Flow.mapAsyncUnordered synchronization

Akka Akka Streams & Alpakka
1

I need to synchronize a Flow.mapAsyncUnordered such that it does not pull until the future has completed. I have a cyclical graph where the next element depends on the results of the previous, therefore I need to way until the downstream has finished before pulling again. Flow.mapAsyncUnordered currently always buffers at least one element, so it is pulling too early. I have come up with the following:

val ref: ActorRef = <some dummy actor>

Sink.fromGraph(GraphDSL.create() { implicit b =>
  import GraphDSL.Implicits._

  val broadcast = b.add(Broadcast[DummyMessage](2))
  val zip = b.add(Zip[DummyMessage, Done])
  val flow = b.add(Flow[DummyMessage].mapAsyncUnordered(1)(ref ? _))

  broadcast         ~> zip.in0
  broadcast ~> flow ~> zip.in1
                       zip.out ~> Sink.ignore

  SinkShape(broadcast.in)
})

Is there a better way to pull messages one at a time? I had also considered the following:

Flow[DummyMessage].flatMapConcat(m => Source.fromFuture(ref ? m)).to(Sink.ignore)

Thanks
Jeff