Add orElse to RxScala

Author: zsxwing

language-adaptors/rxjava-scala/src/main/scala/rx/lang/scala/Observable.scala

@@ -2975,6 +2975,26 @@ trait Observable[+T]
     }
   }
 
+  /**
+   * Returns an Observable that emits the items emitted by the source Observable or a specified default item
+   * if the source Observable is empty.
+   *
+   * <img width="640" height="305" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/defaultIfEmpty.png">
+   *
+   * @param default the item to emit if the source Observable emits no items. This is a by-name parameter, so it is
+   *                only evaluated if the source Observable doesn't emit anything.
+   * @return an Observable that emits either the specified default item if the source Observable emits no
+   *         items, or the items emitted by the source Observable
+   */
+  def orElse[U >: T](default: => U): Observable[U] = {
+    val jObservableOption = map(Some(_)).asJavaObservable.asInstanceOf[rx.Observable[Option[T]]]
+    val o = toScalaObservable[Option[T]](jObservableOption.defaultIfEmpty(None))
+    o map {
+      case Some(element) => element
+      case None => default
+    }
+  }
+
   /**
    * Returns an Observable that forwards all sequentially distinct items emitted from the source Observable.
    *

language-adaptors/rxjava-scala/src/test/scala/rx/lang/scala/CompletenessTest.scala

@@ -79,6 +79,7 @@ class CompletenessTest extends JUnitSuite {
       "contains(Any)" -> "contains(U)",
       "count()" -> "length",
       "debounce(Func1[_ >: T, _ <: Observable[U]])" -> "debounce(T => Observable[Any])",
+      "defaultIfEmpty(T)" -> "orElse(=> U)",
       "delay(Func0[_ <: Observable[U]], Func1[_ >: T, _ <: Observable[V]])" -> "delay(() => Observable[Any], T => Observable[Any])",
       "delay(Func1[_ >: T, _ <: Observable[U]])" -> "delay(T => Observable[Any])",
       "dematerialize()" -> "dematerialize(<:<[Observable[T], Observable[Notification[U]]])",

language-adaptors/rxjava-scala/src/test/scala/rx/lang/scala/ObservableTest.scala

@@ -216,4 +216,16 @@ class ObservableTests extends JUnitSuite {
     o.subscribe()
     assertFalse(called)
   }
+
+  @Test
+  def testOrElse() {
+    val o = Observable.items(1, 2, 3).orElse(4)
+    assertEquals(List(1, 2, 3), o.toBlocking.toList)
+  }
+
+  @Test
+  def testOrElseWithEmpty() {
+    val o = Observable.empty.orElse(-1)
+    assertEquals(List(-1), o.toBlocking.toList)
+  }
 }