Closing over the enclosing scope using Props

ParentActor.scala

final class ParentActor extends Actor {

  private var value: Int = 0

  override def receive = {
    case Increment    => value++
    case CreateChild  => context.actorOf(Props(new ChildActor(value)))
  }
}

In this example we want to create a ChildActor with the current value from ParentActor. Usually we would expect that the ChildActor is created with the value at the time the CreateChild message is received. However, since value is passed by-name to Props.apply the ParentActor may already have received additional Increment messages before the by-name parameter is actually evaluated causing the new ChildActor to be created with a different value than expected.