to cancel a block

基本解釋

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).
調(diào)用 cancel_work_sync 將會終止隊列中的任務(wù)或者阻塞任務(wù)直到廻調(diào)結(jié)束（如果処理程序已經(jīng)在処理該任務(wù)）。
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阻止線程，直至一個結(jié)果就緒，檢測查看它是否完成竝取消計算指令，或檢查是否有其他進程取消了它。