Add a sample for Source.unfoldResource #25468

akka-docs/src/test/java/jdocs/stream/operators/source/UnfoldResource.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+import akka.NotUsed;
+import akka.actor.ActorSystem;
+import akka.stream.javadsl.Source;
+
+import java.util.Optional;
+
+interface UnfoldResource {
+  // imaginary blocking API we need to use
+  // #unfoldResource-blocking-api
+  interface Database {
+    // blocking query
+    QueryResult doQuery();
+  }
+
+  interface QueryResult {
+    boolean hasMore();
+    // potentially blocking retrieval of each element
+    DatabaseEntry nextEntry();
+
+    void close();
+  }
+
+  interface DatabaseEntry {}
+
+  // #unfoldResource-blocking-api
+
+  default void unfoldResourceExample() {
+    ActorSystem system = null;
+
+    // #unfoldResource
+    // we don't actually have one, it was just made up for the sample
+    Database database = null;
+
+    Source<DatabaseEntry, NotUsed> queryResultSource =
+        Source.unfoldResource(
+            // open
+            () -> database.doQuery(),
+            // read
+            (queryResult) -> {
+              if (queryResult.hasMore()) return Optional.of(queryResult.nextEntry());
+              else return Optional.empty();
+            },
+            // close
+            QueryResult::close);
+
+    queryResultSource.runForeach(entry -> System.out.println(entry.toString()), system);
+    // #unfoldResource
+  }
+}