cancel method
void
cancel()
取消正在进行的任务
Implementation
void cancel() {
log(_tag, 'cancel');
if (_isCanceled) {
log(_tag, 'work has been canceled');
return;
}
if (_completer.isCompleted) {
log(_tag, 'work finished');
return;
}
_isCanceled = true;
onCanceled();
}