为什么RelayCommand RaiseCanExecuteChanged不能在单元测试中工作？

Question

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

public RelayCommand FooCommand { get; private set; }

我在视图模型构造函数中新建它并将其指向一些私有方法:

FooCommand = new RelayCommand(Foo, CanFoo);

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

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

然后在setter中SomeRequiredProperty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工作.

Answer 1

固定!

nemesv在FooCommand.RaiseCanExecuteChanged()简单的电话中是正确的CommandManager.InvalidateRequerySuggested().

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

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

问题的原因是CommandManager类中的以下代码行:

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

    ...
}

此行在Dispatcher工作项队列中放置带有DispatcherPriority Background的工作项.该工作项应该通知所有处理程序事件.CommandManager.RequerySuggested

问题是这个工作项永远不会运行.

解决方案是强制调度程序运行工作项.

我在MVVM Foundation CodePlex页面的讨论中找到了解决方案.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:)