Adds sample code for the monitor operator

akka-docs/src/test/scala/docs/stream/operators/sourceorflow/Monitor.scala

@@ -0,0 +1,63 @@
+package docs.stream.operators.sourceorflow
+
+import akka.Done
+import akka.NotUsed
+import akka.actor.ActorSystem
+import akka.stream.FlowMonitor
+import akka.stream.FlowMonitorState
+import akka.stream.scaladsl.Keep
+import akka.stream.scaladsl.Sink
+import akka.stream.scaladsl.Source
+
+import scala.concurrent.Await
+import scala.concurrent.ExecutionContextExecutor
+import scala.concurrent.Future
+import scala.concurrent.duration._
+
+/**
+ * 
+ */
+class Monitor {
+
+  implicit val system = ActorSystem("monitor-sample-sys2")
+  implicit val executionContext: ExecutionContextExecutor = system.dispatcher
+
+  // #monitor
+  val source: Source[Int, NotUsed] =
+    Source.fromIterator(() => Iterator.from(0))
+
+  def printMonitorState(flowMonitor: FlowMonitor[Int]) = {
+    flowMonitor.state match {
+      case FlowMonitorState.Initialized =>
+        println("Stream is initialized but hasn't processed any element")
+      case FlowMonitorState.Received(msg) =>
+        println(s"Last element processed: $msg")
+      case FlowMonitorState.Failed(cause) =>
+        println(s"Stream failed with cause $cause")
+      case FlowMonitorState.Finished => println(s"Stream completed already")
+    }
+  }
+
+  val monitoredSource: Source[Int, FlowMonitor[Int]] = source
+    .take(6)
+    .throttle(5, 1.second)
+    .monitorMat(Keep.right)
+  val monitoredStream: (FlowMonitor[Int], Future[Done]) =
+    monitoredSource
+      .toMat(Sink.foreach(println))(Keep.both)
+      .run()
+
+  val flowMonitor = monitoredStream._1
+  // if we peek on the stream early enough it probably won't have processed any element.
+  printMonitorState(flowMonitor)
+  // wait a few millis and peek in the stream again to see what's the latest element processed
+  Thread.sleep(500)
+  printMonitorState(flowMonitor)
+  // wait until the stream completed
+  Await.result(monitoredStream._2, 3.seconds)
+  printMonitorState(flowMonitor)
+  // #monitor
+
+  monitoredStream._2.onComplete(_ => system.terminate())
+
+}