睡在线程内的替代方法 [英] Alternative to sleep inside a thread

问题描述

各种答案表明，睡在线程内是个坏主意，例如：避免睡眠。为什么？通常给出的一个原因是，如果它正在睡觉，很难优雅地退出线程（通过信号通知它终止）。

Various answers suggest it is a bad idea to sleep inside a thread, for example: Avoid sleep. Why exactly? One reason often given is that it is difficult to gracefully exit the thread (by signalling it to terminate) if it is sleeping.

假设我想定期检查新的文件在一个网络文件夹，也许每10秒一次。这对于优先级设置为低（或最低）的线程似乎是完美的，因为我不希望潜在的耗时的文件I / O影响我的主线程。

Let's say I wanted to periodically check for new files in a network folder, maybe once every 10s. This seems perfect for a thread with the priority set to low (or lowest) because I don't want the potentially time-consuming file I/O to impact my main thread.

有哪些替代方案？代码在Delphi中给出，但同样适用于任何多线程应用程序：

What are the alternatives? Code is given in Delphi but would apply equally to any multi-threaded application:

procedure TNetFilesThrd.Execute();
begin
    try
        while (not Terminated) do
            begin
            // Check for new files
            // ...

            // Rest a little before spinning around again
            if (not Terminated) then
                Sleep(TenSeconds);
            end;
    finally
        // Terminated (or exception) so free all resources...
    end;
end;

稍作修改可能是：

// Rest a little before spinning around again
nSleepCounter := 0;
while (not Terminated) and (nSleepCounter < 500) do
    begin
    Sleep(TwentyMilliseconds);
    Inc(nSleepCounter);
    end;

但这还涉及到休眠...

but this still involves a Sleep...

推荐答案

这样做的标准方法是等待取消事件。在伪代码看起来像这样：

The standard way to do this is to wait on a cancellation event. In pseudo code that looks like this:

while not Terminated do
begin
  // Check for new files
  // ...

  // Rest a little before spinning around again
  FTerminationEvent.WaitFor(TenSeconds);
end;

为了终止你将覆盖 TerminatedSet ：

procedure TMyThread.TerminatedSet;
begin
  inherited;
  FTerminationEvent.SetEvent; // abandon the wait in the thread method
end;

对事件的等待超时或由于事件被发出信号而终止。这允许你的线程暂停一段时间，并不负担CPU，并且仍然响应终止请求。

The wait on the event either times out, or terminates because the event is signaled. This allows your thread to pause for a while and not burden the CPU, and yet also remain responsive to requests to terminate.

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