: Animal {
    func bark() { print("Woof!") }
}

let animal: Animal = Dog()

Which of the following is the correct way to safely downcast animal to a Dog and call its bark() method?

Options:

Which Swift skills should you evaluate during the interview phase?

It's impossible to fully assess a candidate's capabilities in a single interview. However, focusing on core Swift skills ensures you identify individuals who can contribute effectively to your team. Here's a breakdown of the key skills to evaluate.

Which Swift skills should you evaluate during the interview phase?

Swift Fundamentals

Use an assessment to test their understanding of Swift's syntax and basic concepts. Our Swift online test includes questions to filter candidates with a solid Swift base.

To evaluate their understanding of Swift fundamentals, present a simple coding problem.

Write a Swift function that takes an array of integers and returns the sum of all even numbers in the array.

Look for the correct use of loops, conditional statements, and data types. The candidate should also demonstrate an understanding of optionals when handling empty arrays.

Object-Oriented Programming (OOP)

Gauge their OOP knowledge with targeted MCQs that assess understanding of these concepts. Explore our online Swift test which includes OOP questions.

To assess a candidate's grasp of OOP principles, try this question:

Design a class hierarchy representing different types of vehicles (e.g., Car, Truck, Motorcycle). Each vehicle should have properties like number of wheels and maximum speed, and a method to calculate fuel consumption.

The candidate should demonstrate proper use of inheritance, polymorphism (method overriding), and encapsulation. Look for a clear understanding of how these concepts apply to practical code design.

Memory Management

An assessment focusing on memory management can quickly identify candidates with strong understanding. Consider using our iOS online test which covers ARC and memory management concepts.

To directly assess their knowledge of memory management:

Explain how ARC works in Swift and give an example of a potential memory leak scenario, and how to prevent it.

The candidate should explain the role of strong and weak references in ARC. They should also be able to identify common scenarios that can lead to retain cycles and propose solutions using weak or unowned references.

3 Tips for Using Swift Interview Questions

Before you start putting what you've learned to use, here are our top three tips to help you conduct more effective Swift interviews. These tips will help you streamline your process and identify the best candidates.

1. Leverage Swift Skills Tests for Objective Evaluation

Using skills tests can significantly improve the objectivity and effectiveness of your candidate screening process. Skill tests can help identify candidates with practical Swift skills before the interview stage, saving you time and effort.

Consider using our Swift online test or Swift iOS test to assess candidates' coding abilities, problem-solving skills, and understanding of Swift concepts. These tests provide data on a candidate's capabilities, allowing you to focus your interview questions on areas that require further exploration.

By implementing skills tests, you can create a streamlined process, reduce bias, and ensure you're only interviewing candidates with the skills needed to succeed. This data driven approach helps you make informed hiring decisions.

2. Carefully Curate Your Interview Questions

Time is limited during interviews, so choosing the right questions is imperative. Focus on questions that reveal a candidate's practical knowledge, problem-solving approach, and understanding of real-world scenarios.

Ensure that the selected interview questions help evaluate candidates on important skills. Consider exploring our Data Structure interview questions or iOS interview questions to broaden your interview question arsenal and evaluate candidates beyond Swift.

Remember to balance technical questions with behavioral questions to assess their communication skills and cultural fit. This blended approach provides a more view of the candidate's potential.

3. Ask Targeted Follow-Up Questions

Simply asking the initial interview questions is not enough to fully assess a candidate's capabilities. Follow-up questions are important to gauge the depth of their knowledge and confirm their understanding.

For example, if a candidate describes using a specific Swift feature, ask them about the potential performance implications or alternative approaches. This uncovers the candidate's true skill depth, verifying their claims and revealing their critical thinking abilities.

Hire Top Swift Developers with Confidence

Looking to hire Swift developers with the right skills? Accurately assessing their abilities is key. Using skills tests is the most effective way to gauge a candidate's true expertise. Consider leveraging a Swift online test or an iOS online test for a clear picture.

Once you've identified top performers with skills tests, it's time for in-depth interviews. Shortlist the best candidates and head over to our online assessment platform to streamline the hiring process.

Swift Online Test

45 mins | 8 MCQs and 1 Coding Question
The Swift Online test uses scenario-based MCQs to evaluate candidates on their knowledge of Swift programming language, including syntax, semantics, and data types. The test also evaluates a candidate's ability to write efficient and optimized Swift code, and their familiarity with iOS development concepts, such as UI design, iOS SDK, and Xcode. The test includes a coding question to evaluate hands-on Swift programming skills.
Try Swift Online Test

Download Swift interview questions template in multiple formats

Swift Interview Questions FAQs

What are some good Swift interview questions for freshers?

Good Swift interview questions for freshers often focus on basic Swift syntax, data structures, and fundamental programming concepts. Questions about optionals, basic control flow, and simple class implementation are suitable.

How can I assess a junior Swift developer's understanding of UIKit?

To assess a junior Swift developer's understanding of UIKit, ask questions about UI lifecycle methods, basic UI components (like UIButton, UILabel), and how they handle user interactions.

What should I ask an intermediate Swift developer about memory management?

For intermediate Swift developers, questions about memory management, ARC (Automatic Reference Counting), and avoiding retain cycles are appropriate. Also, assess their understanding of strong and weak references.

What are some challenging questions for an expert Swift developer?

Challenging questions for expert Swift developers might include topics like concurrency, advanced data structures, performance optimization, and deep understanding of Swift's internals, such as the Swift runtime.

Why should I use Swift interview questions?

Swift interview questions help you evaluate candidates' knowledge and skills in Swift programming, ensuring they can contribute effectively to your iOS or macOS development projects.

How can I use Swift interview questions effectively?

Use a mix of theoretical and practical questions, assess problem-solving skills, and focus on understanding rather than rote memorization. Tailor the questions to match the specific requirements of the role.

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