I have a function returning a Flow
whose logic involves passing some elements of the graph to an auxiliary Sink
passed as a parameter. I want to retain the auxiliary Sink
's materialized value so I'm able to act upon its value when the constructed stream is launched.
Here's a rough picture of the flow I'm building:
IN ~> (logic: In => Out) ~> Broadcast ~> AuxFilter ~> AuxSink
~> OutFilter ~> OUT
Sample code:
case class Incoming()
trait Element
case class Outcoming() extends Element
case class Persistent() extends Element
def flow[Mat](auxSink: Sink[Persistent, Mat]): Flow[Incoming, Outcoming, NotUsed] = {
val isPersistent = Flow[Element].collect {
case persistent: Persistent => persistent
}
val isRunning = Flow[Element].collect {
case out: Outcoming => out
}
val magicFlow: Flow[Incoming, Element, NotUsed] = Flow[Incoming]
.map(_ => if (Random.nextBoolean()) Outcoming() else Persistent())
Flow.fromGraph {
GraphDSL.create() { implicit b =>
import GraphDSL.Implicits._
val magic = b.add(magicFlow)
val bcast = b.add(Broadcast[Element](2))
val sink = b.add(isRunning)
bcast.out(0) ~> isPersistent ~> auxSink
magic.out ~> bcast.in
bcast.out(1) ~> isRunning ~> sink.in
FlowShape(magic.in, sink.out)
}
}
}
Is there a way to somehow pass the auxSink
's Mat
to the resulting Flow
?
Thanks.
Answering my own question...
Found it! The source of Flow.alsoToMat
pointed me to exactly the logic I needed - to access the materialized value of an auxiliary graph (in my case auxSink
), one has to import its shape into the graph being constructed by passing it as a parameter to GraphDSL.create()
.
def flow[Mat](auxSink: Sink[Persistent, Mat]): Flow[Incoming, Outcoming, Mat] = {
val isPersistent = ...
val isRunning = ...
val magicFlow = ...
Flow.fromGraph {
GraphDSL.create(auxSink) { implicit b => aux =>
import GraphDSL.Implicits._
val magic = b.add(magicFlow)
val bcast = b.add(Broadcast[Element](2))
val sink = b.add(isRunning)
magic ~> bcast ~> isPersistent ~> aux
bcast ~> isRunning ~> sink
FlowShape(magic.in, sink.out)
}
}
}
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