Example for unfoldResourceAsync (#28080)

* Example for unfoldResourceAsync

Co-Authored-By: Enno <458526+ennru@users.noreply.github.com>

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

@@ -0,0 +1,73 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+import akka.Done;
+import akka.NotUsed;
+import akka.actor.ActorSystem;
+import akka.stream.javadsl.Source;
+
+import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
+
+public class UnfoldResourceAsync {
+  // imaginary async API we need to use
+  // #unfoldResource-async-api
+  interface Database {
+    // async query
+    CompletionStage<QueryResult> doQuery();
+  }
+
+  interface QueryResult {
+
+    // are there more results
+    CompletionStage<Boolean> hasMore();
+
+    // async retrieval of each element
+    CompletionStage<DatabaseEntry> nextEntry();
+
+    CompletionStage<Void> close();
+  }
+
+  interface DatabaseEntry {}
+
+  // #unfoldResource-async-api
+
+  void unfoldResourceExample() {
+    ActorSystem system = null;
+
+    // #unfoldResourceAsync
+    // we don't actually have one, it was just made up for the sample
+    Database database = null;
+
+    Source<DatabaseEntry, NotUsed> queryResultSource =
+        Source.unfoldResourceAsync(
+            // open
+            database::doQuery,
+            // read
+            this::readQueryResult,
+            // close
+            queryResult -> queryResult.close().thenApply(__ -> Done.done()));
+
+    queryResultSource.runForeach(entry -> System.out.println(entry.toString()), system);
+    // #unfoldResourceAsync
+  }
+
+  // #unfoldResourceAsync
+  private CompletionStage<Optional<DatabaseEntry>> readQueryResult(QueryResult queryResult) {
+    return queryResult
+        .hasMore()
+        .thenCompose(
+            more -> {
+              if (more) {
+                return queryResult.nextEntry().thenApply(Optional::of);
+              } else {
+                return CompletableFuture.completedFuture(Optional.empty());
+              }
+            });
+  }
+  // #unfoldResourceAsync
+}