mvvm - 使用 TPL 时如何在 UI 线程上调用方法？

Question

我正在开发一个使用 TPL 在后台执行多个任务的 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(bind)到名为 Progress 的 View 模型属性，该属性由带有签名 UpdateProgress(int increment) 的 View 模型方法更新.后台任务需要调用该方法上报进度。

我使用一种方法来更新属性，因为它允许每个任务将 Progress 属性增加不同的数量。所以，如果我有两个任务，第一个任务的时间是第二个的四倍，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 View 模型方法？代码如下。谢谢你的帮助。

任务使用 Parallel.ForEach() ，在如下代码中:

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)没有抛出异常，但它没有进入主线程。从 MVVM ICommand 调用任务对象，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:由 View 或响应 View 相关事件)，这几乎总是 IMO 的情况，您可以将其添加到您的构造函数中:

// Add to class:
TaskFactory uiFactory;

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

然后，当你得到你的事件时，你可以使用它来编码它:

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

编辑:
我做了一个实用类。它是静态的，但如果您愿意，可以为它创建一个接口(interface)并使其成为非静态的:

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() );
}