Foundation Set 5: Concurrency & Async/Await (30 Q&A)

⚡ Concurrency & Async/Await - 30 Quick Q&A

Rapid-fire format: Short questions, short answers. Perfect for mock interviews and quiz systems.

🔹 GCD & DispatchQueues (Q1-10)

Q1: What is GCD?

A: Grand Central Dispatch - Apple’s low-level API for managing concurrent code execution using queues and tasks.

Q2: What’s the difference between serial and concurrent queues?

A: Serial executes tasks one at a time in order. Concurrent executes multiple tasks simultaneously.

Q3: What’s the main queue?

A: A serial queue where all UI updates must happen. Access via DispatchQueue.main.

Q4: How do you dispatch to background?

DispatchQueue.global().async {
    // Background work
}

A: Use DispatchQueue.global().async { } for background thread execution.

Q5: What are QoS (Quality of Service) levels?

A: Priority levels for tasks:

• .userInteractive - highest (UI)
• .userInitiated - high (user-triggered)
• .utility - low (downloads)
• .background - lowest (maintenance)

Q6: How do you update UI from background thread?

DispatchQueue.main.async {
    self.label.text = "Updated"
}

A: Always dispatch UI updates to main queue using DispatchQueue.main.async { }.

Q7: What’s the difference between sync and async?

A: sync blocks current thread until task completes. async returns immediately, task runs in background.

Q8: When would you use DispatchQueue.sync?

A: Rarely - only when you need to wait for result before continuing. Can cause deadlocks if misused.

Q9: What is a race condition?

A: When multiple threads access shared data simultaneously, causing unpredictable results.

Q10: How do you create a custom queue?

let queue = DispatchQueue(label: "com.app.myqueue")

A: Use DispatchQueue(label:) with unique identifier.

🔹 Async/Await (Q11-20)

Q11: What is async/await?

A: Swift’s modern concurrency feature that makes asynchronous code look and behave like synchronous code.

Q12: How do you mark a function as async?

func fetchData() async -> Data {
    // async work
}

A: Add async keyword before return type.

Q13: How do you call an async function?

let data = await fetchData()

A: Use await keyword when calling. Can only call from async context.

Q14: What does await do?

A: Suspends execution without blocking thread until async operation completes.

Q15: Can you use await in a regular function?

A: No - only in functions marked async or in Task { } blocks.

Q16: What is Task in Swift?

Task {
    let result = await fetchData()
}

A: Creates a new asynchronous task. Entry point for calling async code from sync context.

Q17: What’s the difference between Task and Task.detached?

A: Task inherits priority and context. Task.detached runs independently without inheriting anything.

Q18: How do you make multiple async calls in parallel?

async let a = fetch1()
async let b = fetch2()
let results = await [a, b]

A: Use async let or TaskGroup for parallel execution.

Q19: What is TaskGroup?

A: API for running multiple async tasks in parallel and collecting their results.

Q20: How do you cancel a Task?

let task = Task { await longOperation() }
task.cancel()

A: Call .cancel() on Task. Check Task.isCancelled inside to respond.

🔹 Actors & Thread Safety (Q21-30)

Q21: What is an actor?

A: A reference type that protects its mutable state from data races by serializing access automatically.

Q22: How do you define an actor?

actor Counter {
    var value = 0
    func increment() { value += 1 }
}

A: Use actor keyword instead of class. Properties are automatically protected.

Q23: How do you access actor properties?

let counter = Counter()
await counter.increment()  // Must use await

A: Must use await when calling actor methods or accessing properties from outside.

Q24: Why use actors?

A: Prevents data races at compile time. No need for manual locks or queues.

Q25: What is @MainActor?

A: Marks code that must run on main thread. Used for UI updates.

Q26: How do you use @MainActor?

@MainActor
class ViewModel: ObservableObject {
    @Published var data: [Item] = []
}

A: Annotate class or function with @MainActor to guarantee main thread execution.

Q27: What’s the difference between actor and class?

A: Actors provide automatic thread-safety. Classes require manual synchronization (locks, queues).

Q28: Can actors have synchronous methods?

A: Yes, but they’re still protected. Called with await from outside the actor.

Q29: What is Sendable protocol?

A: Marks types safe to pass across concurrency boundaries. Value types and actors conform automatically.

Q30: What’s a data race?

A: When two threads access same memory simultaneously and at least one modifies it, causing corruption.

🎯 Bonus: Common Pitfalls

Q31: Main thread blocking - bad or good?

A: Bad! Never block main thread. Causes UI freezes and ANR (App Not Responding).

Q32: Can you call sync on main from main?

// On main thread:
DispatchQueue.main.sync { } // DEADLOCK!

A: Never! Causes deadlock. Main waits for main to finish.

Q33: How do you avoid retain cycles with async?

Task { [weak self] in
    await self?.doWork()
}

A: Use capture lists [weak self] in Task closures.

Q34: What is structured concurrency?

A: Parent tasks automatically manage and cancel child tasks. Prevents task leaks.

Q35: How do you handle errors in async?

do {
    let data = try await fetchData()
} catch {
    print(error)
}

A: Use try await with do-catch, just like synchronous error handling.

📊 Quick Reference Table

🎮 Quiz-Ready Format

These questions are perfect for:

• ✅ Multiple choice conversion
• ✅ Flash cards
• ✅ Mock interview drills
• ✅ Rapid review sessions
• ✅ Automated quiz systems

Example quiz format:

Q: What does DispatchQueue.main represent?
A) Background queue
B) Serial queue for UI updates ✓
C) Concurrent queue
D) Custom queue

🎯 Topics Covered

• GCD & DispatchQueues (10 questions)
• Async/Await (10 questions)
• Actors & Thread Safety (10 questions)
• Bonus pitfalls (5 questions)

Difficulty: Medium-Hard
Time to Complete: 1-2 hours
Best Format: Flash cards or rapid-fire review
2025 Relevance: async/await & actors are now standard interview topics

💡 Study Tip: Cover answers, read questions, see how many you can answer in 60 seconds each!

📝 Next: Set 6: Networking & APIs - Same rapid-fire format!