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

akka-docs/src/test/scala/docs/stream/cookbook/RecipeSourceFromFunction.scala

@@ -0,0 +1,26 @@
+/**
+ * Copyright (C) 2016-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package docs.stream.cookbook
+
+import java.util.UUID
+
+import akka.NotUsed
+import akka.stream.scaladsl._
+
+class RecipeSourceFromFunction extends RecipeSpec {
+
+  "A source that repeatedly evaluates a function" must {
+
+    "be a mapping of Source.repeat" in {
+      def builderFunction(): String = UUID.randomUUID.toString
+
+      //#source-from-function
+      val source = Source.repeat(NotUsed).map(_ ⇒ builderFunction())
+      //#source-from-function
+
+      val f = source.take(2).runWith(Sink.seq)
+      f.futureValue.distinct.size should ===(2)
+    }
+  }
+}