cancellationHandler

Declaration

var cancellationHandler: (@Sendable () -> (any Error)?)? { get set }

Discussion

FSKit calls the cancellation handler within an independent execution context.

If the handler can’t complete its work successfully, it can return an error from the block or closure. FSKit logs any returned error and then terminates all activity in the container.

The task object clears its cancellationHandler property after the task’s cancellation or completion. This helps accelerate the cleanup of retained state.

The exact structuring of the completion handler depends on the structuring of the code implementing the task. As a concrete example, consider a check operation with the following class:

and a startCheckWithTask method with a helper method performCheck like the following:

When canceled, the handler block in this example sets the checker’s interrupted property, and then calls the DispatchGroup method wait() (Swift) or the function dispatch_group_wait (Objective-C) on the checker’s work group. Because neither of these operations can fail, the handler returns nil to indicate it didn’t encounter an error.

For simplicity, this example doesn’t account for errors, whereas production code must do so. Furthermore, when fully implemented, the performCheck method should perform a check operation. Specifically, it should periodically update the progress object and check its interrupted variable. The check can either complete successfully, complete with an error, or enter the interrupted state. It should then call didComplete(error:) with the appropriate error value or nil. Finally it should call context.work_group.leave() (Swift) or dispatch_group_leave(context.work_group) (Objective-C) to remove itself from its dispatch group.