Example for Source.unfoldAsync (#28111)

akka-docs/src/test/scala/docs/stream/operators/source/UnfoldAsync.scala

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.source
+
+import akka.NotUsed
+import akka.actor.typed.ActorRef
+import akka.actor.typed.ActorSystem
+import akka.stream.scaladsl.Source
+import akka.util.ByteString
+import akka.actor.typed.scaladsl.AskPattern._
+import akka.util.Timeout
+
+import scala.concurrent.Future
+import scala.concurrent.duration._
+
+object UnfoldAsync {
+
+  // #unfoldAsync-actor-protocol
+  object DataActor {
+    sealed trait Command
+    case class FetchChunk(offset: Long, replyTo: ActorRef[Chunk]) extends Command
+    case class Chunk(bytes: ByteString)
+    // #unfoldAsync-actor-protocol
+
+  }
+  implicit val system: ActorSystem[Nothing] = ???
+
+  def unfoldAsyncExample(): Unit = {
+    // #unfoldAsync
+    // actor we can query for data with an offset
+    val dataActor: ActorRef[DataActor.Command] = ???
+    import system.executionContext
+
+    implicit val askTimeout: Timeout = 3.seconds
+    val startOffset = 0L
+    val byteSource: Source[ByteString, NotUsed] =
+      Source.unfoldAsync(startOffset) { currentOffset =>
+        // ask for next chunk
+        val nextChunkFuture: Future[DataActor.Chunk] =
+          dataActor.ask(DataActor.FetchChunk(currentOffset, _))
+
+        nextChunkFuture.map { chunk =>
+          val bytes = chunk.bytes
+          if (bytes.isEmpty) None // end of data
+          else Some((currentOffset + bytes.length, bytes))
+        }
+      }
+    // #unfoldAsync
+  }
+
+}