Main.scala

package point

import akka.actor.{Actor, ActorRef, ActorSystem, Props}
import point.MainActor.Start
import point.Point.{Distance, DistanceResult}

import scala.concurrent.Await
import scala.concurrent.duration._

object MainActor {

  def props(pointRef1: ActorRef, pointRef2: ActorRef): Props = Props(new MainActor(pointRef1, pointRef2))

  case object Start

}

class MainActor(pointRef1: ActorRef, pointRef2: ActorRef) extends Actor {

  override def receive: Receive = {
    case Start =>
      pointRef1 ! Distance(pointRef2) // !メソッドは、tellメソッドの別名！
    case DistanceResult(value) =>
      println(s"result = $value")
  }

}

object Main extends App {
  val system: ActorSystem = ActorSystem()
  val pointRef1: ActorRef = system.actorOf(Point.props(1, 1))
  val pointRef2: ActorRef = system.actorOf(Point.props(2, 2))
  val mainRef: ActorRef = system.actorOf(MainActor.props(pointRef1, pointRef2))

  // pointRef.x pointRef.yはコンパイルエラー
  // pointRef.state にもアクセスできない
  // そもそもActorRefからはActorの内部状態にアクセスできない

  mainRef ! Start

  sys.addShutdownHook {
    system.terminate()
    Await.result(system.whenTerminated, 60 seconds)
  }
}

Point.scala

package point

import akka.actor.{Actor, ActorRef, Props}
import point.Point.{Distance, DistanceResult, GetPoint, GetPointResult}

object Point {

  def props(x: Double, y: Double): Props = Props(new Point(x, y))

  case class Distance(pointRef: ActorRef)
  case class DistanceResult(value: Double)
  private case object GetPoint
  private case class GetPointResult(x: Double, y: Double)

}

class Point(val x: Double, val y: Double) extends Actor { // 本来 val は不要

  // val state: State = ...

  override def receive: Receive = {
    case GetPoint =>
      sender() ! GetPointResult(x, y)
    case Distance(pointRef) =>
      context.become(waitingPointResult(sender()))
      pointRef ! GetPoint
  }

  private def waitingPointResult(_sender: ActorRef): Receive = {
    case GetPointResult(x, y) =>
      val dx = this.x - x
      val dy = this.y - y
      _sender ! DistanceResult(Math.sqrt(dx * dx + dy * dy))
      context.unbecome()
  }

}