The block to invoke when progress is cancelled.
- iOS 7.0+
- macOS 10.9+
- Mac Catalyst 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
If the receiver is a child of another progress object, the
cancellation block will be invoked when the parent is cancelled.
You are responsible for cancelling any work associated with the progress object.
The cancellation handler may be invoked on any queue. If you must do work on a specific queue, you should dispatch to that queue from within the cancellation handler block.