Akka BehaviorTestKit - Check for anonymous actor spawned of specific type, ignoring order

Question

I'm trying to uss the Akka BehaviorTestKit to verify that an anonymous actor of a specific type is spawned.

I tried to use:

testKit.expectEffectType[SpawnedAnonymous[MyActor.Request]]

When I did this, I got an AssertionError because there were other effects that came before the the one I'm testing for. It seems that expectEffectType() only looks at the effects in the order that they occurred. I want to ignore any other previous effects and only care that the one I'm testing for happened at some point. Relying on specific other prior effects would make the test brittle.

So, I then switched gears and tried the following:

val effects: Seq[Effect] = testKit.retrieveAllEffects()
assert(!effects.filter(effect => effect.isInstanceOf[SpawnedAnonymous[Behavior[MyActor.Request]]]).isEmpty)

This turned out to not be a valid test. It always succeeds, regardless of the Behavior type that I check for, due to JVM generic type erasure.

How can I verify that an anonymous actor of a specific type was spawned at any point of time?

Answer 1

Yes, it's problematic. You have 2 choices.

val spawned = effects.find{case _: SpawnedAnonymous[_] => true ...}
spawned.ref ! someMessageThatWillHaveAKnownEffect

Or... as it says in https://doc.akka.io/docs/akka/current/typed/testing-sync.html

Interactions with other actors must be stubbed.

It's not explicit, but part of the meaning to me is that you can't leave behaviors be defined by the actor under test. So instead of ctx.spawn(Behaviors.supervise[MyActor.Request](Behavior(args)).onFailure(...)), I'll instead construct MyActor with an

trait MyActorConstructors {
   def requestHandler(args): Behavior[MyActor.Request]
}

and then call ctx.spawn(constructors.requestHandler(args))

When you construct MyActor you pass in the standard implementations, and from your test you pass in mock actors. That way you can easily determine effects.contains(SpawnedAnonymous(myMockedRequestHandler))

It's a bit of a hassle to set up, but it allows you to explicitly perform synchronous behavior testing (which is what BehaviorTestKit is primarily for), and also gives you a simplified testing point for the standard behaviour implementations in isolation. IdiomaticMockito (from mockito-scala) makes it nicer to deal with as well.