Example for Source.unfoldAsync (#28111)

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

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+import akka.NotUsed;
+import akka.actor.typed.ActorRef;
+import akka.actor.typed.ActorSystem;
+import akka.japi.Pair;
+import akka.stream.javadsl.Source;
+import akka.util.ByteString;
+import akka.util.Timeout;
+import akka.actor.typed.javadsl.AskPattern;
+
+import java.time.Duration;
+import java.util.Optional;
+import java.util.concurrent.CompletionStage;
+
+interface UnfoldAsync {
+
+  // #unfoldAsync-actor-protocol
+  class DataActor {
+    interface Command {}
+
+    static final class FetchChunk implements Command {
+      public final long offset;
+      public final ActorRef<Chunk> replyTo;
+
+      public FetchChunk(long offset, ActorRef<Chunk> replyTo) {
+        this.offset = offset;
+        this.replyTo = replyTo;
+      }
+    }
+
+    static final class Chunk {
+      public final ByteString bytes;
+
+      public Chunk(ByteString bytes) {
+        this.bytes = bytes;
+      }
+    }
+    // #unfoldAsync-actor-protocol
+  }
+
+  default void unfoldAsyncSample() {
+    ActorSystem<Void> system = null;
+    // #unfoldAsync
+    ActorRef<DataActor.Command> dataActor = null; // let's say we got it from somewhere
+
+    Duration askTimeout = Duration.ofSeconds(3);
+    long startOffset = 0L;
+    Source<ByteString, NotUsed> byteSource =
+        Source.unfoldAsync(
+            startOffset,
+            currentOffset -> {
+              // ask for next chunk
+              CompletionStage<DataActor.Chunk> nextChunkCS =
+                  AskPattern.ask(
+                      dataActor,
+                      (ActorRef<DataActor.Chunk> ref) ->
+                          new DataActor.FetchChunk(currentOffset, ref),
+                      askTimeout,
+                      system.scheduler());
+
+              return nextChunkCS.thenApply(
+                  chunk -> {
+                    ByteString bytes = chunk.bytes;
+                    if (bytes.isEmpty()) return Optional.empty();
+                    else return Optional.of(Pair.create(currentOffset + bytes.size(), bytes));
+                  });
+            });
+    // #unfoldAsync
+  }
+}