如何停止 WPF 中的后台工作人员？

Question

我在我的 WPF 应用程序中使用 MVVM 模型。我有一个绑定到取消按钮的命令。我有一个启动按钮，可以启动一些后台工作人员。当我单击取消按钮时，我希望所有后台工作人员停止/退出。 当我点击取消按钮时，使用我当前的代码，后台工作人员不会停止并且“StartEngineeringOperation”完成。谁能帮我解决我在这里做错了什么？

当前代码：

对于 EngineeringViewModel.cs：

公共类 EngineeringViewModel{

public EngineeringViewModel()
{
            StartEngineering= new DelegateCommand(o =>
            {
                worker = new BackgroundWorker
                {
                    WorkerReportsProgress = true,
                    WorkerSupportsCancellation = true
                };
                worker.ProgressChanged += Worker_ProgressChanged;
                worker.RunWorkerCompleted += worker_RunWorkerCompleted;
                if (worker.IsBusy != true) worker.RunWorkerAsync();
                worker.DoWork += (s, e) =>
                {
                    StartEngineeringOperation();
                    if (worker.CancellationPending)
                    {
                        e.Cancel = true;
                        return;
                    }
                };
            },
                k => true);
            Cancel = new DelegateCommand(CancelEngineeringOperation);
}

private void StartEngineeringOperation()
   {
      startAlarmService();
      startTrendQualityCheck();
   }

private void CancelEngineeringOperation(object param)
    {           
        worker.DoWork += (s, e) =>
        {
            if (worker.IsBusy)
            {
                worker.CancelAsync();
                e.Cancel = true;
                return;
            }
           
        };
       
    }
}

我试过这个： 但似乎不起作用：

private void StartEngineeringOperation()
   {
      startAlarmService();                                                                                                      
           if (worker.CancellationPending)
            {
                e.Cancel = true;
                return;
            }
      startTrendQualityCheck();
   }

Answer 1

正如您可能从 te cmets 中了解到的，您需要在您想要支持取消的操作中轮询 BackgroundWorker 的状态。然后采取措施优雅地取消正在进行的操作。

该示例显示了如何在单击按钮时取消后台线程。第一个示例使用旧的BackgroundWorker，第二个示例使用现代且更简洁的任务库。

后台工作人员

private BackgroundWorker Worker { get; set; }

private void StartWorker()
{
  this.Worker = new BackgroundWorker
  {
    WorkerReportsProgress = true,
    WorkerSupportsCancellation = true
  };
   
  this.Worker.DoWork += BackgroundWorker_DoWork;
}

private void BackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
  BackgroundWorker worker = sender as BackgroundWorker;

  DoCancellableWork();  

  // Stop BackgroundWorker from executing
  if (worker.CancellationPending)
  {
    e.Cancel = true;
  }     
}

private void DoCancellableWork()
{      
  // Check for cancellation before executing the cancellable operation and allocating resources etc..
  if (this.Worker.CancellationPending)
  {
    return;
  }

  // Periodically/regularly check for the cancellation flag
  for (int i = 0; i <= 10000000000; i++)
  {
    if (this.Worker.CancellationPending)
    {
      // Cancel operation gracefully e.g., do some cleanup, free resources etc.

      return;
    }

    // Do some work
  }
}

// Alternatively use a command e.g., in a view model class
private void CancelBackgroundWorker_Click(object sender, EventArgs e)
{
  if (this.Worker.WorkerSupportsCancellation)
  {
    this.Worker.CancelAsync();
  }
}

任务库

该示例使用Progress<T> 从后台线程向 UI 线程报告进度。

private CancellationTokenSource CancellationTokenSource { get; set; }

private async Task StartWorker()
{
  this.CancellationTokenSource = new CancellationTokenSource();

  // Prepare callback to update UI from the background thread.
  // The Progress<T> instance MUST be created on the UI thread
  IProgress<int> progressReporter = new Progress<int>(progress => this.ProgressBar.Value = progress);

  await Task.Run(
    () => DoWork(progressReporter, this.CancellationTokenSource.Token), 
    this.CancellationTokenSource.Token);

  this.CancellationTokenSource.Dispose();
}

private void DoWork(IProgress<int> progressReporter, CancellationToken cancellationToken)
{
  DoCancellableWork(progressReporter, cancellationToken);
}

private void DoCancellableWork(IProgress<int> progressReporter, CancellationToken cancellationToken)
{
  // Check for cancellation before executing the operation and allocating resources etc..
  if (cancellationToken.IsCancellationRequested)
  {
    return;
  }

  // Periodically/regularly check for the cancellation flag
  for (int i = 0; i <= 10000000000; i++)
  {
    if (cancellationToken.IsCancellationRequested)
    {
      // Cancel operation gracefully e.g., do some cleanup, free resources etc.

      return;
    }

    // Do some work

    // Report progress
    progressReporter.Report(20);
  }
}

// Alternatively use a command e.g., in a view model class
private void CancelBackgroundThread_Click(object sender, EventArgs e)
{
  this.CancellationtokenSource?.Cancel();
}

Answer 2

由于 OP 将正在完成的任务描述为“检查服务”，我假设完成的工作如下所示：

while(true){
    // check service
    // Post result back to UI thread
    Thread.Sleep(...);
}

这不是编写此类检查的最佳方式。在大多数使用 Thread.Sleep 的情况下，计时器会是更好的选择：

var myTimer  = new System.Timers.Timer(...);
myTimer .Elapsed += OnTimedEvent;
myTimer .AutoReset = true;
myTimer .Enabled = true;

...

private void OnTimedEvent(Object source, ElapsedEventArgs e)
{
    // check service
    // Post result back to UI thread
}

这使得停止/启动正在完成的任务的问题变得简单，只需更改计时器的启用标志即可。也可以使用计时器或同步上下文直接在 UI 线程上运行事件，如果“检查服务”只需要几毫秒，这可能是最好的解决方案。