on demand actor get or else create

Question

I can create actors with actorOf and look them with actorFor. I now want to get an actor by some id:String and if it doesnt exist, I w

Answer 1

I based my solution to this problem on oxbow_lakes' code/suggestion, but instead of creating a simple collection of all the children actors I used a (bidirectional) map, which might be beneficial if the number of child actors is significant.

import play.api._
import akka.actor._
import scala.collection.mutable.Map 

trait ResponsibleActor[K] extends Actor {
  val keyActorRefMap: Map[K, ActorRef] = Map[K, ActorRef]()
  val actorRefKeyMap: Map[ActorRef, K] = Map[ActorRef, K]()

  def getOrCreateActor(key: K, props: => Props, name: => String): ActorRef = {
    keyActorRefMap get key match {
      case Some(ar) => ar
      case None =>  {
        val newRef: ActorRef = context.actorOf(props, name)
        //newRef shouldn't be present in the map already (if the key is different)
        actorRefKeyMap get newRef match{
          case Some(x) => throw new Exception{}
          case None =>
        }
        keyActorRefMap += Tuple2(key, newRef)
        actorRefKeyMap += Tuple2(newRef, key)
        newRef
      }
    }
  }

  def getOrCreateActorSimple(key: K, props: => Props): ActorRef = getOrCreateActor(key, props, key.toString)

  /**
   * method analogous to Actor's receive. Any subclasses should implement this method to handle all messages
   * except for the Terminate(ref) message passed from children
   */
  def responsibleReceive: Receive

  def receive: Receive = {
    case Terminated(ref) => {
      //removing both key and actor ref from both maps
      val pr: Option[Tuple2[K, ActorRef]] = for{
        key <- actorRefKeyMap.get(ref)
        reref <- keyActorRefMap.get(key)
      } yield (key, reref)

      pr match {
        case None => //error
        case Some((key, reref)) => {
          actorRefKeyMap -= ref
          keyActorRefMap -= key
        }
      }
    }
    case sth => responsibleReceive(sth)
  }
}

To use this functionality you inherit from ResponsibleActor and implement responsibleReceive. Note: this code isn't yet thoroughly tested and might still have some issues. I ommited some error handling to improve readability.