Question

I’m using Reactive Extensions for .NET (Rx) to expose events as IObservable<T>. I want to create an unit test where I assert that a particular event is fired. Here is a simplified version of the class I want to test:

public sealed class ClassUnderTest : IDisposable {

  Subject<Unit> subject = new Subject<Unit>();

  public IObservable<Unit> SomethingHappened {
    get { return this.subject.AsObservable(); }
  }

  public void DoSomething() {
    this.subject.OnNext(new Unit());
  }

  public void Dispose() {
    this.subject.OnCompleted();
  }

}

Obviously my real classes are more complex. My goal is to verify that performing some actions with the class under test leads to a sequence of events signaled on the IObservable. Luckily the classes I want to test implement IDisposable and calling OnCompleted on the subject when the object is disposed makes it much easier to test.

Here is how I test:

// Arrange
var classUnderTest = new ClassUnderTest();
var eventFired = false;
classUnderTest.SomethingHappened.Subscribe(_ => eventFired = true);

// Act
classUnderTest.DoSomething();

// Assert
Assert.IsTrue(eventFired);

Using a variable to determine if an event is fired isn’t too bad, but in more complex scenarios I may want to verify that a particular sequence of events are fired. Is that possible without simply recording the events in variables and then doing assertions on the variables? Being able to use a fluent LINQ-like syntax to make assertions on an IObservable would hopefully make the test more readable.

Answer 1

This answer has been updated to the now released version 1.0 of Rx.

Official documentation is still scant but Testing and Debugging Observable Sequences on MSDN is a good starting place.

The test class should derive from ReactiveTest in the Microsoft.Reactive.Testing namespace. The test is based around a TestScheduler that provides virtual time for the test.

The TestScheduler.Schedule method can be used to queue up activities at certain points (ticks) in virtual time. The test is executed by TestScheduler.Start. This will return an ITestableObserver<T> that can be used for asserting for instance by using the ReactiveAssert class.

public class Fixture : ReactiveTest {

  public void SomethingHappenedTest() {
    // Arrange 
    var scheduler = new TestScheduler();
    var classUnderTest = new ClassUnderTest();

    // Act 
    scheduler.Schedule(TimeSpan.FromTicks(20), () => classUnderTest.DoSomething());
    var actual = scheduler.Start(
      () => classUnderTest.SomethingHappened,
      created: 0,
      subscribed: 10,
      disposed: 100
    );

    // Assert
    var expected = new[] { OnNext(20, new Unit()) };
    ReactiveAssert.AreElementsEqual(expected, actual.Messages);
  }

}

TestScheduler.Schedule is used to schedule a call to DoSomething at time 20 (measured in ticks).

Then TestScheduler.Start is used to perform the actual test on the observable SomethingHappened. The lifetime of the subscription is controlled by the arguments to the call (again measured in ticks).

Finally ReactiveAssert.AreElementsEqual is used to verify that OnNext was called at time 20 as expected.

The test verifies that calling DoSomething immediately fires the observable SomethingHappened.