cancel method
Implementation
void cancel({required String reason}) {
assert(_completeOrCancelReason == null);
_completeOrCancelReason = reason;
assert(state == TaskState.init || state == TaskState.readyToComplete);
state = TaskState.cancel;
manager.removeTask(this);
doneState(2);
mtLog("任务$debugIdentify 取消:$_completeOrCancelReason");
}