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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