Cancellation handling in Swift - Step-by-Step Execution

Concept Flow - Cancellation handling

Start async task

↓

Check for cancellation?

Yes→Stop task early

No↓

Continue work

↓

Complete task

↓

Return result or error

This flow shows how an async task checks if it was cancelled and stops early if so, otherwise it continues and completes.

Execution Sample

Swift

func fetchData() async throws -> String {
  try Task.checkCancellation()
  // simulate work
  try await Task.sleep(nanoseconds: 1_000_000_000)
  return "Data loaded"
}

This async function checks for cancellation before sleeping 1 second, then returns a string.

Execution Table

Step	Action	Cancellation Checked	Result	Next Step
1	Start fetchData()	No	No cancellation	Sleep 1 second
2	Sleep for 1 second	No	Sleep completes	Return result
3	Return "Data loaded"	N/A	Success	End
4	If cancelled before step 1	Yes	Throws CancellationError	Exit early

💡 Task ends either by completing normally or throwing CancellationError if cancelled early.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
isCancelled	false	false	false	false or true if cancelled
result	nil	nil	nil	"Data loaded" or error

Key Moments - 2 Insights

Why does the function call Task.checkCancellation() at the start?

It immediately checks if the task was cancelled before doing any work, so it can stop early as shown in execution_table row 4.

What happens if the task is cancelled during the sleep?

The sleep is cancellable, so the function throws a CancellationError and exits early, as explained in execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 1 if the task is cancelled?

A The function throws CancellationError and exits early

B The function continues sleeping

C The function returns "Data loaded" immediately

D The function ignores cancellation

💡 Hint

See execution_table row 4 where cancellation is checked at step 1.

According to variable_tracker, what is the value of 'result' after step 2 if not cancelled?

A "Data loaded"

B CancellationError

C nil

D false

💡 Hint

Result is assigned only after step 3, so after step 2 it is still nil.

If we remove Task.checkCancellation(), what changes in the execution flow?

A The task sleeps for zero seconds

B The task never stops early on cancellation

C The task throws CancellationError immediately

D The task returns an error immediately

💡 Hint

Without explicit cancellation check, the task continues until it hits a cancellable suspension point.

Concept Snapshot

Cancellation handling in Swift async tasks:
- Use Task.checkCancellation() to detect early cancellation.
- Throw CancellationError to stop work early.
- Cancellable suspension points (like Task.sleep) also throw on cancellation.
- Always check cancellation to avoid wasted work.
- Handle cancellation errors to clean up if needed.

Full Transcript

This visual execution trace shows how Swift async functions handle cancellation. The function fetchData() first calls Task.checkCancellation() to see if the task was cancelled before doing any work. If cancelled, it throws CancellationError and stops early. Otherwise, it continues to sleep for 1 second, which is also cancellable. If cancellation happens during sleep, the function throws and exits early. If no cancellation occurs, it returns the string "Data loaded". The variable tracker shows how the cancellation state and result change step by step. Key moments clarify why checking cancellation early is important and what happens if cancellation occurs during sleep. The quiz tests understanding of cancellation checks, variable states, and effects of removing cancellation checks.