How to add elements to Source dynamically?

With Akka Streams 2 you can use a sourceQueue : How to create a Source that can receive elements later via a method call?

One way to have a non-finite source is to use a special kind of actor as the source, one that mixes in the ActorPublisher trait. If you create one of those kinds of actors, and then wrap with a call to ActorPublisher.apply, you end up with a Reactive Streams Publisher instance and with that, you can use an apply from Source to generate a Source from it. After that, you just need to make sure your ActorPublisher class properly handles the Reactive Streams protocol for sending elements downstream and you are good to go. A very trivial example is as follows:

import akka.actor._
import akka.stream.actor._
import akka.stream.ActorFlowMaterializer
import akka.stream.scaladsl._

object DynamicSourceExample extends App{

  implicit val system = ActorSystem("test")
  implicit val materializer = ActorFlowMaterializer()

  val actorRef = system.actorOf(Props[ActorBasedSource])
  val pub = ActorPublisher[Int](actorRef)

  Source(pub).
    map(_ * 2).
    runWith(Sink.foreach(println))

  for(i <- 1 until 20){
    actorRef ! i.toString
    Thread.sleep(1000)
  }

}

class ActorBasedSource extends Actor with ActorPublisher[Int]{
  import ActorPublisherMessage._
  var items:List[Int] = List.empty

  def receive = {
    case s:String =>
      if (totalDemand == 0) 
        items = items :+ s.toInt
      else
        onNext(s.toInt)    

    case Request(demand) =>  
      if (demand > items.size){
        items foreach (onNext)
        items = List.empty
      }
      else{
        val (send, keep) = items.splitAt(demand.toInt)
        items = keep
        send foreach (onNext)
      }


    case other =>
      println(s"got other $other")
  }


}