Simple sample how to connect a Flow to Source and keep going the stream using Akka Stream

MapSourceToFlow.scala

package main

import akka.actor.ActorSystem
import akka.stream.{ActorMaterializer, ClosedShape}
import akka.stream.scaladsl.{Flow, GraphDSL, RunnableGraph, Sink, Source}
import GraphDSL.Implicits._
import com.typesafe.config.{ Config, ConfigFactory }

object SrcToFlow extends App {

  // implicit actor system
  implicit val system = ActorSystem()

  // implicit actor materializer
  implicit val materializer = ActorMaterializer()

  val graph = RunnableGraph.fromGraph(GraphDSL.create(Sink.ignore) { implicit b => sink =>

    //Simple Source
    val src = Source(List(1, 2, 3))

    val fromSrc = b.add(Flow[Int].log("from-Src"))

    //Emit new list per element received
    val flow = b.add(Flow[Int].flatMapConcat{ element =>
      Source(List(4, 5, 6))
    })

    //¯\_(ツ)_/¯ nothing to do, just for sample purposes
    val map = b.add(Flow[Int].map{ element =>
      element
    })

    src ~> fromSrc ~>flow ~> map ~> sink
    ClosedShape
  })

  graph.run()

}