+doc #15239: Stream cookbook

akka-docs-dev/rst/scala/code/docs/stream/cookbook/RecipeLoggingElements.scala

@@ -0,0 +1,61 @@
+package docs.stream.cookbook
+
+import akka.event.Logging
+import akka.stream.scaladsl.{ Sink, Source, Flow }
+import akka.testkit.{ EventFilter, TestProbe }
+
+class RecipeLoggingElements extends RecipeSpec {
+
+  "Simple logging recipe" must {
+
+    "work with println" in {
+      val printProbe = TestProbe()
+      def println(s: String): Unit = printProbe.ref ! s
+
+      val mySource = Source(List("1", "2", "3"))
+
+      //#println-debug
+      val loggedSource = mySource.map { elem => println(elem); elem }
+      //#println-debug
+
+      loggedSource.runWith(Sink.ignore)
+      printProbe.expectMsgAllOf("1", "2", "3")
+    }
+
+    "work with PushStage" in {
+      val mySource = Source(List("1", "2", "3"))
+
+      //#loggingadapter
+      import akka.stream.stage._
+      class LoggingStage[T] extends PushStage[T, T] {
+        private val log = Logging(system, "loggingName")
+
+        override def onPush(elem: T, ctx: Context[T]): Directive = {
+          log.debug("Element flowing through: {}", elem)
+          ctx.push(elem)
+        }
+
+        override def onUpstreamFailure(cause: Throwable,
+                                       ctx: Context[T]): TerminationDirective = {
+          log.error(cause, "Upstream failed.")
+          super.onUpstreamFailure(cause, ctx)
+        }
+
+        override def onUpstreamFinish(ctx: Context[T]): TerminationDirective = {
+          log.debug("Upstream finished")
+          super.onUpstreamFinish(ctx)
+        }
+      }
+
+      val loggedSource = mySource.transform(() => new LoggingStage)
+      //#loggingadapter
+
+      EventFilter.debug(start = "Element flowing").intercept {
+        loggedSource.runWith(Sink.ignore)
+      }
+
+    }
+
+  }
+
+}