Akka Stream Materialized Value - akka

Question

I want to reference the materialized value from the flow. Below is the code snippet, but its not compiling, error:
type mismatch;
found : (akka.NotUsed, scala.concurrent.Future[akka.Done])
required: (Playground.DomainObj, scala.concurrent.Future[akka.Done])
Code:
import akka.actor.ActorSystem
import akka.stream.scaladsl._
import scala.concurrent.Future
import akka.NotUsed
import akka.Done
implicit val actorSystem = ActorSystem("example")
case class DomainObj(name: String, age: Int)
val customFlow1:Flow[String,DomainObj,NotUsed] = Flow[String].map(s => {
DomainObj(s, 50)
})
val customFlow2 = Flow[DomainObj].map(s => {
s.age + 10
})
val printAnySink: Sink[Any, Future[Done]] = Sink.foreach(println)
val c1 = Source.single("John").viaMat(customFlow1)(Keep.right).viaMat(customFlow2)(Keep.left).toMat(printAnySink)(Keep.both)
val res: (DomainObj, Future[Done]) = c1.run()
Find the code in playground: https://scastie.scala-lang.org/P9iSx49cQcaOZfKtVCzTPA
I want to reference the DomainObj after the stream completes/

Answer 1

The materialized value of a Flow[String, DomainObj, NotUsed] is NotUsed, not a DomainObj, therefore c1's materialized value is (NotUsed, Future[Done]).
It looks like the intent here is to capture the DomainObj which is created in customFlow1. That can be accomplished with
val customFlow1: Flow[String, DomainObj, Future[DomainObj]] =
Flow[String]
.map { s => DomainObj(s, 50) }
.alsoTo(Sink.head)
val res: (Future[DomainObj], Future[Done]) = c1.run()
Note that Sink.head effectively requires that customFlow1 can only be used downstream of something that only emits once.