=str - Introduces a TCK-verified SingleElementPublisher to optimize Source.single and things depending on it

akka-stream-tck/src/test/scala/akka/stream/tck/SingleElementPublisherTest.scala

@@ -0,0 +1,20 @@
+/**
+ * Copyright (C) 2014 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.stream.tck
+
+import akka.stream.impl.SingleElementPublisher
+
+import scala.collection.immutable
+import akka.stream.scaladsl.Sink
+import akka.stream.scaladsl.Source
+import org.reactivestreams._
+
+class SingleElementPublisherTest extends AkkaPublisherVerification[Int] {
+
+  def createPublisher(elements: Long): Publisher[Int] = {
+    Source(SingleElementPublisher(0, "single-element-publisher")).runWith(Sink.publisher)
+  }
+
+  override def maxElementsFromPublisher(): Long = 1
+}

akka-stream/src/main/scala/akka/stream/impl/CompletedPublishers.scala

@@ -3,8 +3,7 @@
  */
 package akka.stream.impl
 
-import org.reactivestreams.{ Subscriber, Publisher }
+import org.reactivestreams.{ Subscriber, Publisher, Subscription }
-import org.reactivestreams.Subscription
 
 /**
  * INTERNAL API
@@ -40,6 +39,36 @@ private[akka] final case class ErrorPublisher(t: Throwable, name: String) extend
   override def toString: String = name
 }
 
+private[akka] final case class SingleElementPublisher[T](value: T, name: String) extends Publisher[T] {
+  import ReactiveStreamsCompliance._
+
+  private[this] class SingleElementSubscription(subscriber: Subscriber[_ >: T]) extends Subscription {
+    private[this] var done: Boolean = false
+    override def cancel(): Unit = done = true
+
+    override def request(elements: Long): Unit = if (!done) {
+      if (elements < 1) rejectDueToNonPositiveDemand(subscriber)
+      done = true
+      try {
+        tryOnNext(subscriber, value)
+        tryOnComplete(subscriber)
+      } catch {
+        case _: SpecViolation ⇒ // TODO log?
+      }
+    }
+  }
+
+  override def subscribe(subscriber: Subscriber[_ >: T]): Unit =
+    try {
+      requireNonNullSubscriber(subscriber)
+      tryOnSubscribe(subscriber, new SingleElementSubscription(subscriber))
+    } catch {
+      case _: SpecViolation ⇒ // nothing we can do
+    }
+  def apply[T]: Publisher[T] = this.asInstanceOf[Publisher[T]]
+  override def toString: String = name
+}
+
 /**
  * INTERNAL API
  * This is only a legal subscription when it is immediately followed by

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -192,11 +192,8 @@ object Source extends SourceApply {
    * Elements are pulled out of the iterator in accordance with the demand coming
    * from the downstream transformation steps.
    */
-  def apply[T](f: () ⇒ Iterator[T]): Source[T, Unit] = {
+  def apply[T](f: () ⇒ Iterator[T]): Source[T, Unit] =
-    apply(new immutable.Iterable[T] {
+    apply(new immutable.Iterable[T] { override def iterator: Iterator[T] = f() })
-      override def iterator: Iterator[T] = f()
-    })
-  }
 
   /**
    * A graph with the shape of a source logically is a source, this method makes
@@ -214,9 +211,8 @@ object Source extends SourceApply {
    * stream will see an individual flow of elements (always starting from the
    * beginning) regardless of when they subscribed.
    */
-  def apply[T](iterable: immutable.Iterable[T]): Source[T, Unit] = {
+  def apply[T](iterable: immutable.Iterable[T]): Source[T, Unit] =
-    Source.single(()).mapConcat((_: Unit) ⇒ iterable).withAttributes(DefaultAttributes.iterableSource)
+    Source.single(iterable).mapConcat(identity).withAttributes(DefaultAttributes.iterableSource)
-  }
 
   /**
    * Start a new `Source` from the given `Future`. The stream will consist of
@@ -242,7 +238,7 @@ object Source extends SourceApply {
    * Every connected `Sink` of this stream will see an individual stream consisting of one element.
    */
   def single[T](element: T): Source[T, Unit] =
-    apply(SynchronousIterablePublisher(List(element), "SingleSource")).withAttributes(DefaultAttributes.singleSource) // FIXME optimize
+    apply(SingleElementPublisher(element, "SingleSource")).withAttributes(DefaultAttributes.singleSource) // FIXME optimize
 
   /**
    * Create a `Source` that will continually emit the given element.