Unit test IObservable<T> with ObserveOnDispatcher

Question

I need to test piece of code

        var watcher = new FakeIFileSystemWatcher();
        watcher.FilesToBeImported
            .ObserveOnDispatcher()
            .Subscribe(list.Add);

so I created this little unit test but I can't make it pass cause list.Count is always 0

    [Test]
    public void Foo()
    {
        var list = new List<string>();

        var watcher = new FakeIFileSystemWatcher();
        watcher.FilesToBeImported
            .ObserveOnDispatcher()
            .Subscribe(list.Add);

        Task task = Task.Factory.StartNew(() =>
        {                
            watcher.AddFile("cc");
            watcher.AddFile("cc");
            watcher.AddFile("cc");
        }, TaskCreationOptions.LongRunning);
        Task.WaitAll(task);

        Assert.AreEqual(3, list.Count);
    }

if I comment out the method

            .ObserveOnDispatcher()

it pass but how can I get it working also with ObserveOnDispatcher() ?

Answer 1

If you use ObserveOnDispatcher you create a dependency to the "dispatcher" which means that you need a window and a message loop. To get around this problem in a unit test you can instead use the ObserveOn method that uses a scheduler and then use dependency injection to inject the correct scheduler. For unit testing you could use Scheduler.Immediate and for the actual application you could use DispatcherScheduler.Instance. Notice that there also exists a TestScheduler which is really useful for running unit tests in virtual time.

Answer 2

I solved using the class DispatcherUtil I found here Using the WPF Dispatcher in unit tests

now my code is the following

    [Test]
    public void Foo()
    {
        var list = new List<string>();

        var watcher = new FakeIFileSystemWatcher();
        watcher.FilesToBeImported
            .ObserveOnDispatcher()
            .Subscribe(list.Add);

        Task task = Task.Factory.StartNew(() =>
        {
            watcher.AddFile("cc");
            watcher.AddFile("cc");
            watcher.AddFile("cc");
            watcher.AddFile("cc");
        }, TaskCreationOptions.LongRunning);
        Task.WaitAll(task);
        DispatcherUtil.DoEvents();
        Assert.AreEqual(4, list.Count);
    }

and it works like a charm