statefulMapConcat.scala

implicit val system = ActorSystem()
implicit val executor = system.dispatcher
implicit val materializer = ActorMaterializer()

val source: Source[Int, NotUsed] = Source(1 to 10)
val flow1: Flow[Int, Int, NotUsed] = Flow[Int].map(_ * 10)
val flow2: Flow[Int, Int, NotUsed] = Flow[Int].filter(_ % 3 == 0)
val flow3: Flow[Int, Int, NotUsed] = Flow[Int].statefulMapConcat { () =>
  var sequence = 0
  i =>
    sequence = sequence + i
    List(sequence)
}

source.via(flow1).via(flow2).via(flow3).runForeach(println)

// 30
// 90
// 180