gistfile1.scala

val users = List(
  User(1, "Aashrita", 22),
  User(2, "Mark", 12),
  User(3, "Ron", 19),
  User(4, "Adam", 20)
)

val userSource: Source[User, NotUsed] = Source(users)

def writeToFile(filename: String): Sink[User, Future[IOResult]] = {
  Flow[User]
   .map(_.toString)
   .map(ByteString(_))
   .toMat(FileIO.toPath(Paths.get(filename)))(Keep.right)
}

val g = RunnableGraph.fromGraph(GraphDSL.create() { implicit builder =>
  import GraphDSL.Implicits._

  val bcast = builder.add(Broadcast[User](2))
  userSource ~> bcast.in
  bcast.out(0) ~> Flow[User].filter(_.age >= 18) ~> writeToFile("a.txt")
  bcast.out(1) ~> Flow[User].filter(_.age < 18) ~>  writeToFile("b.txt")
  ClosedShape
})
g.run()