Splitter variant that handles demand an termination of the outlets individually.

Splitter.scala

import akka.stream._
import akka.stream.stage._

/**
 * Fan-out the stream of `scala.util.Either[L, R]` elements to `L` and `R` streams.
 *
 * '''Emits when''' emits when an element is available from the input and the chosen output has demand
 *
 * '''Backpressures when''' the currently chosen output back-pressures
 *
 * '''Completes when''' upstream completes and no output is pending
 *
 * '''Cancels when''' when both downstreams cancel
 */
class Splitter[L, R] extends GraphStage[FanOutShape2[Either[L, R], L, R]] {

  val in: Inlet[Either[L, R]] = Inlet("in")

  val left: Outlet[L] = Outlet("left")

  val right: Outlet[R] = Outlet("right")

  override val shape = new FanOutShape2(in, left, right)

  def createLogic(inheritedAttributes: Attributes) = new GraphStageLogic(shape) {

    var pendingElem: Any = null

    var pendingOutlet: Outlet[_] = null

    var downstreamRunning = 2

    class SplitterInHandler extends InHandler {

      def maybePull(): Unit = {
        if (isAvailable(left) || isAvailable(right)) {
          pull(in)
        }
      }

      def handlePush[T](elem: T, outlet: Outlet[T]): Unit = {
        if (!isClosed(outlet)) {
          if (isAvailable(outlet)) {
            push(outlet, elem)
            maybePull()
          } else {
            pendingElem = elem
            pendingOutlet = outlet
          }
        } else {
          maybePull()
        }
      }

      override def onPush: Unit = {
        grab(in) match {
          case Left(elem) => handlePush(elem, left)
          case Right(elem) => handlePush(elem, right)
        }
      }

      override def onUpstreamFinish(): Unit = {
        if (pendingElem == null) {
          completeStage()
        }
      }
    }

    class SplitterOutHandler[T](outlet: Outlet[T]) extends OutHandler {

      override def onPull: Unit = {
        if (pendingElem != null && pendingOutlet == outlet) {
          push(outlet, pendingElem.asInstanceOf[T])
          pendingElem = null
          if (!isClosed(in)) {
            if (!hasBeenPulled(in)) {
              pull(in)
            }
          } else {
            completeStage()
          }
        } else {
          if (!hasBeenPulled(in)) {
            pull(in)
          }
        }
      }

      override def onDownstreamFinish(): Unit = {
        downstreamRunning -= 1
        if (downstreamRunning == 0) {
          completeStage()
        } else if (pendingElem != null && pendingOutlet == outlet) {
          pendingElem = null
          if (!hasBeenPulled(in)) {
            pull(in)
          }
        }
      }
    }

    setHandler(in, new SplitterInHandler)
    setHandler(left, new SplitterOutHandler(left))
    setHandler(right, new SplitterOutHandler(right))
  }
}