c# - 为什么 RelayCommand RaiseCanExecuteChanged 在单元测试中不起作用？

Question

我正在使用 Nuget (4.1.23.0) 上可用的当前版本的 MvvmLight，调用 RaiseCanExecuteChanged 似乎在单元测试中没有做任何事情。场景很简单，我有一个命令:

public RelayCommand FooCommand { get; private set; }

我在 View 模型构造函数中新建它并将它指向一些私有(private)方法:

FooCommand = new RelayCommand(Foo, CanFoo);

private void Foo()
{
    // do some fooing.
}

private bool CanFoo()
{
    return SomeRequiredProperty != null;
}

然后在 SomeRequiredProperty 的 setter 中我调用 RaiseCanExecuteChanged:

public object SomeRequiredProperty
{
    get
    {
        return someRequiredProperty;
    }

    set
    {
        someRequiredProperty = value;
        FooCommand.RaiseCanExecuteChanged();
    }
}

现在在单元测试中，我执行以下操作:

// Arrange
var canExecuteChanged = false;
viewModel.FooCommand.CanExecuteChanged += (sender, args) => canExecuteChanged = true;

// Act
viewModel.SomeRequiredProperty = new object();

// Assert
Assert.That(canExecuteChanged, Is.True);

测试失败，因为我的事件处理程序没有触发。这是为什么？

更新:该行为确实在运行时有效。

Answer 1

已修复!

nemesv是正确的，因为 FooCommand.RaiseCanExecuteChanged() 只是调用 CommandManager.InvalidateRequerySuggested()。

除此之外，FooCommand.CanExecuteChanged 只是将处理程序转发给 CommandManager.RequerySuggested 事件:

public event EventHandler CanExecuteChanged
{
    add
    {
        ...
        CommandManager.RequerySuggested += value;
    }
    ... 
}

问题的原因是 CommandManager 中的以下代码行类:

private void RaiseRequerySuggested()
{
    ...
    _requerySuggestedOperation = dispatcher.
        BeginInvoke(
            DispatcherPriority.Background,
            new DispatcherOperationCallback(RaiseRequerySuggested),
            null); // dispatcher is the Dispatcher for the current thread.

    ...
}

此行放置一个 DispatcherPriority 的工作项背景 关于 Dispatcher工作项队列。工作项应该通知 CommandManager.RequerySuggested 事件的所有处理程序。

问题 是此工作项从未运行过。

解决方案是强制调度员运行工作项。

我在 this discussion 中找到了解决方案在 MVVM 基金会 CodePlex 页面上。我设法将代码简化为以下帮助程序类。

public static class DispatcherTestHelper
{
    private static DispatcherOperationCallback exitFrameCallback = ExitFrame;

    /// <summary>
    /// Synchronously processes all work items in the current dispatcher queue.
    /// </summary>
    /// <param name="minimumPriority">
    /// The minimum priority. 
    /// All work items of equal or higher priority will be processed.
    /// </param>
    public static void ProcessWorkItems(DispatcherPriority minimumPriority)
    {
        var frame = new DispatcherFrame();

        // Queue a work item.
        Dispatcher.CurrentDispatcher.BeginInvoke(
            minimumPriority, exitFrameCallback, frame);

        // Force the work item to run.
        // All queued work items of equal or higher priority will be run first. 
        Dispatcher.PushFrame(frame);
    }

    private static object ExitFrame(object state)
    {
        var frame = (DispatcherFrame)state;

        // Stops processing of work items, causing PushFrame to return.
        frame.Continue = false;
        return null;
    }
}

我的测试现在看起来像这样:

// Arrange
var canExecuteChanged = false;
viewModel.FooCommand.CanExecuteChanged += 
    (sender, args) => canExecuteChanged = true;

// Act
viewModel.SomeRequiredProperty = new object();
DispatcherTestHelper.ProcessWorkItems(DispatcherPriority.Background);

// Assert
Assert.That(canExecuteChanged, Is.True);

而且，最重要的是，它通过了 :)