# cancel() Asynchronously cancels the dispatch source, preventing any further invocation of its event handler block. ``` func cancel() ``` ## Discussion Cancellation prevents any further invocation of the event handler block for the dispatch source, but does not interrupt any work that is already in progress. If a cancellation handler was set before cancellation, it is sub mitted to the target queue once any in-progress event handler work is finished. Once the cancellation handler is submitted, it is safe to close the source’s handle (file descriptor or mach port). It is invalid to close a file descriptor or deallocate a mach port that is currently being tracked by a dispatch source object before the cancellation handler is invoked. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)