NSubstitute vs PRISM EventAggregator: Assert that calling a method triggers event with correct payload

Question

Consider the below method that updates a person and publishes an event through the PRISM EventAggregator to indicate that the person has been updated.

I would like t

Answer 1

Just add a subscriber to your unit test and check the result of the delegate. See Prism's IEventAggregator unit tests as an example

https://github.com/PrismLibrary/Prism/blob/master/Source/Prism.Tests/Events/PubSubEventFixture.cs#L14

Answer 2

I'm not an experienced unit tester so I'm not sure this is the correct unit test to write. But anyway, this is how I tested it so far and it seems to do what I want.

[Test]
public void TestPersonUpdateSendsEventWithCorrectPayload()
{
    // ARRANGE
    PersonUpdatedEventArgs payload = null;
    _eventAggregator
        .GetEvent<PersonUpdatedEvent>()
        .Subscribe(message => payload = message);

    // ACT
    _personService.UpdatePerson(5);

    // ASSERT
    payload.Should().NotBeNull();
    payload.Id.Should().Be(5);
}

Feedback welcome.

Answer 3

You can actually substitute away the event aggregator like this:

public class ToBeTested
{
    private readonly IEventAggregator _eventAggregator;

    public ToBeTested( IEventAggregator eventAggregator )
    {
        _eventAggregator = eventAggregator;
    }

    public void DoStuff()
    {
        _eventAggregator.GetEvent<OneEvent>().Publish( "test" );
    }
}

public class OneEvent : PubSubEvent<string>
{
}

[TestFixture]
internal class Test
{
    [Test]
    public void DoStuffTest()
    {
        var myEvent = new MyOneEvent();
        var myEventAggregator = Substitute.For<IEventAggregator>();
        myEventAggregator.GetEvent<OneEvent>().Returns( myEvent );

        var toBeTested = new ToBeTested( myEventAggregator );
        toBeTested.DoStuff();

        Assert.That( myEvent.ReceivedPayload, Is.EqualTo( "test" ) );
    }

    private class MyOneEvent : OneEvent
    {
        public override void Publish( string payload )
        {
            ReceivedPayload = payload;
        }

        public string ReceivedPayload
        {
            get;
            private set;
        }
    }
}

Note the fake event class that's foisted on the subject under test to get access to the payload.