cancel method
Cancel the downloading, if deleteOnCancel is true
then file will be deleted
will return false
if downloading was already finished or cancelled
Implementation
Future<bool> cancel() async {
if (_doneOrCancelled) return false;
_addEvent(TaskEvent(
state: TaskState.canceled,
bytesReceived: _bytesReceived,
totalBytes: _totalBytes));
await _subscription?.cancel();
_subscription = null;
_dispose(TaskState.canceled);
return true;
}