=doc graphstage sink example (#20955) (#21030)

akka-docs/rst/scala/code/docs/stream/GraphStageDocSpec.scala

@@ -4,15 +4,14 @@
 package docs.stream
 
 import akka.NotUsed
-import akka.stream.scaladsl.{ Keep, Sink, Flow, Source }
+import akka.stream.scaladsl.{ Flow, Keep, Sink, Source }
 import akka.stream.stage._
 import akka.stream._
-
 import akka.stream.testkit.{ TestPublisher, TestSubscriber }
 import akka.testkit.{ AkkaSpec, TestLatch }
 
 import scala.collection.mutable
-import scala.concurrent.{ Promise, Await, Future }
+import scala.concurrent.{ Await, Future, Promise }
 import scala.concurrent.duration._
 import scala.collection.immutable.Iterable
 
@@ -86,6 +85,35 @@ class GraphStageDocSpec extends AkkaSpec {
     Await.result(result2, 3.seconds) should ===(5050)
   }
 
+  "Demonstrate creation of GraphStage Sink" in {
+    //#custom-sink-example
+    import akka.stream.SinkShape
+    import akka.stream.stage.GraphStage
+    import akka.stream.stage.InHandler
+
+    class StdoutSink extends GraphStage[SinkShape[Int]] {
+      val in: Inlet[Int] = Inlet("StdoutSink")
+      override val shape: SinkShape[Int] = SinkShape(in)
+
+      override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
+        new GraphStageLogic(shape) {
+
+          // This requests one element at the Sink startup.
+          override def preStart(): Unit = pull(in)
+
+          setHandler(in, new InHandler {
+            override def onPush(): Unit = {
+              println(grab(in))
+              pull(in)
+            }
+          })
+        }
+    }
+    //#custom-sink-example
+
+    Source(List(0, 1, 2)).runWith(Sink.fromGraph(new StdoutSink))
+  }
+
   //#one-to-one
   class Map[A, B](f: A => B) extends GraphStage[FlowShape[A, B]] {