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.
