C# AutoResetEvent WaitOne stopped by dispatcher

本文关键字：by dispatcher stopped WaitOne AutoResetEvent | 更新日期: 2023-09-27 18:07:46

private void WaitForDriveToBecomeReady()
{
    AutoResetEvent syncEvent = new AutoResetEvent(false); //set wait signal to use later
    //dispatcher to be able to change stuff in xaml from within thread
    Action action1 = new Action(delegate() { grdMain.Children.Add(notification); });
    Action action2 = new Action(delegate() { grdMain.Children.Remove(notification); });
    Thread restoreThread1 = new Thread(()=>{
        grdMain.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Background, action1); //show a notification
        Thread.Sleep(1500); //sleep a bit...
        grdMain.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Background, action2); //hide a notification
        syncEvent.Set(); //signal to continue at *.WaitOne()
    });
    restoreThread1.Start();
    syncEvent.WaitOne(); //let main thread wait until *.Set(); is called
}

如果你注释掉这两个grdMain.Dispatcher.Invoke(…);如果注释掉*.Set()，它也可以完美地工作;和* .WaitOne ();但WHYYYY呢?我两个都需要^^。我不明白…

C# AutoResetEvent WaitOne stopped by dispatcher

假设在Dispatcher的线程上调用WaitForDriveToBecomeReady，则显式地引入了死锁。

考虑执行过程

  • 你设置了重置事件
  • 线程开始执行
  • 调度线程调用syncEvent.WaitOne()，该线程现在被阻塞，直到该事件被设置
  • 第二个线程执行Dispatcher.Invoke;这将一个消息放入Dispatcher的队列中，并等待Dispatcher处理它(在主线程上)

因此主线程阻塞等待事件，该事件最终将由第二个线程设置。第二个线程被阻塞，等待主线程处理消息。教科书僵局。

一般来说，等待UI线程是不好的;这样很容易产生死锁，但即使它有效，您仍然会阻止UI更新，从而创建一个无响应的程序。根据上面的代码片段，很难说如何最好地重新组织代码，以便不必阻塞UI线程，但似乎这个概念(准备驱动器，并在准备好后执行某些操作)将是异步方法的候选。

我终于有时间继续阅读有关async和await的更多内容。谢谢@Jacob指出问题所在。

这是我的工作异步代码toast通知出现只要你不连接你的驱动器。

//the method
public async Task WaitForDriveAsync(string path, string waitingToastText)
{
    int width = 300;
    int height = 125;
    TextBlock toastTextBlock = new TextBlock() { Text = waitingToastText, HorizontalAlignment = HorizontalAlignment.Center, VerticalAlignment = VerticalAlignment.Center, FontSize = 23, Width = (width - 15), TextWrapping = TextWrapping.WrapWithOverflow };
    Grid notification = new Grid();
    notification.Width = width;
    notification.Height = height;
    notification.Background = Brushes.Red;
    notification.Margin = new System.Windows.Thickness(0, 27, 0.4, 0);
    notification.VerticalAlignment = VerticalAlignment.Top;
    notification.HorizontalAlignment = HorizontalAlignment.Right;
    notification.Children.Add(toastTextBlock);
    grdMain.Children.Add(notification);
    while (!Directory.Exists(path))
    {
        await Task.Delay(1000);
    }
    grdMain.Children.Remove(notification);
}
//to call it
private async void btnBackupNow_Click(object sender, RoutedEventArgs e)
{
    await WaitForDriveAsync(@"B:'", "Please connect your drive.");
}
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