How to use stackable trait pattern with Akka actors?

Question

I'm trying to implement a Pub/Sub trait to mix into other akka actors using a stackable trait.

Here is what I came up with:

trait PubSubActor extends Actor {
  abstract override def receive = 
    super.receive orElse {
      case Subscribe(topic) => /* ... */
      case Publish(topic, msg) => /* ... */
    }
}

class MyActor extends Actor with PubSubActor {
  override def receive = {
    case SomeMessage(a, b, c) => /* ... */
  }
}

At which point, the compiler throws back an error: error: overriding method receive in trait MyActor... method receive needs `abstract override' modifiers.

Can you explain to me why this isn't working? How can I fix it so it works?

Thanks!

UPDATE

The following works:

trait PubSubActor extends Actor {
  abstract override def receive = 
    super.receive orElse {
      case Subscribe(topic) => /* ... */
      case Publish(topic, msg) => /* ... */
    }
}

class MyActor extends Actor {
  override def receive = {
    case SomeMessage(a, b, c) => /* ... */
  }
}

class MyActorImpl extends MyActor with PubSubActor

But why? Why can I get the behavior I want this way but not the other? Any reasons? I can't seem to figure out the underlying difference between these two samples that makes the difference.

Answer 1

There's a simple and concise solution:

Define a Receiving trait that chains multiple receive functions using orElse :

trait Receiving { 
  var receivers: Receive = Actor.emptyBehavior 
  def receiver(next: Actor.Receive) { receivers = receivers orElse next }
  def receive = receivers // Actor.receive definition
}

Using this in actors is easy:

trait PubSubActor extends Receiving {
  receiver {
    case Publish => /* I'm the first to handle messages */
  }
}

class MyActor extends PubSubActor with Receiving {
  receiver {
    case SomeMessage => /* PubSubActor didn't handle, I receive the message */ 
  }
}

First PubSubActor's receive will be called. If message wasn't handled it will be passed to MyActor's receive.

Answer 2

You can certainly achieve what you are looking for using Akka's composable actor feature. This is described a bit in Extending Actors using PartialFunction chaining.

First, the infrastructure code (straight from the docs):

class PartialFunctionBuilder[A, B] {
  import scala.collection.immutable.Vector

  // Abbreviate to make code fit
  type PF = PartialFunction[A, B]

  private var pfsOption: Option[Vector[PF]] = Some(Vector.empty)

  private def mapPfs[C](f: Vector[PF] => (Option[Vector[PF]], C)): C = {
    pfsOption.fold(throw new IllegalStateException("Already built"))(f) match {
      case (newPfsOption, result) => {
        pfsOption = newPfsOption
        result
      }
    }
  }

  def +=(pf: PF): Unit =
    mapPfs { case pfs => (Some(pfs :+ pf), ()) }

  def result(): PF =
    mapPfs { case pfs => (None, pfs.foldLeft[PF](Map.empty) { _ orElse _ }) }
}

trait ComposableActor extends Actor {
  protected lazy val receiveBuilder = new PartialFunctionBuilder[Any, Unit]
  final def receive = receiveBuilder.result()
}

Then the behaviors you want to be able to compose into actors:

trait PubSubActor { self:ComposableActor =>
  receiveBuilder += {
    case Subscribe(topic) => /* ... */
    case Publish(topic, msg) => /* ... */
  }
}

trait MyActor  { self:ComposableActor =>
  receiveBuilder += {
    case SomeMessage(a, b, c) => /* ... */
  }
}

And lastly, an actual actor that uses these composable behaviors:

class MyActorImpl extends ComposableActor with PubSubActor with MyActor