Add streams recipe for 'source from function' (#22074) (#24132)

akka-docs/src/test/java/jdocs/stream/javadsl/cookbook/RecipeSourceFromFunction.java

@@ -0,0 +1,63 @@
+/**
+ *  Copyright (C) 2015-2017 Lightbend Inc. <http://www.lightbend.com/>
+ */
+package jdocs.stream.javadsl.cookbook;
+
+import akka.NotUsed;
+import akka.actor.ActorSystem;
+import akka.stream.ActorMaterializer;
+import akka.stream.Materializer;
+import akka.stream.javadsl.Sink;
+import akka.stream.javadsl.Source;
+import akka.testkit.javadsl.TestKit;
+import com.typesafe.config.ConfigFactory;
+import org.junit.AfterClass;
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.List;
+import java.util.UUID;
+import java.util.concurrent.TimeUnit;
+
+public class RecipeSourceFromFunction extends RecipeTest {
+  static ActorSystem system;
+  static Materializer mat;
+
+  @BeforeClass
+  public static void setup() {
+    system = ActorSystem.create("RecipeSourceFromFunction", ConfigFactory.parseString("akka.loglevel=DEBUG\nakka.loggers = [akka.testkit.TestEventListener]"));
+    mat = ActorMaterializer.create(system);
+  }
+
+  @AfterClass
+  public static void tearDown() {
+    TestKit.shutdownActorSystem(system);
+    system = null;
+    mat = null;
+  }
+
+  @Test
+  public void beMappingOfRepeat() throws Exception {
+    new TestKit(system) {
+      final String builderFunction() {
+        return UUID.randomUUID().toString();
+      }
+
+      {
+        //#source-from-function
+        final Source<String, NotUsed> source = Source
+          .repeat(NotUsed.getInstance())
+          .map(elem -> builderFunction());
+        //#source-from-function
+
+        final List<String> result = source
+          .take(2)
+          .runWith(Sink.seq(), mat)
+          .toCompletableFuture().get(3, TimeUnit.SECONDS);
+
+        Assert.assertEquals(2, result.size());
+      }
+    };
+  }
+}