setCancelHandler(handler:)
Sets the cancellation handler block for the dispatch source.
Declaration
func setCancelHandler(handler: DispatchWorkItem)Parameters
- handler:
The event handler block to submit to the source’s target queue.
Discussion
The cancellation handler (if specified) is submitted to the source’s target queue in response to a call to a call to the cancel() method once the system has released all references to the source’s underlying handle and the source’s event handler block has returned.
To safely close a file descriptor or destroy a Mach port, a cancellation handler is required for that descriptor or port. Closing the descriptor or port before the cancellation handler runs can result in a race condition. If a new descriptor is allocated with the same value as the recently closed descriptor while the source’s event handler is still running, the event handler may read/write data using the wrong descriptor.