Adding a Scala and a Java guide to Akka Extensions

akka-docs/scala/code/akka/docs/extension/ExtensionDocSpec.scala

@@ -0,0 +1,74 @@
+package akka.docs.extension
+
+import org.scalatest.WordSpec
+import org.scalatest.matchers.MustMatchers
+
+//#imports
+import akka.actor._
+import java.util.concurrent.atomic.AtomicLong
+
+//#imports
+
+//#extension
+class CountExtensionImpl extends Extension {
+  //Since this Extension is a shared instance
+  // per ActorSystem we need to be threadsafe
+  private val counter = new AtomicLong(0)
+
+  //This is the operation this Extension provides
+  def increment() = counter.incrementAndGet()
+}
+//#extension
+
+//#extensionid
+object CountExtension
+  extends ExtensionId[CountExtensionImpl]
+  with ExtensionIdProvider {
+  //The lookup method is required by ExtensionIdProvider,
+  // so we return ourselves here, this allows us
+  // to configure our extension to be loaded when
+  // the ActorSystem starts up
+  override def lookup = CountExtension
+
+  //This method will be called by Akka
+  // to instantiate our Extension
+  override def createExtension(system: ActorSystemImpl) = new CountExtensionImpl
+}
+//#extensionid
+
+//#extension-usage-actor
+import akka.actor.Actor
+
+class MyActor extends Actor {
+  def receive = {
+    case someMessage ⇒
+      CountExtension(context.system).increment()
+  }
+}
+//#extension-usage-actor
+
+//#extension-usage-actor-trait
+import akka.actor.Actor
+
+trait Counting { self: Actor ⇒
+  def increment() = CountExtension(context.system).increment()
+}
+class MyCounterActor extends Actor with Counting {
+  def receive = {
+    case someMessage ⇒ increment()
+  }
+}
+//#extension-usage-actor-trait
+
+class ExtensionDocSpec extends WordSpec with MustMatchers {
+
+  "demonstrate how to create an extension in Scala" in {
+    val system: ActorSystem = null
+    intercept[Exception] {
+      //#extension-usage
+      CountExtension(system).increment
+      //#extension-usage
+    }
+  }
+
+}

akka-docs/scala/extending-akka.rst

@@ -0,0 +1,53 @@
+.. _extending-akka:
+
+Akka Extensions
+===============
+
+.. sidebar:: Contents
+
+   .. contents:: :local:
+
+
+Building an Extension
+---------------------
+
+So let's create a sample extension that just lets us count the number of times something has happened.
+
+First, we define what our ``Extension`` should do:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocSpec.scala
+   :include: imports,extension
+
+Then we need to create an ``ExtensionId`` for our extension so we can grab ahold of it.
+
+.. includecode:: code/akka/docs/extension/ExtensionDocSpec.scala
+   :include: imports,extensionid 
+
+Wicked! Now all we need to do is to actually use it:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocSpec.scala
+   :include: extension-usage
+
+Or from inside of an Akka Actor:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocSpec.scala
+   :include: extension-usage-actor
+
+You can also hide extension behind traits:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocSpec.scala
+   :include: extension-usage-actor-trait
+
+That's all there is to it!
+
+Loading from Configuration
+--------------------------
+
+To be able to load extensions from your Akka configuration you must add FQCNs of implementations of either ``ExtensionId`` or ``ExtensionIdProvider``
+in the "akka.extensions" section of the config you provide to your ``ActorSystem``.
+
+Applicability
+-------------
+
+The sky is the limit!
+By the way, did you know that Akka's ``Typed Actors``, ``Serialization`` and other features are implemented as Akka Extensions?

akka-docs/scala/index.rst

@@ -19,3 +19,4 @@ Scala API
    routing
    fsm
    testing
+   extending-akka