How do I test Prism event aggregator subscriptions, on the UIThread?

Question

I have a class, that subscribes to an event via PRISMs event aggregator.

As it is somewhat hard to mock the event aggregator as noted here, I just instantiate a real

Answer 1

You may not like this as it may involve what you feel is an "ugly hack", but my preference IS to use a real EventAggregator rather than mocking everything. While ostensibly an external resource, the EventAggregator runs in memory and so does not require much set-up, clear down, and is not a bottle neck like other external resources such as databases, web-services, etcetera would be and therefore I feel it is appropriate to use in a unit test. On that basis I have used this method to overcome the UI thread issue in NUnit with minimal change or risk to my production code for the sake of the tests.

Firstly I created an extension method like so:

public static class ThreadingExtensions
{
    private static ThreadOption? _uiOverride;

    public static ThreadOption UiOverride
    {
        set { _uiOverride = value; }
    }

    public static ThreadOption MakeSafe(this ThreadOption option)
    {
        if (option == ThreadOption.UIThread && _uiOverride != null)
            return (ThreadOption) _uiOverride;

        return option;
    }

}

Then, in all my event subscriptions I use the following:

EventAggregator.GetEvent().Subscribe
(
    x => // do stuff, 
    ThreadOption.UiThread.MakeSafe()
);

In production code, this just works seamlessly. For testing purposes, all I have to do is add this in my set-up with a bit of synchronisation code in my test:

[TestFixture]
public class ExampleTest
{
    [SetUp]
    public void SetUp()
    {
        ThreadingExtensions.UiOverride = ThreadOption.Background;
    }

    [Test]
    public void EventTest()
    {
        // This doesn't actually test anything useful.  For a real test
        // use something like a view model which subscribes to the event
        // and perform your assertion on it after the event is published.
        string result = null;
        object locker = new object();
        EventAggregator aggregator = new EventAggregator();

        // For this example, MyEvent inherits from CompositePresentationEvent
        MyEvent myEvent = aggregator.GetEvent();

        // Subscribe to the event in the test to cause the monitor to pulse,
        // releasing the wait when the event actually is raised in the background
        // thread.
        aggregator.Subscribe
        (
            x => 
            {
                result = x;
                lock(locker) { Monitor.Pulse(locker); }
            },
            ThreadOption.UIThread.MakeSafe()
        );

        // Publish the event for testing
        myEvent.Publish("Testing");

        // Cause the monitor to wait for a pulse, but time-out after
        // 1000 millisconds.
        lock(locker) { Monitor.Wait(locker, 1000); }

        // Once pulsed (or timed-out) perform your assertions in the real world
        // your assertions would be against the object your are testing is
        // subscribed.
        Assert.That(result, Is.EqualTo("Testing"));
    }
}

To make the waiting and pulsing more succinct I have also added the following extension methods to ThreadingExtensions:

    public static void Wait(this object locker, int millisecondTimeout)
    {
        lock (locker)
        {
            Monitor.Wait(locker);
        }
    }

    public static void Pulse(this object locker)
    {
        lock (locker)
        {
            Monitor.Pulse(locker);
        }
    }

Then I can do:

// 
aggregator.Subscribe(x => locker.Pulse(), ThreadOption.UIThread.MakeSafe());

myEvent.Publish("Testing");

locker.Wait(1000);
// 

Again, if your sensibilities mean you want to use mocks, go for it. If you'd rather use the real thing, this works.