How to create Akka Sink from a function that consumes Source - akka

Question

I have a function something like
def send[T, R](src: Source[T, _]): Future[R]
Conceptually, it is a sink that drains the source and streams it out to another system. It should be simple to make an actual Akka Sink out of it, but I cannot find a way. Can you please help me write a Sink-generating function along these lines:
def fromFunction[T, Mat](fn: Source[T, _] => Mat): Sink[T, Mat] = ???
I tried to wire up a graph, but it gets stuck on function trying to materialise the value before the graph is connected. I guess I need to defer the function call, but do not know how.
class FuncSinkShape[T, R](fn: Source[T, _] ⇒ Future[R])
extends GraphStageWithMaterializedValue[SinkShape[T], Future[R]] {
val in = Inlet[T]("SourceFlowShape.in")
override val shape = SinkShape(in)
override def createLogicAndMaterializedValue(inheritedAttributes: Attributes): (GraphStageLogic, Future[R]) = {
val sourceStage = new GraphStage[SourceShape[T]]() {
override val shape = SourceShape(Outlet[T]("FuncSinkShape.out"))
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = {
new GraphStageLogic(shape) {
setHandler(shape.out, new OutHandler {
override def onPull(): Unit = {
//pull(in)
}
})
}
}
}
val logic = new GraphStageLogic(shape) {
setHandler(in, new InHandler {
override def onPush(): Unit = {
val elm=grab(in)
err.println(s"-- onPush $elm")
emit(sourceStage.shape.out, elm)
}
})
}
(logic, fn(Source.fromGraph(sourceStage)))
}
}