Akka.Net test Child actor creation/supervision with TestKit

Question

I have Akka.Net code similar to the following and I am trying to write tests for it:

public class DoesSomethingActor : UntypedActor
{
    protected override void OnReceive(object message)
    {

    }
}

public class ForwardsMessagesActor : UntypedActor
{
    protected override void OnReceive(object message)
    {
        var actor = Context.ActorOf(Context.DI().Props<DoesSomethingActor>(), "DoesSomethingWorker");

        for (int i = 0; i < 5; i++)
        {
            actor.Tell(message + " " + i);
        }
    }
}

I have got this test working but I am clearly missing something since I am not using much of TestKit at all. Is there still no official documentation for how to test this using TestKit ?

//creating actor mocks with Moq seems to confuse Akka - it just doesn't work 
//but creating mock classes manually like this, 
//then configuring them in the DI container works
public class DoesSomethingActorSpy : DoesSomethingActor
{
    public static List<object> ReceivedMessages = new List<object>();

    protected override void OnReceive(object message)
    {
        ReceivedMessages.Add(message);
    }
}

    [TestMethod]
    public void ForwardsMessagesActor_Creates5Messages()
    {
        //set up DI container to use DoesSomethingActorSpy as a child actor
        ContainerBuilder builder = new ContainerBuilder();
        builder.RegisterType<ForwardsMessagesActor>();
        builder.RegisterType<DoesSomethingActorSpy>().As<DoesSomethingActor>();
        IContainer container = builder.Build();

        var propsResolver = new AutoFacDependencyResolver(container, Sys);

        var actor = ActorOfAsTestActorRef<ForwardsMessagesActor>(propsResolver.Create<ForwardsMessagesActor>());

        actor.Tell("Test");

        //this looks wrong, I probably should be using something from TestKit
        Thread.Sleep(10);

        CollectionAssert.AreEquivalent(
            new[] { "Test 0", "Test 1", "Test 2", "Test 3", "Test 4" },
            DoesSomethingActorSpy.ReceivedMessages);
    }

How should I create mock actors? Is there any method on TestKit I can call to wait until all messages have been processed?

Answer 1

Quoted from How to Test Akka.NET Actors: Unit Testing w/ Akka.TestKit :

Testing the parent/child relationship is more complicated. This is one case where Akka.NET's commitment to providing simple abstractions makes testing harder.

The simplest way to test this relationship is with messaging. For example, you can create a parent actor whose child messages another actor once it starts. Or you may have the parent forward a message to the child, and then the child can reply to the original sender, eg:

 public class ChildActor : ReceiveActor { public ChildActor() { ReceiveAny(o => Sender.Tell("hello!")); } } public class ParentActor : ReceiveActor { public ParentActor() { var child = Context.ActorOf(Props.Create(() => new ChildActor())); ReceiveAny(o => child.Forward(o)); } } [TestFixture] public class ParentGreeterSpecs : TestKit { [Test] public void Parent_should_create_child() { // verify child has been created by sending parent a message // that is forwarded to child, and which child replies to sender with var parentProps = Props.Create(() => new ParentActor()); var parent = ActorOfAsTestActorRef<ParentActor>(parentProps, TestActor); parent.Tell("this should be forwarded to the child"); ExpectMsg("hello!"); } } 

A Word of Caution On Testing Parent/Child Relationships

Avoid over-coupling your code to your hierarchy!

Over-testing parent/child relationships can couple your tests to your hierarchy implementation. This increases the costs of refactoring your code later on by forcing many test rewrites. You need to strike a balance between verifying your intent and testing your implementation.