akka_join.scala

val g = RunnableGraph.fromGraph(GraphDSL.create() { implicit b: GraphDSL.Builder[NotUsed] =>
  import GraphDSL.Implicits._

  val merger = b.add(Merge[(List[BloodPressureEvent], List[HeartRateEvent])](2))

  bloodPressureSource.map(toEvent[BloodPressureEvent]).map(event => (List(event), List())) ~> merger.in(0)
  heartRateSource.map(toEvent[HeartRateEvent]).map(event => (List(), List(event))) ~> merger.in(1)

  merger ~> Sink.foreach(println)

  ClosedShape
})