Cancellation handling in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift async cancellation check?

Consider this Swift async function that checks for cancellation inside a loop. What will be printed when the task is cancelled during execution?

Swift

func performTask() async {
    for i in 1...5 {
        try Task.checkCancellation()
        print("Step \(i) completed")
        try await Task.sleep(nanoseconds: 500_000_000) // 0.5 seconds
    }
    print("Task finished")
}

let task = Task {
    await performTask()
}

Task {
    try await Task.sleep(nanoseconds: 1_200_000_000) // 1.2 seconds
    task.cancel()
}

// Wait for task to finish
try await task.value

A

Step 1 completed
Step 2 completed
Task finished

B

Step 1 completed
Step 2 completed
Step 3 completed

C

Step 1 completed
Step 2 completed
Step 3 completed
Task finished

D

Step 1 completed
Step 2 completed

💡 Hint

Cancellation is checked before each step and the task is cancelled after 1.2 seconds.

The task runs steps with 0.5 seconds delay each. After 1.2 seconds, cancellation is requested. The cancellation check stops the loop after step 2 completes, so only steps 1 and 2 print. The task does not reach step 3 or finish message.

🧠 Conceptual

intermediate

What happens when a Swift Task is cancelled but cancellation is not checked?

In Swift concurrency, if a Task is cancelled but the code inside does not check for cancellation, what is the behavior?

A The Task continues running until it finishes or checks for cancellation explicitly.

B The Task immediately stops execution and throws a cancellation error.

C The Task restarts automatically from the beginning.

D The Task pauses and waits for manual resume.

💡 Hint

Cancellation requires explicit checks in Swift concurrency.

In Swift, cancelling a Task sets its cancellation flag, but the Task code must check for cancellation using Task.checkCancellation() or similar. Without checks, the Task runs normally.

🔧 Debug

advanced

Why does this Swift async function not stop when cancelled?

Given this Swift async function, why does it not stop when the Task is cancelled? func fetchData() async { for i in 1...3 { print("Fetching chunk \(i)") try? await Task.sleep(nanoseconds: 1_000_000_000) } print("Fetch complete") } let task = Task { await fetchData() } Task { try? await Task.sleep(nanoseconds: 1_500_000_000) task.cancel() } try? await task.value

A Because Task.sleep ignores cancellation requests.

B Because the function does not call Task.checkCancellation() to detect cancellation.

C Because the Task.cancel() call is ignored in async functions.

D Because the loop range is fixed and cannot be interrupted.

💡 Hint

Cancellation requires explicit checks in Swift async code.

The function never checks for cancellation explicitly. Task.sleep can throw if cancelled, but try? ignores errors, so cancellation is not detected and the loop continues.

📝 Syntax

advanced

Which option correctly cancels a Swift Task and handles cancellation error?

Which code snippet correctly cancels a Swift Task and handles the cancellation error properly?

Swift

func longRunningTask() async throws {
    for i in 1...5 {
        try Task.checkCancellation()
        print("Processing \(i)")
        try await Task.sleep(nanoseconds: 500_000_000)
    }
}

let task = Task {
    try await longRunningTask()
}

Task {
    try await Task.sleep(nanoseconds: 1_200_000_000)
    task.cancel()
    do {
        try await task.value
    } catch {
        print("Task was cancelled")
    }
}

A Use try await task.value inside do-catch to catch cancellation error and print message.

B Use task.cancel() and ignore task.value completely.

C Use task.cancel() and then immediately call task.value without await or try.

D Use try? await task.value without catch block to ignore cancellation error.

💡 Hint

Cancellation throws an error that must be caught.

Cancellation throws a CancellationError. Using try await task.value inside a do-catch block allows catching this error and handling it gracefully.

🚀 Application

expert

How many times will the print statement execute before cancellation stops the task?

Analyze this Swift async function that checks cancellation and sleeps. How many times will "Working..." print before the task is cancelled? func work() async throws { var count = 0 while true { try Task.checkCancellation() print("Working... \(count)") count += 1 try await Task.sleep(nanoseconds: 300_000_000) // 0.3 seconds } } let task = Task { try await work() } Task { try await Task.sleep(nanoseconds: 1_000_000_000) // 1 second task.cancel() } try? await task.value

A 5 times

B 3 times

C 4 times

D 6 times

💡 Hint

Each loop iteration takes about 0.3 seconds, cancellation happens after 1 second.

Each iteration prints then sleeps 0.3 seconds. After 1 second, cancellation is requested. The task completes 3 full iterations and starts the 4th before cancellation is detected, so 4 prints occur.