Unit testing with Moq, Prism 6, and Event Aggregation - unit-testing

Question

I want to unit test a module by throwing messages at it via Event Aggregation to make sure it responds appropriately, either by setting properties appropriately, or by publishing other messages as a result. I am using Prism 6.
In my project, the infrastructure project has:
public class ImportantMessage : PubSubEvent<string>
{
}
ModuleA publishes a message like this:
eventAggregator.GetEvent<ImportantMessage>().Publish(importantString);
ModuleB receives the message like this:
eventAggregator.GetEvent<ImportantMessage>().Subscribe(HandleImportantMessage);
Here is HandleImportantMessage:
public void HandleImportantMessage(string importantString)
{
. . .
}
The ModuleB constructor is called as follows:
ModuleB(IEventAggregator EventAggregator)
This constructor is called by the Prism framework. For unit testing, I need to create an instance of ModuleB, and pass an IEventAggregator, probably a fake one created by Moq. And I want to do this in such a way that the message I publish carries importantString with it.
If I Google the phrase “unit tests with moq and event aggregation,” there are
several references, but I didn’t see how to use any of these approaches to pass “importantString” from ModuleA To ModuleB. The sample code for Prism 5 creates a fake event aggregator, but without using Moq. I don't understand how it works, and don't see how to pass a string with it.
My test code starts off something like this:
var moqEventAggregator = new Mock(IEventAggregator);
var moqImportantMessage = new Mock<ImportantMessage>();
moqEventAggregator.Setup(x => x.GetEvent<ImportantMessage>());
Some of the references I have seen apply something like .Returns(eventBeingListenedTo.Object);
to moqEventAggregator after Setup is applied.
I obviously need to apply .Setup(something) to moqImportantMessage in order to pass importantString, but I don't see exactly what yet.
What am I missing? How do I pass a string with the fake published message?

Answer 1

Basically you need to mock 2 things here:
The event aggregator
The event itself
Given you have the mock of the event you need to do as you said:
moqEventAggregator.Setup(x => x.GetEvent<ImportantMessage>()).Returns(moqImportantMessage);
Mocking the event itself should be like so:
Action<string> action;
moqImportantMessage.Setup(_ => _.Subscribe(It.IsAny<Action<string>>>()))
.Callback(_action =>
{
action = _action;
});
And then you can raise the subscription like so:
action("some string");