to cancel a block

基本解释

A call to cancel_work_sync will terminate the work in the queue or block until the callback has finished (if the work is already in progress in the handler).
调用 cancel_work_sync 将会终止队列中的任务或者阻塞任务直到回调结束（如果处理程序已经在处理该任务）。
The recipient can ask the Future to block until a result is ready, poll to see whether it's complete, cancel the computation, or check whether another thread has canceled it.
接收者可以要求 Future阻止线程，直至一个结果就绪，检测查看它是否完成并取消计算指令，或检查是否有其他进程取消了它。