js test driver - Page 547

Rx Reactive extensions: Unit testing with FromAsyncPattern

- by Andrew Anderson

The Reactive Extensions have a sexy little hook to simplify calling async methods: var func = Observable.FromAsyncPattern<InType, OutType>( myWcfService.BeginDoStuff, myWcfService.EndDoStuff); func(inData).ObserveOnDispatcher().Subscribe(x => Foo(x)); I am using this in an WPF project, and it works great at runtime. Unfortunately, when trying to unit test methods that use this technique I am experiencing random failures. ~3 out of every five executions of a test that contain this code fails. Here is a sample test (implemented using a Rhino/unity auto-mocking container): [TestMethod()] public void SomeTest() { // arrange var container = GetAutoMockingContainer(); container.Resolve<IMyWcfServiceClient>() .Expect(x => x.BeginDoStuff(null, null, null)) .IgnoreArguments() .Do( new Func<Specification, AsyncCallback, object, IAsyncResult>((inData, asyncCallback, state) => { return new CompletedAsyncResult(asyncCallback, state); })); container.Resolve<IRepositoryServiceClient>() .Expect(x => x.EndRetrieveAttributeDefinitionsForSorting(null)) .IgnoreArguments() .Do( new Func<IAsyncResult, OutData>((ar) => { return someMockData; })); // act var target = CreateTestSubject(container); target.DoMethodThatInvokesService(); // Run the dispatcher for everything over background priority Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Background, new Action(() => { })); // assert Assert.IsTrue(my operation ran as expected); } The problem that I see is that the code that I specified to run when the async action completed (in this case, Foo(x)), is never called. I can verify this by setting breakpoints in Foo and observing that they are never reached. Further, I can force a long delay after calling DoMethodThatInvokesService (which kicks off the async call), and the code is still never run. I do know that the lines of code invoking the Rx framework were called. Other things I've tried: I have attempted to modify the second last line according to the suggestions here: Reactive Extensions Rx - unit testing something with ObserveOnDispatcher No love. I have added .Take(1) to the Rx code as follows: func(inData).ObserveOnDispatcher().Take(1).Subscribe(x = Foo(x)); This improved my failure rate to something like 1 in 5, but they still occurred. I have rewritten the Rx code to use the plain jane Async pattern. This works, however my developer ego really would love to use Rx instead of boring old begin/end. In the end I do have a work around in hand (i.e. don't use Rx), however I feel that it is not ideal. If anyone has ran into this problem in the past and found a solution, I'd dearly love to hear it.

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