Add Actor.immutablePartial (closes #24015)

akka-typed-tests/src/test/scala/akka/typed/scaladsl/ImmutablePartial.scala

@@ -0,0 +1,53 @@
+/**
+ * Copyright (C) 2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.typed
+package scaladsl
+
+import akka.typed.testkit.{ EffectfulActorContext, TestKitSettings }
+import akka.typed.testkit.scaladsl.TestProbe
+import org.junit.runner.RunWith
+import org.scalatest.junit.JUnitRunner
+import scala.concurrent.duration.DurationInt
+
+@RunWith(classOf[JUnitRunner])
+final class ImmutablePartialSpec extends TypedSpec {
+
+  final object `An Actor.immutablePartial behavior (native)`
+    extends Tests
+    with NativeSystem
+
+  final object `An Actor.immutablePartial behavior (adapted)`
+    extends Tests
+    with AdaptedSystem
+
+  trait Tests extends StartSupport {
+
+    private implicit val testSettings = TestKitSettings(system)
+
+    override implicit def system: ActorSystem[TypedSpec.Command]
+
+    def `must correctly install the message handler`(): Unit = {
+      val probe = TestProbe[Command]("probe")
+      val behavior =
+        Actor.immutablePartial[Command] {
+          case (_, Command2) ⇒
+            probe.ref ! Command2
+            Actor.same
+        }
+      val context = new EffectfulActorContext("ctx", behavior, 42, null)
+
+      context.run(Command1)
+      context.currentBehavior shouldBe behavior
+      probe.expectNoMsg(100.milliseconds)
+
+      context.run(Command2)
+      context.currentBehavior shouldBe behavior
+      probe.expectMsg(Command2)
+    }
+  }
+
+  private sealed trait Command
+  private case object Command1 extends Command
+  private case object Command2 extends Command
+}

akka-typed/src/main/scala/akka/typed/scaladsl/Actor.scala

@@ -185,6 +185,12 @@ object Actor {
       new BehaviorImpl.ImmutableBehavior(onMessage, onSignal)
   }
 
+  /**
+   * Construct an immutable actor behavior from a partial message handler which treats undefined messages as unhandled.
+   */
+  def immutablePartial[T](onMessage: PartialFunction[(ActorContext[T], T), Behavior[T]]): Immutable[T] =
+    Actor.immutable[T] { (ctx, t) ⇒ onMessage.applyOrElse((ctx, t), (_: (ActorContext[T], T)) ⇒ Actor.unhandled[T]) }
+
   /**
    * Construct an actor behavior that can react to lifecycle signals only.
    */