How does task cancellation work?

Middle PRO

The essence

Cancellation in Swift Concurrency is cooperative: task.cancel() only sets a flag and marks every child task in the tree. Nothing gets interrupted — the code must check the flag and wind down on its own. A task that never looks at Task.isCancelled runs to completion as if no cancellation happened.

Ways to react:

• Task.isCancelled — check and decide for yourself what to do.
• try Task.checkCancellation() — throw CancellationError if cancelled.
• withTaskCancellationHandler(operation:onCancel:) — run code immediately at the moment of cancellation (close a connection, stop a driver), even if the task is currently suspended.
• Most system async APIs (URLSession, Task.sleep, AsyncSequence) check cancellation themselves and throw CancellationError or URLError.cancelled.