One of the component that helps the ViewModels to communicate with other ViewModels is the MessageBroker (AKA EventAggregator, AKA Mediator): a manager class that route the messages from the ViewModels.

One good implementation of the Mediator for the MVVM is this http://sachabarber.net/?p=477:

To make it more testable we extract an interface:

public interface IMessageBroker
{
  void NotifyColleagues(MessageType messageType);
  void Register<T>(Action<T> action, MessageType messageType);
}

If you TDD your application or if you write unit test you could need to mock the IMessageBroker and it’s not immediate because the Action<T>.

Alessandro wrote a solution that is interesting using Moq:

[Fact]
public void Sample_Test()
{
  Mock<IMessageBroker> broker = new Mock<IMessageBroker>();
  Action<String> action = null;

  broker
    .Setup(b => b.Subscribe(It.IsAny<Action<String>>()))
    .Callback<Action<String>>(a => action = a);

  new FakeViewModel(broker.Object);

  action.Invoke("TestMessage");

  // Asserts...
}

Moq is a powerful mock framework and it’s simplicity make the elegant and easy to read!