Cancellation handling in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is cancellation handling in Swift concurrency?

Cancellation handling is the process of stopping a running asynchronous task when it is no longer needed, allowing the program to free resources and avoid unnecessary work.

beginner

How do you check if a Swift Task has been cancelled?

You check if a task has been cancelled by using the property Task.isCancelled inside the task's code.

intermediate

What happens if you throw CancellationError in a Swift async task?

Throwing CancellationError signals that the task was cancelled and allows the task to exit early and clean up resources.

intermediate

Why should you check for cancellation regularly inside long-running async tasks?

Because it lets the task stop work early if cancellation is requested, saving CPU time and improving app responsiveness.

beginner

How do you cancel a running Swift Task?

You call the cancel() method on the Task instance you want to stop.

Which property tells you if a Swift Task has been cancelled?

A Task.isCancelled

B Task.isRunning

C Task.isComplete

D Task.isPaused

Task.isCancelled is true when a cancellation request has been made for the task.

What method do you call to request cancellation of a Swift Task?

A stop()

B cancel()

C terminate()

D end()

The cancel() method requests the task to stop as soon as possible.

What error type is commonly thrown to indicate a cancelled task in Swift concurrency?

A AsyncError

B TaskError

C CancellationError

D StopError

CancellationError is the standard error used to signal task cancellation.

Why is it important to check for cancellation inside a long-running async task?

A To log task progress

B To restart the task automatically

C To increase task priority

D To free resources and stop unnecessary work

Checking for cancellation lets the task stop early and save resources.

What happens if you ignore cancellation requests in a Swift Task?

A The task will continue running until completion

B The task will stop immediately

C The task will throw an error automatically

D The task will pause

Ignoring cancellation means the task keeps running and wastes resources.

Explain how to implement cancellation handling in a Swift async task.

Think about how to detect and respond to cancellation requests inside the task.

Why is cancellation handling important in asynchronous programming?

Consider what happens if tasks run without stopping when they should.

Practice

1. What is the main purpose of cancellation handling in Swift concurrency?

easy

A. To create new tasks continuously

B. To speed up all running tasks

C. To restart tasks automatically

D. To stop tasks that are no longer needed

Solution

1. Step 1: Understand cancellation handling concept

   Cancellation handling is used to stop tasks that are no longer needed to save resources.

2. Step 2: Compare options with concept

   Only To stop tasks that are no longer needed matches the purpose of stopping unnecessary tasks.

3. Final Answer:

   To stop tasks that are no longer needed -> Option D

4. Quick Check:

   Cancellation handling = stop unnecessary tasks [OK]

Hint: Cancellation means stopping tasks early to save resources [OK]

Common Mistakes:

• Thinking cancellation speeds up tasks
• Confusing cancellation with restarting tasks
• Assuming cancellation creates new tasks

2. Which of the following is the correct way to check for cancellation inside a Swift async task?

easy

A. if Task.cancelled() { return }

B. if Task.isCancelled { return }

C. if Task.checkCancel() { return }

D. if Task.isCancel { return }

Solution

1. Step 1: Recall correct API for cancellation check

   The correct property to check cancellation is Task.isCancelled.

2. Step 2: Verify syntax correctness

   if Task.isCancelled { return } uses Task.isCancelled correctly; others use invalid method or property names.

3. Final Answer:

   if Task.isCancelled { return } -> Option B

4. Quick Check:

   Use Task.isCancelled to check cancellation [OK]

Hint: Remember: Task.isCancelled is a property, not a method [OK]

Common Mistakes:

• Using incorrect method names like cancelled()
• Confusing property names with similar words
• Missing capitalization or spelling errors

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

Task {
    for i in 1...3 {
        try Task.checkCancellation()
        print("Running step \(i)")
        if i == 2 {
            Task.current.cancel()
        }
    }
}

medium

A. Running step 1
Running step 2

B. Running step 1
Running step 2
Running step 3

C. Running step 1

D. No output due to immediate cancellation

Solution

1. Step 1: Understand Task.checkCancellation() behavior

   Task.checkCancellation() throws if the task is cancelled, stopping further execution.

2. Step 2: Analyze loop and cancellation timing

   Cancellation happens when i == 2, so steps 1 and 2 print, but step 3 is skipped due to cancellation.

3. Final Answer:

   Running step 1\nRunning step 2 -> Option A

4. Quick Check:

   Cancellation after step 2 stops step 3 [OK]

Hint: Cancellation stops loop after current iteration [OK]

Common Mistakes:

• Assuming all steps run despite cancellation
• Ignoring that checkCancellation throws on cancel
• Thinking cancellation happens before step 2

4. Identify the error in this Swift async task cancellation handling code:

Task {
    if Task.isCancelled {
        print("Cancelled")
    }
    try Task.checkCancellation()
    print("Running")
}

medium

A. Task.isCancelled check is redundant before checkCancellation()

B. Missing try keyword before Task.checkCancellation()

C. No error; code handles cancellation correctly

D. Task.isCancelled should be checked after checkCancellation()

Solution

1. Step 1: Understand difference between Task.isCancelled and checkCancellation()

   Task.checkCancellation() throws if cancelled, so it already handles cancellation detection.

2. Step 2: Analyze redundancy of Task.isCancelled check

   Checking Task.isCancelled before checkCancellation() is redundant because checkCancellation() will throw if cancelled.

3. Final Answer:

   Task.isCancelled check is redundant before checkCancellation() -> Option A

4. Quick Check:

   checkCancellation() alone suffices to detect cancellation [OK]

Hint: Use checkCancellation() to handle cancellation, no need for isCancelled first [OK]

Common Mistakes:

• Thinking isCancelled must come before checkCancellation()
• Missing try keyword (actually present here)
• Assuming no error when redundancy exists

5. You want to cancel a long-running Swift async task when a user taps a cancel button. Which approach correctly handles cancellation and cleans up resources?

func performTask() async {
    let task = Task {
        do {
            for i in 1...5 {
                try Task.checkCancellation()
                print("Processing \(i)")
                try await Task.sleep(nanoseconds: 1_000_000_000)
            }
        } catch {
            print("Task was cancelled")
        }
        print("Cleanup done")
    }
    // User taps cancel button
    task.cancel()
}

hard

A. Task.cancel() should be called inside the task, not outside

B. Cleanup code will not run after cancellation

C. This code correctly cancels the task and runs cleanup after cancellation

D. Task.checkCancellation() does not throw, so cancellation is ignored

Solution

1. Step 1: Analyze cancellation handling in the task

   Task.checkCancellation() throws when cancelled, caught by catch block which prints cancellation message.

2. Step 2: Verify cleanup code execution

   Cleanup code runs after catch block, ensuring resources are cleaned up even after cancellation.

3. Final Answer:

   This code correctly cancels the task and runs cleanup after cancellation -> Option C

4. Quick Check:

   Cancellation triggers catch and cleanup runs [OK]

Hint: Use try/catch with checkCancellation() to handle cancel and cleanup [OK]

Common Mistakes:

• Assuming cleanup won't run after cancellation
• Calling cancel() inside the task incorrectly
• Believing checkCancellation() does not throw