How to verify the EventAggregator's unsubscribe method is called when disposing a ViewModel with Prism

I'm struggling to write a test that confirms that I am correctly unsubscribing from an EventAggregator's message when it is closed. Anyone able to point out the (simple) answer?!

Here is the code:

public class ViewModel : BaseViewModel, IViewModel
{
    private readonly IEventAggregator eventAggregator;
    private SubscriptionToken token;
    IssuerSelectedEvent issuerSelectedEvent;

    public ViewModel(IView view, IEventAggregator eventAggregator)
    {
        this.eventAggregator = eventAggregator;
        View = view;        

        issuerSelectedEvent = eventAggregator.GetEvent<IssuerSelectedEvent>();
        token = issuerSelectedEvent.Subscribe(SelectedIssuerChanged, true);            
    }

    private void SelectedIssuerChanged(IssuerSelectedCommand obj)
    {
        Console.WriteLine(obj);
    }

    public IView View { get; set; }

    public ove开发者_运维技巧rride void Dispose()
    {
        issuerSelectedEvent.Unsubscribe(token);
    }
}

The test fails with:

Moq.MockVerificationException : The following setups were not matched: IssuerSelectedEvent x => x.Unsubscribe(It.IsAny())

Here is the test:

[Test]
public void UnsubscribeFromEventAggregatorOnDispose()
{            
    var view = new Mock<ICdsView>();
    var ea = new Mock<EventAggregator>();
    var evnt = new Mock<IssuerSelectedEvent>();

    evnt.Setup(x => x.Unsubscribe(It.IsAny<SubscriptionToken>()));

    var vm = new CdsIssuerScreenViewModel(view.Object, ea.Object);

    vm.Dispose();

    evnt.VerifyAll();
}

Here I am verifying that the Unsubscribe was called on the mocked IssuerSelectedEvent

[Test]
public void UnsubscribeFromEventAggregatorOnDispose()
{            
    var view = new Mock<ICdsView>();
    var ea = new Mock<IEventAggregator>();
    var evnt = new Mock<IssuerSelectedEvent>();

    ea.Setup(x => x.GetEvent<IssuerSelectedEvent>()).Returns(evnt.Object);

    var vm = new CdsIssuerScreenViewModel(view.Object, ea.Object);

    vm.Dispose();

    evnt.Verify(x => x.Unsubscribe(It.IsAny<SubscriptionToken>());
}

If you want to check that the exact same token is passed into the Unsubscribe then you will need a Setup for the Subscribe method that returns a token you create in your test.

You need to tell your EventAggregator mock to return your mocked IssuerSelectedEvent:

ea.Setup(x => x.GetEvent<IssuerSelectedEvent>()).Return(evnt.Object);

The tests needs to be changed to:

[Test]
public void UnsubscribeFromEventAggregatorOnDispose()
{            
    var view = new Mock<ICdsView>();
    var ea = new Mock<IEventAggregator>();
    var evnt = new Mock<IssuerSelectedEvent>();

    ea.Setup(x => x.GetEvent<IssuerSelectedEvent>()).Returns(evnt.Object);
    evnt.Setup(x => x.Unsubscribe(It.IsAny<SubscriptionToken>()));

    var vm = new CdsIssuerScreenViewModel(view.Object, ea.Object);

    vm.Dispose();

    evnt.VerifyAll();
}