Hi,

I came up with a solution (below) however it is sub optimal - it has to 
materialise the graph every time to get Sink.lastOption - ideally what I 
want is to avoid materialisation on each iteration:

    "Loop on Source on Sink.optionLast" in {
      def dynamicSource = Source(1 to Random.nextInt(4)) // represents a 
cursor from a database that we want to re-run if not empty


      def maybeLastFromMaterialisedGraph: Future[Option[Int]] = {
        val source = dynamicSource
        val sink = Sink.foreach(println)
        val sink2 = Sink.lastOption[Int]
        val dataFlow =
          source.alsoToMat(sink)(Keep.right)
            .toMat(sink2)(Keep.both)
        val (fIgnore, fInt) = dataFlow.run()
        fInt
      }


      // TODO: make this tail recursive
      def loop(eventuallyMaybeInt: Future[Option[Int]]): Future[Option[Int]] 
= {
        eventuallyMaybeInt.flatMap { o =>
          o match {
            case None => eventuallyMaybeInt
            case _ => loop(maybeLastFromMaterialisedGraph)
          }
        }
      }


      val i = await(loop(maybeLastFromMaterialisedGraph))
      println(s"Sink.last i=$i")
    }







On Tuesday, 21 August 2018 10:09:29 UTC+1, Mr B wrote:
>
> Hi,
>
> I am an akka streams newbie and was wondering how to create a Source that 
> restarts if it was not empty (Source would be dynamic eg the result of a 
> database fetch)?
>
> My initial thoughts were to broadcast the source to say Sink.optionFirst  
> and if the materialised value of Sink.optionFirst is Some(element), then 
> restart the source, if it is None then complete as normal. However I am 
> struggling with how I would do this.
>
> Thanks
>

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