Is this an acceptable way to test MVVM-Light Toolkit Messages?

Question

    [TestMethod()]
    public void ShowSetup_SendsMessage_WhenShowSetupCommandCalled()
    {

        //Arrange
        Messenger.Reset();
        MainViewModel target = new MainViewModel();
        bool wasCalled = false;
        Messenger.Default.Register<NotificationMessage>(this,"Settings",(msg) => wasCalled = true);

        //Act
        target.ShowSetupCommand.Execute(null);

        //Assert
        Assert.IsTrue(wasCalled);
    }

I see there is an IMessenger interface and I tried to mock it and set Messenger.OverrideDefault to the mock like this:

var mock = new Mock<IMessenger>();      
Messenger.OverrideDefault((Messenger)mock.Object);

But I got an invalid cast error. Is the OverrideDefault method not for that purpose or more likely I'm using it incorrectly.

Or would I have an interface for the classes that are receiving the messages and mock those? All I really want to test is that a RelayCommand sends a message when it is called.

Answer 1

I just started to look at this myself. I'm a little surprised that Messenger.OverrideDefault doesn't take an IMessenger as a parameter. You have to inherit Messenger.

I suppose you could create a class that internally uses your mock object and then do a Verify.

        [Test]
    public void ShowSetup_SendsMessage_WhenShowSetupCommandCalled() {
        Messenger.Reset();
        MaintenanceViewModel target = new MainViewModel();
        IMessenger mockMessenger = MockRepository.GenerateMock<IMessenger>();
        mockMessenger.Expect(m => m.Send("Settings"));
        TestMessenger testMessenger = new TestMessenger(mockMessenger);
        Messenger.OverrideDefault(testMessenger);
        bool wasCalled = false;
        Messenger.Default.Register<NotificationMessage>(this, "Settings", (msg) => wasCalled = true);
        target.ShowSetupCommand.Execute(null);

        mockMessenger.VerifyAllExpectations();
    }

You may or may not need a stub on the Register method.

The TestMessenger class:

    public class TestMessenger : Messenger {
    private IMessenger _mockMessenger;
    public TestMessenger(IMessenger mock) {
        _mockMessenger = mock;
    }
    public override void Register<TMessage>(object recipient, bool receiveDerivedMessagesToo, Action<TMessage> action) {
        _mockMessenger.Register<TMessage>(recipient, receiveDerivedMessagesToo, action);
    }

    public override void Register<TMessage>(object recipient, Action<TMessage> action) {
        _mockMessenger.Register<TMessage>(recipient, action);
    }

    public override void Send<TMessage, TTarget>(TMessage message) {
        _mockMessenger.Send<TMessage, TTarget>(message);
    }

    public override void Send<TMessage>(TMessage message) {
        _mockMessenger.Send<TMessage>(message);
    }

    public override void Unregister<TMessage>(object recipient, Action<TMessage> action) {
        _mockMessenger.Unregister<TMessage>(recipient, action);
    }

    public override void Unregister<TMessage>(object recipient) {
        _mockMessenger.Unregister<TMessage>(recipient);
    }

    public override void Unregister(object recipient) {
        _mockMessenger.Unregister(recipient);
    }
}