Question

是否可以编写一个调用Messenger.Default.Register方法的单元测试，然后编写一个Assertion以供Action使用？

我想在我的一个命令上调用Execute之后确定我的ViewModel是否正在发送正确的消息。

我曾尝试将Assert.AreEqual编写为Action，但这似乎并没有正常工作。

Answer 1

听起来像mocking的工作！假设您正在将messenger接口传递给您的viewmodel（因为dependency inversion is a Good Thing，出于这个原因），如果我理解正确，您的代码应该看起来像这样的：

public class YourViewModel
{
    readonly IMessenger messenger;

    public YourViewModel(IMessenger messenger)
    {
        this.messenger = messenger;
        // setup of your delegate command to call Execute
    }

    void Execute(object parameter)
    {
        messenger.Send(new YourMessageType());
    }
}

然后在您的单元测试中，您将模拟信使并验证是否调用了正确的方法，在本例中为Send。因此，使用流行的模拟框架Moq：

public class YourViewModelTests
{
    [Test]
    public void Execute_Always_SendsYourMessageType()
    {
        // arrange
        var mockRepository = new MockRepository(MockBehavior.Loose);
        var mockMessenger = mockRepository.Create<IMessenger>();
        var systemUnderTest = new YourViewModel(mockMessenger.Object);

        // act
        systemUnderTest.YourCommand.Execute(null);

        // assert
        mockMessenger.Verify(p => p.Send<YourMessageType>(
                          It.Is(m => /* return true if it's the right message */)));
    }
}

通常我会将所有“安排”阶段转移到测试设置方法中，但你应该明白这一点。

如果您仍然想在不嘲笑信使的情况下这样做并使用Messenger.Default，您可以执行以下操作：

public class YourViewModelTests
{
    [Test]
    public void Execute_Always_SendsYourMessageType()
    {
        // arrange
        var systemUnderTest = new YourViewModel();

        // Set the action to store the message that was sent
        YourMessageType actual;
        Messenger.Default.Register<YourMessageType>(this, t => actual = t);


        // act
        systemUnderTest.YourCommand.Execute(null);


        // assert
        YourMessageType expected = /* set up your expected message */;
        Assert.That(actual, Is.EqualTo(expected));
    }
}

Answer 2

Alternatively, for each test it is possible to create a separate copy of the Messenger. For the runtime you want to use the Default instance of the Messenger, but for Unit Tests, as I said, create a separate copy for each test:

        return new GalaSoft.MvvmLight.Messaging.Messenger(); // Unit Tests

        return GalaSoft.MvvmLight.Messaging.Messenger.Default; // Runtime

Otherwise one might end up re-inventing the wheel, since in more complex situations where there is a need to test ViewModel communications, you will have to manage Messenger subscribers, message types an so on. Then probably writing unit tests for the messenger mock making sure it works in the same way as the original messenger. There is nothing in the engine of the Messenger that should be different when comparing Runtime and Test executions.

So for testing a factory returns the same instance of the Messenger. Test method subscribes and waits, ViewModel publishes; then Test accepts and exits. Otherwise Test times out and reports an error. I found this approach more "closer to reality" than mocking the messenger and verifying through the mock that the method was called.

单元测试MVVMLight Messenger

2 个答案: