Unnesting a Scala Actors nested receive example

UnnestedReceives.scala

/**
* I had a question directed at me, on how to encode the following scenario in Akka Actors,
* as for Scala Actors one would simply nest the receives.
*
* Recuirements are as follows:
* The first thing the actor needs to do, is to subscribe to a channel of events,
* Then it must replay (process) all "old" events
* Then it has to wait for a GoAhead signal to begin processing the new events
* It mustn't "miss" events that happen between catching up with the old events and getting the GoAhead signal
*/

import akka.actor._
import akka.actor.Actor._
import scala.collection.mutable.ListBuffer

class MyActor extends Actor {
 //If you need to store sender/senderFuture you can change it to ListBuffer[(Any, Channel)]
 val queue = new ListBuffer[Any]()

 //This message processes a message/event
 def process(msg: Any): Unit = println("processing: " + msg)
 //This method subscribes the actor to the event bus
 def subscribe() {} //Your external stuff
 //This method retrieves all prior messages/events
 def allOldMessages() = List()

 override def preStart {
   //We override preStart to be sure that the first message the actor gets is
   //'Replay, that message will start to be processed _after_ the actor is started
   self ! 'Replay
   //Then we subscribe to the stream of messages/events
   subscribe()
 }

 def receive = {
   case 'Replay => //Our first message should be a 'Replay message, all others are invalid
     allOldMessages() foreach process //Process all old messages/events
     become  { //Switch behavior to look for the GoAhead signal
       case 'GoAhead => //When we get the GoAhead signal we process all our buffered messages/events
         queue foreach process
         queue.clear
         become { //Then we change behaviour to process incoming messages/events as they arrive
           case msg => process(msg)
         }
       case msg => //While we haven't gotten the GoAhead signal, buffer all incoming messages
         queue += msg //Here you have full control, you can handle overflow etc
     }
 }
}