为什么BackgroundWorker总是很忙？

Question

我在我的 WPF 应用程序的后台工作人员中发现了一些奇怪的东西。

我现在想要完成的是等到 BW 完成以启动另一个线程。

检查以下代码：

if (bw.IsBusy)
{
    bw.CancelAsync();

    System.Threading.ThreadStart WaitThread = 
        new System.Threading.ThreadStart(delegate() {
            while (bw.IsBusy)
            {
                System.Threading.Thread.Sleep(100);
            }

            bw.RunWorkerAsync();
        });

    System.Windows.Application.Current.Dispatcher.Invoke(
        System.Windows.Threading.DispatcherPriority.Normal,
        WaitThread);  // if I remove this line, bw fires RunWorkerAsyncEvent
}
else
{
    bw.RunWorkerAsync();
}

请注意，我添加了一个 Dispatcher.Invoke 以等到 bw 不忙，但如果我调用它并且从不触发 RunWorkerAsyncCompleted 事件，则所有时间都是忙碌的。虽然，如果我删除那条线，会触发事件，这真的很奇怪。

我怎样才能等到 bw 完成？

Answer 1

这称为“死锁”，是一种非常常见的线程问题。在 RunWorkerCompleted 事件运行之前，BGW 无法停止忙碌。该事件在您的应用程序的主线程上运行，它只能在您的主线程不忙于做其他事情时运行。它必须是空闲的，在调度程序循环内运行。

但是你的主线程不是空闲的，它卡在 while() 循环中，等待 IsBusy 返回 false。所以事件永远无法运行，因为主线程正忙于等待 BGW 完成，BGW 无法完成，因为主线程永远不会空闲。 “致命的拥抱”，又名僵局。

您将不得不以不同的方式执行此操作，您不能等待。通过创建另一个 BGW 实例来说。或者将等待后的代码移动到 RunWorkerCompleted 事件处理程序中。如果它是由按钮的 Click 事件激活的，那么请确保在调用 RunWorkerAsync() 时禁用该按钮，然后在 RunWorkerCompleted 中重新启用它。

Answer 2

由于应用程序的“主”线程无法运行（和处理事件）而发生死锁，因此 Backgroundworker 永远无法触发它的已完成功能。您可以做些什么来让应用程序触发它是添加一个 Application.DoEvents();

这是我目前在类似场景中使用的，它看起来就像一个魅力！

    while (bw.IsBusy)
    {
        Application.DoEvents();
        System.Threading.Thread.Sleep(100);
    }

Answer 3

我不确定我是否理解正确，但我认为您正在尝试重新启动 BackgroundWorker（因此，如果它很忙，请先停止它，然后再重新启动它），如果这是你想要的，试试这个：

在你的班级某处声明这个委托：

    private delegate bool StateChecker();

并使用此代码重新启动您的 BackgroundWorker：

    StateChecker stillWorking = () => { return bw.IsBusy; };

    if (bw.IsBusy)
    {
        bw.CancelAsync();
        while ((bool)this.Dispatcher.Invoke(stillWorking, null)) Thread.Sleep(15);
    }
    bw.RunWorkerAsync();

Answer 4

这是完整且有效的代码。

C#（窗口类）

 public partial class ForceSyncWindow : Window
    {
        BackgroundWorker backgroundWorker = new BackgroundWorker();

        public ForceSyncWindow()
        {
            InitializeComponent();

            ProgressBar1.Visibility = System.Windows.Visibility.Hidden;

            backgroundWorker.WorkerSupportsCancellation = true;

            // To report progress from the background worker we need to set this property
            backgroundWorker.WorkerReportsProgress = true;

            // This event will be raised on the worker thread when the worker starts
            backgroundWorker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);

            // This event will be raised when we call ReportProgress
            backgroundWorker.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker_ProgressChanged);

            backgroundWorker.RunWorkerCompleted += backgroundWorker_RunWorkerCompleted;
        }

        void backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            // First, handle the case where an exception was thrown. 
            if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message);
            }
            else if (e.Cancelled)
            {
                // Next, handle the case where the user canceled  
                // the operation. 
                // Note that due to a race condition in  
                // the DoWork event handler, the Cancelled 
                // flag may not have been set, even though 
                // CancelAsync was called.
                ProgressBar1.Value = 0;
                // TODO LOG  = "Canceled";

            }
            else
            {
                // Finally, handle the case where the operation  
                // succeeded.
                // TODO LOG e.Result.ToString();

            }

            ProgressBar1.Value = 0;
            ProgressBar1.Visibility = System.Windows.Visibility.Hidden;

            // Enable the Synchronize button. 
            this.Synchronize.IsEnabled = true;

            // Disable the Cancel button.
            this.Cancel.IsEnabled = false;
        }

        // On worker thread so do our thing!
        void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
        {
              // Your background task goes here
                for (int i = 0; i <= 100; i++)
                {
                    if (backgroundWorker.CancellationPending == true)
                    {
                        e.Cancel = true;
                        break;
                    }
                    else
                    {
                        // Perform a time consuming operation and report progress.
                        // Report progress to 'UI' thread
                        backgroundWorker.ReportProgress(i);
                        // Simulate long task
                        System.Threading.Thread.Sleep(100); ;
                    }                              
                }            
        }

        // Back on the 'UI' thread so we can update the progress bar
        void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            // The progress percentage is a property of e
            ProgressBar1.Value = e.ProgressPercentage;
        }

        private void Synchronize_Click(object sender, RoutedEventArgs e)
        {
            ProgressBar1.Value = 0;
            ProgressBar1.Visibility = System.Windows.Visibility.Visible;

            // Disable the Synchronize button. 
            this.Synchronize.IsEnabled = false;

            // Enable the Cancel button.
            this.Cancel.IsEnabled = true;

            // Start the background worker
            backgroundWorker.RunWorkerAsync();
        }

        private void Cancel_Click(object sender, RoutedEventArgs e)
        {
            if (backgroundWorker.IsBusy)
            {
                // Cancel the background worker
                backgroundWorker.CancelAsync();
            }
        }
    }

XAML

<Window x:Class="MySyncManager.Views.ForceSyncWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        Title="ForceSyncWindow" Height="300" Width="509" ResizeMode="NoResize" WindowStartupLocation="CenterScreen">
    <Grid>

        <Button Content="Synchronize" Name="Synchronize" HorizontalAlignment="Left" Margin="411,10,0,0" VerticalAlignment="Top" Width="75" Click="Synchronize_Click"/>
        <RichTextBox HorizontalAlignment="Left" Height="132" Margin="10,116,0,0" VerticalAlignment="Top" Width="476">

        </RichTextBox>
        <Button Content="Cancel" x:Name="Cancel" HorizontalAlignment="Left" Margin="411,40,0,0" VerticalAlignment="Top" Width="75" RenderTransformOrigin="0.508,2.154" Click="Cancel_Click"/>
        <ProgressBar Name="ProgressBar1" HorizontalAlignment="Left" Height="10" Margin="10,101,0,0" VerticalAlignment="Top" Width="476"/>

    </Grid>
</Window>

Answer 5

把它放在另一个线程里面：

private void myButton_Click(object sender, RoutedEventArgs e)
{
   BackgroundWorker mainWorker = new BackgroundWorker();
   mainWorker.DoWork += new DoWorkEventHandler(mainWorker_DoWork);
   mainWorker.RunWorkerAsync();
}

void mainWorker_DoWork(object sender, DoWorkEventArgs e)
{
   for(int x = 0; x >= 100; x++)
   {
      BackgroundWorker worker = new BackgroundWorker();
      worker.DoWork += new DoWorkEventHandler(worker_DoWork);
      worker.RunWorkerAsync(argument: x);
      while(worker.IsBusy)
      {
         Thread.Sleep(100);
      }
   }
}

void worker_DoWork(object sender, DoWorkEventArgs e)
{
   string iam = "Hello i'm thread number: " + e.Argument.ToString();
   //do something ...
}