为什么WPF中的BackgroundWorker需要Thread.Sleep来更新UI控件？

Question

namespace WpfApplication1
{
/// <summary>
/// Interaction logic for Window1.xaml
/// </summary>
public partial class Window1 : Window
{
    BackgroundWorker bgWorker;
    Action<int> myProgressReporter;

    public Window1()
    {
        InitializeComponent();
        bgWorker = new BackgroundWorker();
        bgWorker.DoWork += bgWorker_Task;
        bgWorker.RunWorkerCompleted += myWorker_RunWorkerCompleted;

        // hook event to method
        bgWorker.ProgressChanged += bgWorker_ReportProgress;

        // hook the delegate to the method
        myProgressReporter = updateProgress;

        bgWorker.WorkerReportsProgress = true;

    }

    private void myWorker_RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
    {
        object result;
        result = e.Result;
        MessageBox.Show(result.ToString());
        progressBar1.Value = 0;
        button1.IsEnabled = true;
    }

    private void bgWorker_ReportProgress(object sender, ProgressChangedEventArgs e)
    {
        System.Windows.Threading.Dispatcher disp = button1.Dispatcher;
        disp.BeginInvoke(myProgressReporter,e.ProgressPercentage);
        //Dispatcher.BeginInvoke(myProgressReporter, DispatcherPriority.Normal, e.ProgressPercentage);
    }

    private void updateProgress(int progressPercentage)
    {
        progressBar1.Value = progressPercentage;
    }

    private void bgWorker_Task(Object sender, DoWorkEventArgs e)
    {
        int total = 1000;
        for (int i = 1; i <= total; i++)
        {
            if (bgWorker.WorkerReportsProgress)
            {
                int p = (int)(((float)i / (float)total) * 100);
                bgWorker.ReportProgress(p);
            }
            Thread.Sleep(1); // Without Thread.Sleep(x) the main thread freezes or gives stackoverflow exception, 
        }

        e.Result = "Completed";
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {            
        if(!bgWorker.IsBusy)
            bgWorker.RunWorkerAsync("This is a background process");
        button1.IsEnabled = false;
    }
}
}

Answer 1

因为在您的（人为）场景中，您向主线程发送了 1000 个更新请求。
它没有时间进行空闲循环（需要进行屏幕更新）。

但是（感谢 TerrorAustralis），您应该从合并 bgWorker_ReportProgress 和 myProgressReporter 方法开始。您现在正在同步两次，这可能是导致 stackoverflow 的原因。调度 UpdateProgress 事件是Backgroundworker 的主要功能之一：

private void bgWorker_ReportProgress(object sender, ProgressChangedEventArgs e)
{
  //System.Windows.Threading.Dispatcher disp = button1.Dispatcher;
  //disp.BeginInvoke(myProgressReporter,e.ProgressPercentage);
  progressBar1.Value = progressPercentage; // safe because we're on the main Thread here
}

Answer 2

可能性：
Dispatcher.BeginInvoke() 是一个异步操作。既然是这种情况，您可以在它完成操作之前再次尝试点击它。要查看这是否是问题所在，请尝试同步的 Dispatcher.Invoke()

作为一种可能的解决方法，如果您只想更新进度栏，backgroundWorker ProgressChanged 事件可以在不使用显式调度程序的情况下完成此操作。