Why isn't my EventAggregator Subscription Handling This Event? - unit-testing

Question

I have an Autofac DI Container defined as follows:
public class Bootstrapper
{
public IContainer BootStrap()
{
var builder = new ContainerBuilder();
builder.RegisterType<ItemViewModel>().AsSelf();
builder.RegisterType<EventAggregator>()
.As<IEventAggregator>()
.SingleInstance();
}
}
I have a Unit Test defined to test whether a deletion removes the deleted item from the collection:
[Fact]
public void Should_remove_item_from_collection_when_item_is_deleted()
{
const int deletedId = 42;
// adds three items to the collection
_openItemEditViewEvent.Publish(deletedId);
_openItemEditViewEvent.Publish(8);
_openItemEditViewEvent.Publish(9);
// I've tried this:
_eventAggregatorMock.Object.GetEvent<ItemDeletedEvent>().Publish(42);
// and alternatively, this (not at the same time):
_itemDeletedEventMock.Object.Publish(42);
Assert.Equal(2,_vm.ItemEditViewModels.Count); // always fails
Assert.False(_vm.ItemEditViewModels
.Select(vm => vm.Item.Id).Contains(42), "Wrong item deleted");
}
The constructor of the Unit Test initializes and assigns the EventAggregator to the view model:
_eventAggregatorMock = new Mock<IEventAggregator>();
_itemDeletedEventMock = new Mock<ItemDeletedEvent>();
_eventAggregatorMock.Setup(ea => ea.GetEvent<ItemDeletedEvent>())
.Returns(_itemDeletedEventMock.Object);
_vm = new ItemViewModel(_eventAggregatorMock.Object, */ ... /*);
In my actual view model, I Subscribe to the event:
public ItemViewModel(IEventAggregator ea, /* ... */)
{
_eventAggregator.GetEvent<ItemDeletedEvent>()
.Subscribe(OnItemDeleted, true);
}
And we never hit a breakpoint here:
public void OnItemDeleted()
{
// never happens
}
For the life of me, I can't figure out what I'm doing wrong - I'm overlooking something... do I have to Setup the event's Publish event in the Mock? Should I be using a real ItemDeletedEvent instance instead of a Mock? Any help would be greatly appreciated.

Answer 1

=> Hi Scott,
there are 2 ViewModel-scenarios you want to test when using an EventAggregator:
You want to test that your ViewModel is publishing an event
You want to test that your ViewModel does something when an event was published. So the ViewModel has to subscribe to that Event to do something
(Note: The following lines are true for PRISM's EventAggregator, which is the one you're using I guess. For other EventAggregators it could be different)
For the first scenario, you have to create a mock for the event. Then you can verify on that mock-instance that the Publish-method of the Event has been called.
For the second scenario, which is the scenario you have in your question, you have to use the real event in your test. Why?
When you call the Publish-method on a event-mock, that Publish method won't call the subscribers to that Event, as there's no logic behind the Subscribe-method. For sure you could setup both methods and implement that publish/subscribe-logic in your mock. But there's no reason to do so, just use the real Event
When you use the real event, the Publish-method will call all the subscribers. And this is exactly What you need in your test.
It should look like this:
_itemDeletedEvent = new ItemDeletedEvent();
_eventAggregatorMock.Setup(ea => ea.GetEvent<ItemDeletedEvent>())
.Returns(_itemDeletedEvent);
Now your ViewModel will get this itemDeletedEvent-instance from the EventAggregator. In your test you call the Publish-method on this itemDeletedEvent-instance and it will work.
More about this is explained in my Course on Pluralsight about WPF and Test Driven Development: http://www.pluralsight.com/courses/wpf-mvvm-test-driven-development-viewmodels
Thomas
http://www.thomasclaudiushuber.com