How to add elements to Source dynamically? - scala

Question

I have example code to generate an unbound source and working with it:
object Main {
def main(args : Array[String]): Unit = {
implicit val system = ActorSystem("Sys")
import system.dispatcher
implicit val materializer = ActorFlowMaterializer()
val source: Source[String] = Source(() => {
Iterator.continually({ "message:" + ThreadLocalRandom.current().nextInt(10000)})
})
source.runForeach((item:String) => { println(item) })
.onComplete{ _ => system.shutdown() }
}
}
I want to create class which implements:
trait MySources {
def addToSource(item: String)
def getSource() : Source[String]
}
And I need use it with multiple threads, for example:
class MyThread(mySources: MySources) extends Thread {
override def run(): Unit = {
for(i <- 1 to 1000000) { // here will be infinite loop
mySources.addToSource(i.toString)
}
}
}
And expected full code:
object Main {
def main(args : Array[String]): Unit = {
implicit val system = ActorSystem("Sys")
import system.dispatcher
implicit val materializer = ActorFlowMaterializer()
val sources = new MySourcesImplementation()
for(i <- 1 to 100) {
(new MyThread(sources)).start()
}
val source = sources.getSource()
source.runForeach((item:String) => { println(item) })
.onComplete{ _ => system.shutdown() }
}
}
How to implement MySources?

Answer 1

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")
}
}

Answer 2

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

Answer 3

As I mention in this answer, the SourceQueue is the way to go, and since Akka 2.5 there is a handy method preMaterialize which eliminates the need to create a composite source first.
I give an example in my other answer.