import akka.actor.ActorSystem
import akka.stream.Attributes.name
import akka.stream.scaladsl._
import akka.stream.stage.{GraphStage, GraphStageLogic, OutHandler}
import akka.stream.{ActorMaterializer, Attributes, Outlet, SourceShape}

class BaseStreamApp extends App {
  implicit val system = ActorSystem("stream-test")
  implicit val materializer = ActorMaterializer()
}

object SourceSingle extends BaseStreamApp {
  val data = 100
//  val source = Source.single(data)
  val source = Source.fromGraph(new _SingleSource(data))

  source.runForeach(println)
}

object SourceApply extends BaseStreamApp {
  val data = 1 to 10
//  val source = Source(data)
  val source = Source.single(data)
                    .mapConcat(_ConstantFun.scalaIdentityFunction)
                    .withAttributes(_DefaultAttributes.iterableSource)

  source.runForeach(println)
}

final class _SingleSource[T](val elem: T) extends GraphStage[SourceShape[T]] {
  override def initialAttributes: Attributes = _DefaultAttributes.singleSource
  //ReactiveStreamsCompliance.requireNonNullElement(elem)
  val out = Outlet[T]("single.out")
  val shape = SourceShape(out)
  def createLogic(attr: Attributes) =
    new GraphStageLogic(shape) with OutHandler {
      def onPull(): Unit = {
        push(out, elem)
        completeStage()
      }
      setHandler(out, this)
    }

  override def toString: String = "SingleSource"
}

object _DefaultAttributes {
  val iterableSource = name("iterableSource")
  val singleSource = name("singleSource")
}

object _ConstantFun {
  private val conforms = (a: Any) => a
  def scalaIdentityFunction[T]: T => T = conforms.asInstanceOf[Function[T, T]]
}