Question

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
}

如果你注释掉两个grdMain.Dispatcher.Invoke（...）;上面的代码是完美的。如果你注释掉* .Set（），它也可以运行perfekt;和* .WaitOne（）; 但为什么呢？我需要两个^^。我不明白......

Answer 1

假设在Dispatcher的线程上调用WaitForDriveToBecomeReady，您明确地引入了死锁。

考虑执行过程

您设置了重置事件
线程开始执行
调度程序线程调用syncEvent.WaitOne()，该线程现在被阻塞，直到该事件设置为
第二个线程执行Dispatcher.Invoke;这会在Dispatcher的队列中放入一条消息，并等待它处理它（在主线程上）

因此，主线程被阻塞，等待最终由第二个线程设置的事件。并且您阻止第二个线程等待主线程处理消息。课本死锁。

一般来说，等待UI线程是不好的;这样很容易出现死锁，但即使它有效，你仍然会阻止UI更新，创建一个无响应的程序。基于上面的代码段，很难说如何最好地重新组织代码，这样你就不必阻止UI线程，但似乎这个概念（准备一个驱动器，并在它准备就绪后做一些事情）将是一个候选者对于asynchronous method。

Answer 2

我终于有时间继续阅读有关异步和等待的更多信息。感谢@Jacob指出问题所在。

以下是我的工作异常代码，只要您没有连接驱动器就会显示Toast通知。

//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.");
}

C＃AutoResetEvent WaitOne由调度程序停止

2 个答案: