如何在WPF中停止后臺作業人員？

我在 WPF 應用程式中使用了 MVVM 模型。我有一個命令系結到取消按鈕。我有一個啟動按鈕，可以啟動一些后臺作業人員。當我單擊取消按鈕時，我希望所有后臺作業人員停止/退出。當我單擊取消按鈕時，使用我當前的代碼，后臺作業人員不會停止并且“StartEngineeringOperation”完成。任何人都可以幫我解決我在這里做錯了什么嗎？

當前代碼：

對于 EngineeringViewModel.cs：

公共類 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;
            }
           
        };
       
    }
}

我試過這個：但似乎不起作用：

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

uj5u.com熱心網友回復：

正如您可能從 te 評論中了解到的那樣，您需要輪詢BackgroundWorker您想要支持取消的操作的狀態。然后采取措施優雅地取消正在進行的操作。

該示例顯示了如何在按鈕單擊時取消后臺執行緒。第一個示例使用舊的BackgroundWorker，第二個示例使用現代且更干凈的 Task 庫。

后臺作業者

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();
  }
}

任務庫

該示例使用Progress<T>向 UI 執行緒報告從后臺執行緒的進度。

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();
}

uj5u.com熱心網友回復：

由于 OP 將正在完成的任務描述為“檢查服務”，我認為完成的作業如下所示：

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

這不是撰寫此類支票的最佳方式。在大多數使用 Thread.Sleep 的情況下，計時器將是更好的選擇：

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
}

這使得停止/啟動正在完成的任務的問題成為更改計時器的啟用標志的簡單問題。也可以使用計時器或同步背景關系直接在 UI 執行緒上運行事件，如果“檢查服務”只需要幾毫秒，這可能是最好的解決方案。

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