How to write unit tests that use MVVM Light's Messenger?

Question

I'm working on writing some unit tests for a WPF app that uses MVVM Light's Messenger. We've got several methods that look like similar to this:

private void ExecuteViewTemplatesCommand()
{
    OpenViewMessage message = new OpenViewMessage();
    CurrentViewModel = message.ViewModel = ViewModelLocator.TemplateVM;
    Messenger.Default.Send<OpenViewMessage>(message);
}

I figure I've got to write unit tests which work against the side effects that running a method that is of type void will do, like in this case assign the message to a property of the Messenger object. How do I unit tests for this situation, please?

Answer 1

In order to write unit test for mvvm light you will need to first register the message, and know when to send the message Let's say you have 2 classes in your viewmodel that is not tightly couple, we'll call them classA and classB. Here is the process that can be use to register and send the message.

ClassA //register the message
{
          MessengerInstance.Register<//in here you can put the class that you want to register, for the sake to simplify the example, we'll introduce this as classC>(this, DelegateFunctionHanlder);
}

private void DelegateFunctionHanlder(classC message)
{
    //in here, we'll do something
}

Then you'll need to create the classC as a messageclass, this will act as mediator class, which is a messenger

namespace something
{
    public class ClassC
    {
        //it can be blank
    }
}

Then create a ClassB

ClassB
{
    classC _classCMessage = new classC();
    MessengerInstace.Send(_classCMessage);

}

The point here is that all of this prevent ClassA directly talk to ClassB, the reason being is that we try to avoid any type of dependency, assume classB is a WPF framework, we will try to avoid to depend on that in case one day you decide to switch to different framework