10 Must-Know iOS Interview Questions for Beginners (2025) — Part 2

Welcome to Part 2 of the 10 Must-Know iOS Interview Questions for Beginners (2025) series!

In this continuation, we dive deeper into advanced yet essential concepts that often come up in interviews. These topics, like associated types, async/await, KVO and others, will help you solidify your understanding of Swift and iOS development.

This article follows the same structured approach as Part 1:

1. Concept: What it is and why it matters.
2. Use: Where and when to use it.
3. Real-Time Example: A clear, practical scenario.
4. Code Sample: Concise and easy-to-understand code.
5. Best Practices: Tips to master the concept.

If you haven’t checked Part 1 yet, you can find it here

If you’re preparing for your first iOS developer interview as a junior or intern, this article is for you. If you’re experienced, you might find this too basic — but feel free to share it with someone starting their iOS journey!

Let’s continue building your interview confidence. 🚀

6. What is an Associate type in protocol? How do you use them?

a. Concept: What is an Associated Type in Protocol?

An associated type in a protocol is a placeholder type that is defined at the time a protocol is adopted by a conforming type. It allows a protocol to work with generic types without specifying concrete types upfront.

Think of it as a way to make protocols flexible and reusable.

Why It Matters:

• Enables protocols to handle a variety of data types.
• Makes protocols more powerful by combining them with generics.

b. Use: Where and When to Use Associated Types

Use associated types when defining protocols that need to operate on generic types, especially when the exact type isn’t known until the protocol is adopted.

Examples include protocols for collections, data transformations, or reusable utilities.

c. Real-Time Example: Custom Collection Protocol

Consider creating a protocol for collections that can hold any type of data. You don’t want to restrict the collection to a specific data type.

d. Code Sample

protocol Container {
    associatedtype Item
    func add(_ item: Item)
    func getAllItems() -> [Item]
}

class StringContainer: Container {
    private var items: [String] = []

    func add(_ item: String) {
        items.append(item)
    }

    func getAllItems() -> [String] {
        return items
    }
}

class IntContainer: Container {
    private var items: [Int] = []

    func add(_ item: Int) {
        items.append(item)
    }

    func getAllItems() -> [Int] {
        return items
    }
}

let stringContainer = StringContainer()
stringContainer.add("Swift")
stringContainer.add("Programming")
print(stringContainer.getAllItems()) // Output: ["Swift", "Programming"]

let intContainer = IntContainer()
intContainer.add(42)
intContainer.add(99)
print(intContainer.getAllItems()) // Output: [42, 99]

• **StringContainer** works with String as its Item type, storing and returning a list of strings.
• **IntContainer** works with Int as its Item type, storing and returning a list of integers.

Each conforming type decides what the Item will be, enabling flexibility while keeping the protocol definition reusable.

e. Best Practices

Best Practices for Using Associated Types

• Choose Descriptive Associated Type Names: Use meaningful and descriptive names for associated types, like Item, Element, or Value, to convey their purpose. This improves readability and helps maintain clarity in your code.
• Constrain Associated Types When Necessary: When possible, apply constraints to associated types to ensure they meet specific requirements (e.g., conforming to a protocol, being equatable). This enhances safety and usability. swift
• Limit Associated Types Scope: Avoid using too many associated types within a single protocol. Keeping them focused helps simplify the protocol and ensures it serves a specific purpose.
• Use Type Erasure When Necessary: When dealing with heterogeneous collections or wrappers, consider using type erasure to abstract away the associated type, making it easier to work with dynamic types. swift
• Combine with Generics: Use associated types in combination with generics when defining flexible and reusable APIs. This makes your code adaptable to a variety of types while maintaining clarity.

7. How do you handle asynchronous task in Swift using async/await? Compare it to using completion handlers

1. Concept: Handling Asynchronous Tasks in Swift

In Swift, asynchronous tasks are handled using the async/await syntax or the older completion handlers approach.

• Async/Await: Introduced in Swift 5.5, it simplifies asynchronous programming by making code more readable and sequential.
• Completion Handlers: A callback-based mechanism where you pass a closure to be executed once an asynchronous task completes.

2. Use: Where and When to Use Each

• Async/Await: Use when developing apps targeting iOS 15+ or macOS 12+ to write more readable and maintainable code for complex asynchronous workflows.
• Completion Handlers: Use when supporting older iOS versions or working with libraries that haven’t adopted async/await yet.

3. Real-Time Example: Fetching Data from an API

Imagine fetching user data from an API.

4. Code Sample: Using Async/Await vs Completion Handlers

// Using async/await

func fetchUserData() async throws -> String {
    let url = URL(string: "https://api.example.com/user")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return String(data: data, encoding: .utf8) ?? "No Data"
}

Task {
    do {
        let userData = try await fetchUserData()
        print("User Data: \(userData)")
    } catch {
        print("Error: \(error)")
    }
}

//Using Completion Handler

func fetchUserData(completion: @escaping (Result<String, Error>) -> Void) {
    let url = URL(string: "https://api.example.com/user")!
    URLSession.shared.dataTask(with: url) { data, _, error in
        if let error = error {
            completion(.failure(error))
            return
        }
        let userData = String(data: data ?? Data(), encoding: .utf8) ?? "No Data"
        completion(.success(userData))
    }.resume()
}

fetchUserData { result in
    switch result {
    case .success(let userData):
        print("User Data: \(userData)")
    case .failure(let error):
        print("Error: \(error)")
    }
}

5. Best Practices: Handling Asynchronous Tasks

For Async/Await

1. Adopt for Modern Apps: Prefer async/await for better readability and easier debugging in apps targeting iOS 15+.
2. Combine with Error Handling: Use try and catch to handle errors cleanly.
3. Avoid Mixing Paradigms: Refactor old completion handlers to async/await for consistency.

For Completion Handlers

1. Use for Compatibility: Use completion handlers when supporting older platforms or interacting with legacy APIs.
2. Avoid Callback Hell: Use techniques like combining closures or breaking tasks into smaller functions.
3. Wrap for Async/Await: Provide async/await versions of completion-based APIs for modern usage.

8. Explain the purpose of Key-Value Observing (KVO) in Swift. How does it differ from Combine or Delegation?

1. Concept: What is Key-Value Observing (KVO)?

Key-Value Observing (KVO) is a mechanism in Swift and Objective-C that allows an object to observe changes to the properties of another object. When the observed property changes, the observer is notified automatically. It’s widely used for implementing reactive patterns in applications.

• Purpose: To track changes to specific properties and respond to those changes.
• Example Use Case: Monitoring changes in the value of a property in UI or model objects.

2. Use: Where and When to Use KVO

• Legacy Support: KVO is commonly used in Objective-C and Swift projects that rely on Cocoa or UIKit, especially when working with frameworks that still use KVO (e.g., NSManagedObject in Core Data or AVPlayer for media playback).
• Dynamic Properties: KVO works best with properties marked as @objc dynamic.
• Limited Use: In modern Swift, Combine or property observers (willSet/didSet) are preferred due to type safety and better integration with Swift.

3. Real-Time Example: Observing a Property

Imagine tracking the playback progress of a video using KVO.

4. Code Sample: Using KVO in Swift

import Foundation

class VideoPlayer: NSObject {
    @objc dynamic var playbackProgress: Double = 0.0
}

class ProgressObserver: NSObject {
    var observation: NSKeyValueObservation?

    func observe(player: VideoPlayer) {
        observation = player.observe(\.playbackProgress, options: [.new]) { player, change in
            if let newValue = change.newValue {
                print("Playback progress changed to: \(newValue)")
            }
        }
    }
}

// Usage
let player = VideoPlayer()
let observer = ProgressObserver()
observer.observe(player: player)

player.playbackProgress = 0.5 // Prints: Playback progress changed to: 0.5
player.playbackProgress = 1.0 // Prints: Playback progress changed to: 1.0

5. Best Practices for KVO

1. Use Modern Alternatives When Possible: Consider Combine or property observers in Swift for better type safety and integration.
2. Remove Observers: If using the old KVO approach (pre-Swift 4.0), ensure you manually remove observers to avoid crashes. Modern KVO (NSKeyValueObservation) handles deallocation automatically.
3. Use with Care: Avoid overusing KVO as it can lead to hard-to-maintain code.
4. Scope Observations: Keep NSKeyValueObservation in a strong reference to maintain observation until explicitly canceled.
5. Mark Properties as Dynamic: Use the @objc dynamic keyword for properties to enable KVO compatibility.

9. How do you manage thread safety in Swift application?

1. Concept: What is Thread Safety?

Thread safety ensures that shared resources (like variables or objects) are accessed or modified safely when multiple threads or tasks are executing concurrently. In Swift, managing thread safety is crucial to prevent issues like race conditions, data corruption, or crashes.

2. Use: Where and When to Manage Thread Safety

• Shared Resources: Whenever multiple threads access or modify a shared resource, such as a shared counter, cache, or configuration object.
• Concurrency: When using concurrent operations (e.g., DispatchQueue, OperationQueue) or Swift’s structured concurrency features (Task).
• Critical Code Sections: Protect operations that must be executed atomically or in sequence.

3. Real-Time Example: Updating a Shared Counter

Imagine an app tracking the number of active users using a shared counter. Concurrent updates to this counter require thread safety to avoid incorrect values.

4. Code Sample: Managing Thread Safety in Swift

//Without Thread Safety (Risk of Race Condition)

class Counter {
    var count = 0

    func increment() {
        count += 1
    }
}

let counter = Counter()
DispatchQueue.concurrentPerform(iterations: 10) {
    counter.increment()
}
print("Final count: \(counter.count)") // Output may vary due to race conditions

//Using DispatchQueue for Synchronization

class ThreadSafeCounter {
    private var count = 0
    private let queue = DispatchQueue(label: "com.example.counterQueue")

    func increment() {
        queue.sync {
            count += 1
        }
    }

    func getCount() -> Int {
        queue.sync {
            count
        }
    }
}

let counter = ThreadSafeCounter()
DispatchQueue.concurrentPerform(iterations: 10) {
    counter.increment()
}
print("Final count: \(counter.getCount())") // Always correct

//Using NSLock for Synchronization

class ThreadSafeCounter {
    private var count = 0
    private let queue = DispatchQueue(label: "com.example.counterQueue")

    func increment() {
        queue.sync {
            count += 1
        }
    }

    func getCount() -> Int {
        queue.sync {
            count
        }
    }
}

let counter = ThreadSafeCounter()
DispatchQueue.concurrentPerform(iterations: 10) {
    counter.increment()
}
print("Final count: \(counter.getCount())") // Always correct

// Using Actor for Thread Safety (Swift Concurrency)

actor CounterActor {
    private var count = 0

    func increment() {
        count += 1
    }

    func getCount() -> Int {
        count
    }
}

let counter = CounterActor()
await withTaskGroup(of: Void.self) { group in
    for _ in 0..<10 {
        group.addTask {
            await counter.increment()
        }
    }
}
print("Final count: \(await counter.getCount())") // Always correct

5. Best Practices: Ensuring Thread Safety

• Use Serial Queues: For shared resources, use a private serial queue (DispatchQueue) to synchronize access.
• Leverage Locks: Use NSLock or pthread_mutex for critical sections. Be mindful of deadlocks.
• Use Thread-Safe Collections: Use thread-safe collections like NSCache or libraries providing thread-safe containers.
• Adopt Swift Concurrency: Use actors, Task, or await to simplify thread-safe programming.
• Minimize Shared State: Design your app to reduce shared mutable state and use immutable data wherever possible.
• Avoid Blocking Threads: Avoid queue.sync calls on the main thread to prevent deadlocks and UI freezes.
• Test for Race Conditions: Use tools like Thread Sanitizer (enable it in Xcode) to detect concurrency issues during development.

10. What is Dynamic Dispatch, and how does Swift implement it ?

1. Concept: What is Dynamic Dispatch?

Dynamic dispatch is a runtime mechanism where the method or function to execute is determined based on the type of the object, rather than its static type as seen at compile time. It allows polymorphism in object-oriented programming, enabling different implementations of the same method to be called based on the actual type of the object.

In Swift:

• Static dispatch is determined at compile time (e.g., struct methods, final methods, or inline optimizations).
• Dynamic dispatch is determined at runtime (e.g., methods in classes and protocols with @objc).

2. Use: Where and When to Use Dynamic Dispatch

Dynamic dispatch is crucial in scenarios where:

1. Polymorphism is Needed: For overriding methods in class hierarchies or protocol conformance.
2. Interoperability with Objective-C: When working with Cocoa frameworks using @objc methods.
3. Runtime Behavior: When decisions on method execution depend on runtime types, such as in UIKit delegates or Combine publishers.

3. Real-Time Example: Dynamic Dispatch in Action

Consider a scenario with a base class and subclasses implementing their own versions of a method.

4. Code Sample: Demonstrating Dynamic Dispatch

class Animal {
    func sound() {
        print("Animal makes a sound")
    }
}

class Dog: Animal {
    override func sound() {
        print("Dog barks")
    }
}

class Cat: Animal {
    override func sound() {
        print("Cat meows")
    }
}

let animals: [Animal] = [Dog(), Cat()]
for animal in animals {
    animal.sound() // Output: "Dog barks", "Cat meows"
}

Key Points:

1. The sound() method's behavior is determined at runtime based on the actual type of the object (Dog or Cat).
2. This is dynamic dispatch in action.

Swift’s Implementation of Dynamic Dispatch

• Virtual Table (vtable): Swift uses a vtable mechanism for dynamic dispatch in class inheritance. Each class has a vtable that maps method calls to their appropriate implementation at runtime.
• Objective-C Runtime: For methods marked with @objc, Swift uses the Objective-C runtime’s message dispatch system (objc_msgSend) for dynamic dispatch.
• Existential Containers: For protocols without associated types or Self requirements, Swift uses existential containers, enabling dynamic dispatch.
• Witness Tables: For protocols with associated types or Self requirements, Swift employs witness tables for method resolution.
• Final and Static Methods: These are statically dispatched, bypassing the runtime overhead of dynamic dispatch.

5. Best Practices for Dynamic Dispatch

• Use Only When Necessary: Prefer static dispatch (struct, enum, or final) for performance-critical code, as dynamic dispatch introduces a slight runtime overhead.
• Use @objc Judiciously: Add @objc only when interoperating with Objective-C or when required by frameworks. Avoid unnecessary use to keep type safety.
• Design with Protocols: Protocols with witness tables provide an efficient way to achieve dynamic dispatch without sacrificing too much performance.
• Mark Methods final: If a method is not meant to be overridden, mark it as final to enable static dispatch and improve performance.

Conclusion

🎉 Happy New Year 2025! 🎉

What better way to start the year than by leveling up your iOS development skills? In Part 2 of the “10 Must-Know iOS Interview Questions for Beginners (2025)” series, we dived into topics like associated types, async/await, KVO, thread safety, and dynamic dispatch — key concepts for any aspiring iOS developer.

If you missed Part 1, don’t worry — you can catch up here for foundational questions and tips to ace your interviews.

If this article helped you, please leave a 👏 to share the love and encourage others to read it. Don’t forget to follow me for more insights as we gear up for Part 3, with even more exciting iOS interview prep!

Wishing you a year filled with growth, opportunities, and success. Let’s make 2025 your breakthrough year! 🚀