Moqing MVVM Light's IMessenger - moq

Question

While trying to write some unit tests with Moq and MVVM Light's messenger class I've encountered an issue. The issue is how can you Mock<IMessenger>(); and verify that a message was received and the action processed?
What is the correct way write the test if using IMessenger? I've seen plenty of examples using Messenger.Default,
and having Messenger.Reset() in the TestInitialize method. I used IMessenger believing it would help with test-ability.
Using Moq to verify that a message is sent works as I would expect.
[TestMethod]
public void LoadData_SendsUpdateStatusEvents()
{
//Arrange
_mockMessenger.Setup(x => x.Send(It.IsAny<string>(),
It.IsAny<UpdateStatusEvent>()));
//Act
//Assert
_mockMessenger.Verify(x => x.Send(It.IsAny<string>(),
It.IsAny<UpdateStatusEvent>()), Times.Exactly(2));
}
However, if I have this registration in my constructor..
_messenger.Register<IUnitsManager>(this, OnUpdatedUnitsEvent);
And I write a unit test to check that OnUpdatedUnitsEvent runs...
[TestMethod]
public void OnUpdatedUnitsEvent_UpdateUserUnitValueOfAllMaterials()
{
//Arrange
_mockUnits.Object.SetUnitsToMetric();
_mockMessenger.Object.Send(_mockUnits.Object);
//Act
var currentItem = _materialMasterVm.MaterialModels.CurrentItem as MaterialDetailViewModel;
//Assert
Assert.AreEqual(DensitySolidSymbols.KgCm3, currentItem.Density.UserUnitType);
Assert.AreEqual(PressureSymbols.KPa, currentItem.YieldStress);
Assert.AreEqual(PressureSymbols.KPa, currentItem.YoungsModulus);
}
What is the recommended approach to get this to work using when using IMessenger and Moq?

Answer 1

Here's what I would do:
// This is a field in your test fixture
Action<IUnitsManager> _callback;
...
// In your test setup, save the delegate that is registered with the message type
mockMessenger.Setup(x => x.Register(It.IsAny<object>(), It.IsAny<Action<IUnitsManager>>()))
.Callback<object, Action<IUnitsManager>>((o, a) => _callback = a);
...
// When you want to send the message, invoke the callback
_callback(_mockUnits.Object);
However, I want to add that when sending messages in tests using the IMessenger, I do not mock it. The above is a little too much hassle to setup, and besides, this is exactly what the Messenger concrete does anyways. To put it another way, I'm either putting my faith in Moq's Setup/Callback mechanism or in the MVVM Light Messenger's Register/Send mechanism. I choose the latter because I don't need to do any extra setup in my tests to make it work.
So, when I need to test that my SUT registers for and handles messages properly, I assume the IMessenger works as expected and just use a concrete instance, i.e.:
// Create a new messenger every test
IMessenger mockMessenger = new Messenger();
// Send the message
mockMessenger.Send<IUnitsManager>(_mockUnits.Object);