取消后台任务

Question

当我的C#应用​​程序关闭时,它有时会陷入清理例程.具体来说,后台工作人员没有关闭.这基本上就是我试图关闭它的方式:

private void App_FormClosing(object sender,FormClosingEventArgs e){backgroundWorker1.CancelAsync(); 而(backgroundWorker1.IsBusy); //被困在这里 }

我应该采取不同的方式吗？我使用的是Microsoft Visual C#2008 Express Edition.谢谢.

附加信息:

后台工作人员似乎没有退出.这就是我所拥有的:

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
   while (!backgroundWorker1.CancellationPending)
   {
      // Do something.
   }
}

我还修改了清理代码:

private void App_FormClosing(object sender, FormClosingEventArgs e)
{
   while (backgroundWorker1.IsBusy)
   {
      backgroundWorker1.CancelAsync();
      System.Threading.Thread.Sleep(1000);
   }
}

我还应该做些什么吗？

Answer 1

一些非常好的建议,但我不认为它们解决了潜在的问题:取消后台任务.

不幸的是,在使用时BackgroundWorker,终止任务取决于任务本身.while循环终止的唯一方法是,如果后台任务检查其Cancel属性并从当前进程返回或中断.

示例基础

例如,考虑一下

private readonly BackgroundWorker worker = new BackgroundWorker ();

public void SomeFormEventForStartingBackgroundTask ()
{
    worker.DoWork += BackgroundTask_HotelCalifornia;
    worker.WorkerSupportsCancellation = true;
    worker.RunWorkerAsync ();
}

// semantically, you want to perform this task for lifetime of
// application, you may even expect that calling CancelAsync
// will out and out abort this method - that is incorrect.
// CancelAsync will only set DoWorkEventArgs.Cancel property
// to true
private void BackgroundTask_HotelCalifornia (object sender, DoWorkEventArgs e)
{
    for ( ; ;)
    {
        // because we never inspect e.Cancel, we can never leave!
    }
}

private void App_FormClosing(object sender, FormClosingEventArgs e)     
{
    // [politely] request termination
    worker.CancelAsync();

    // [politely] wait until background task terminates
    while (worker.IsBusy);
}

这是默认情况下发生的事情.现在,也许你的任务不是一个无限循环,也许它只是一个长期运行的任务.无论哪种方式,您的主线程将阻止[实际上它正在旋转,但是whatevs]直到任务完成,或者不是视情况而定.

如果您亲自编写并可以修改任务,那么您有几个选择.

示例改进

例如,这是上述示例的更好实现

private readonly BackgroundWorker worker = new BackgroundWorker ();

// this is used to signal our main Gui thread that background
// task has completed
private readonly AutoResetEvent isWorkerStopped = 
    new AutoResentEvent (false);

public void SomeFormEventForStartingBackgroundTask ()
{
    worker.DoWork += BackgroundTask_HotelCalifornia;
    worker.RunWorkerCompleted += BackgroundTask_Completed;
    worker.WorkerSupportsCancellation = true;
    worker.RunWorkerAsync ();
}

private void BackgroundTask_HotelCalifornia (object sender, DoWorkEventArgs e)
{
    // execute until canceled
    for ( ; !e.Cancel;)
    {
        // keep in mind, this task will *block* main
        // thread until cancel flag is checked again,
        // so if you are, say crunching SETI numbers 
        // here for instance, you could still be blocking
        // a long time. but long time is better than 
        // forever ;)
    }
}

private void BackgroundTask_Completed (
    object sender, 
    RunWorkerCompletedEventArgs e)
{
    // ok, our task has stopped, set signal to 'signaled' state
    // we are complete!
    isStopped.Set ();
}

private void App_FormClosing(object sender, FormClosingEventArgs e)     
{
    // [politely] request termination
    worker.CancelAsync();

    // [politely] wait until background task terminates
    isStopped.WaitOne ();
}

虽然这样做更好,但它并不是那么好.如果您[合理]确保您的后台任务将结束,这可能"足够好".

但是,我们[通常]想要的是这样的

private void App_FormClosing(object sender, FormClosingEventArgs e)     
{
    // [politely] request termination
    worker.CancelAsync();

    // [politely] wait until background task terminates
    TimeSpan gracePeriod = TimeSpan.FromMilliseconds(100);
    bool isStoppedGracefully = isStopped.WaitOne (gracePeriod);

    if (!isStoppedGracefully)
    {
        // KILL! KILL! KILL!
    }
}

唉,我们做不到.BackgroundWorker不会暴露任何强制终止的手段.这是因为它是一个构建在一些隐藏线程管理系统之上的抽象,如果它被强制终止,它可能会使应用程序的其他部分失去稳定性.

我实现上述目标的唯一方法是管理自己的线程.

示例理想

所以,例如

private Thread worker = null;

// this time, 'Thread' provides all synchronization
// constructs required for main thread to synchronize
// with background task. however, in the interest of
// giving background task a chance to terminate gracefully
// we supply it with this cancel signal
private readonly AutoResetEvent isCanceled = new AutoResentEvent (false);

public void SomeFormEventForStartingBackgroundTask ()
{
    worker = new Thread (BackgroundTask_HotelCalifornia);
    worker.IsBackground = true;
    worker.Name = "Some Background Task"; // always handy to name things!
    worker.Start ();
}

private void BackgroundTask_HotelCalifornia ()
{
    // inspect cancel signal, no wait period
    // 
    // NOTE: so cheating here a bit, this is an instance variable
    // but could as easily be supplied via parameterized thread
    // start delegate
    for ( ; !isCanceled.WaitOne (0);)
    {
    }
}

private void App_FormClosing(object sender, FormClosingEventArgs e)     
{
    // [politely] request termination
    isCanceled.Set ();

    // [politely] wait until background task terminates
    TimeSpan gracePeriod = TimeSpan.FromMilliseconds(100);
    bool isStoppedGracefully = worker.Join (gracePeriod);

    if (!isStoppedGracefully)
    {
        // wipe them out, all of them.
        worker.Abort ();
    }
}

那就是线程管理的一个不错的介绍.

哪个最适合你？取决于您的申请.最好不要晃动船只,并修改当前的实施以确保这一点

1. 您的后台任务检查并尊重Cancel财产
2. 你的主线程等待完成,而不是轮询

比较和评估每种方法的优缺点非常重要.

如果您必须控制并保证终止其他人的任务,那么编写包含上述内容的线程管理系统可能就是您的选择.但是你会失去开箱即用的功能,如线程池,进度报告,跨线程数据编组[工作人员那样做,没有？],以及其他一些东西.更不用说"滚动你自己"往往容易出错.

无论如何,希望这有助于:)

Answer 2

Kevin Gale正确地指出，BackgroundWorker 的 DoWork 处理程序需要轮询 CancellationPending，并在请求取消时返回。

话虽这么说，如果在您的应用程序关闭时发生这种情况，您也可以安全地忽略它。BackgroundWorker 使用 ThreadPool 线程，根据定义，它是后台线程。保持该线程运行不会阻止您的应用程序终止，并且当您的应用程序关闭时，该线程将自动被拆除。