Question

我正在开发一个使用TPL在后台执行多项任务的MVVM应用程序。任务需要向UI报告进度，以便可以更新进度对话框。由于应用程序是MVVM，因此进度对话框绑定到名为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量增加Progress属性。因此，如果我有两个任务，第一个任务需要四倍，第一个任务调用UpdateProgress(4)，第二个任务调用UpdateProgress(1)。因此，第一个任务完成时进度为80％，第二个任务完成时进度为100％。

我的问题非常简单：如何从后台任务中调用视图模型方法？代码如下。谢谢你的帮助。

任务使用Parallel.ForEach()，代码如下：

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)不会抛出异常，但它没有通过主线程。这些任务是从MVVM ICommand对象调用的，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

假设您的ViewModel是在UI线程上构建的（即：通过View，或者响应View相关事件），这种情况几乎总是IMO，您可以将它添加到构造函数中：

// Add to class:
TaskFactory uiFactory;

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

然后，当你收到你的活动时，可以用它来整理它：

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

修改我做了一个util类。它是静态的，但如果你想要，你可以为它创建一个接口并使其成为非静态接口：

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() ); }

Answer 2

要对主UI线程进行方法调用，可以使用Dispatcher的InvokeMethod方法。如果您使用像Carliburn这样的MVVM框架，它会对Dispatcher进行抽象，因此您可以使用Execute.OnUIThread（Action）执行几乎相同的操作。

检查this Microsoft有关如何使用Dispatcher的文章。

使用TPL时如何在UI线程上调用方法？

2 个答案: