如何在使用TPL时在UI线程上调用方法？

Question

我正在使用TPL在后台执行几个任务的MVVM应用程序。这些任务需要向UI报告进度，以便可以更新进度对话框。由于该应用程序是MVVM，因此进度对话框将绑定到名为Progress的视图模型属性，该属性由具有签名UpdateProgress(int increment)的视图模型方法进行更新。后台任务需要调用此方法来报告进度。

我使用一种方法来更新属性，因为它允许每个任务以不同的数量递增Progress属性。所以，如果我有两个任务，第一个任务需要第二个任务的四倍，第一个任务调用UpdateProgress(4)，第二个任务调用UpdateProgress(1)。因此，第一项任务完成时进度为80％，第二项任务完成时为100％。

我的问题真的很简单：如何从我的后台任务调用视图模型方法？代码如下。谢谢你的帮助。

---

的任务使用Parallel.ForEach()，代码看起来像这样：

private void ResequenceFiles(IEnumerable<string> fileList, ProgressDialogViewModel viewModel) 
{ 
    // Wrap token source in a Parallel Options object 
    var loopOptions = new ParallelOptions(); 
    loopOptions.CancellationToken = viewModel.TokenSource.Token; 

    // Process images in parallel 
    try 
    { 
     Parallel.ForEach(fileList, loopOptions, sourcePath => 
     { 
      var fileName = Path.GetFileName(sourcePath); 
      if (fileName == null) throw new ArgumentException("File list contains a bad file path."); 
      var destPath = Path.Combine(m_ViewModel.DestFolder, fileName); 
      SetImageTimeAttributes(sourcePath, destPath); 

      // This statement isn't working 
      viewModel.IncrementProgressCounter(1); 
     }); 
    } 
    catch (OperationCanceledException) 
    { 
     viewModel.ProgressMessage = "Image processing cancelled."; 
    } 
} 

声明viewModel.IncrementProgressCounter(1)不是抛出一个异常，但它不是通过主线程获得。该任务从MVVM ICommand对象调用，代码如下所示：

public void Execute(object parameter) 
{ 
    ... 

    // Background Task #2: Resequence files 
    var secondTask = firstTask.ContinueWith(t => this.ResequenceFiles(fileList, progressDialogViewModel)); 

    ... 
} 

Answer 1

马歇尔方法调用，您可以使用Dispatcher的方法的InvokeMethod主UI线程。如果您使用像Carliburn这样的MVVM框架，它对Dispatcher具有抽象，所以您可以使用Execute.OnUIThread（Action）执行几乎相同的操作。

检查this关于如何使用分派器的Microsoft文章。

Answer 2

假设你的视图模型构建UI线程（即：经查看，或在响应于查看相关的事件），这是这种情况几乎总是IMO，你可以将它添加到你的构造：

// Add to class: 
TaskFactory uiFactory; 

public MyViewModel() 
{ 
    // Construct a TaskFactory that uses the UI thread's context 
    uiFactory = new TaskFactory(TaskScheduler.FromCurrentSynchronizationContext()); 
} 

然后，当你得到你的活动，你可以用它来名帅它：

void Something() 
{ 
    uiFactory.StartNew(() => DoSomething()); 
} 

编辑： 我做了一个UTIL类。它是静态的，但如果你愿意，你可以为它创建一个接口，并使其非静态：

public static class UiDispatcher 
{ 
    private static SynchronizationContext UiContext { get; set; } 

    /// <summary> 
    /// This method should be called once on the UI thread to ensure that 
    /// the <see cref="UiContext" /> property is initialized. 
    /// <para>In a Silverlight application, call this method in the 
    /// Application_Startup event handler, after the MainPage is constructed.</para> 
    /// <para>In WPF, call this method on the static App() constructor.</para> 
    /// </summary> 
    public static void Initialize() 
    { 
     if (UiContext == null) 
     { 
      UiContext = SynchronizationContext.Current; 
     } 
    } 

    /// <summary> 
    /// Invokes an action asynchronously on the UI thread. 
    /// </summary> 
    /// <param name="action">The action that must be executed.</param> 
    public static void InvokeAsync(Action action) 
    { 
     CheckInitialization(); 

     UiContext.Post(x => action(), null); 
    } 

    /// <summary> 
    /// Executes an action on the UI thread. If this method is called 
    /// from the UI thread, the action is executed immendiately. If the 
    /// method is called from another thread, the action will be enqueued 
    /// on the UI thread's dispatcher and executed asynchronously. 
    /// <para>For additional operations on the UI thread, you can get a 
    /// reference to the UI thread's context thanks to the property 
    /// <see cref="UiContext" /></para>. 
    /// </summary> 
    /// <param name="action">The action that will be executed on the UI 
    /// thread.</param> 
    public static void Invoke(Action action) 
    { 
     CheckInitialization(); 

     if (UiContext == SynchronizationContext.Current) 
     { 
      action(); 
     } 
     else 
     { 
      InvokeAsync(action); 
     } 
    } 

    private static void CheckInitialization() 
    { 
     if (UiContext == null) throw new InvalidOperationException("UiDispatcher is not initialized. Invoke Initialize() first."); 
    } 
} 

用法：

void Something() 
{ 
    UiDispatcher.Invoke(() => DoSomething()); 
}