This ReceiveMessage.Partial keyword can be used in the akka.actor.Actor.Receive domain, to receive the next message that is a Message. The above code creates an actor to echo any string messages.

import akka.actor._
def echoActor = new Actor {
  def receive: Receive = {
    while (true) {
      val lastMessage = !ReceiveMessage.Partial[String]
      sender() ! lastMessage
    }
    throw new Exception("Unreachable code!")
  }
}

val pinger = system.actorOf(Props(echoActor))
pinger ! "hello world"
expectMsg("hello world")

All messages that are not the class of Message will not be handled.

object UnhandledMessage
pinger ! UnhandledMessage
pinger ! "string message"
expectMsg("string message")