1.x: Add Single.toCompletable() (ReactiveX#3866)

Closes ReactiveX#3865.

Author: artem-zinnatullin

src/main/java/rx/Single.java

@@ -2095,6 +2095,31 @@ public final Observable<T> toObservable() {
         return asObservable(this);
     }
 
+    /**
+     * Returns a {@link Completable} that discards result of the {@link Single} (similar to
+     * {@link Observable#ignoreElements()}) and calls {@code onCompleted} when this source {@link Single} calls
+     * {@code onSuccess}. Error terminal event is propagated.
+     * <p>
+     * <img width="640" height="295" src=
+     * "https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.toCompletable.png"
+     * alt="">
+     * <dl>
+     * <dt><b>Scheduler:</b></dt>
+     * <dd>{@code toCompletable} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @return a {@link Completable} that calls {@code onCompleted} on it's subscriber when the source {@link Single}
+     *         calls {@code onSuccess}.
+     * @see <a href="http://reactivex.io/documentation/completable.html">ReactiveX documentation:
+     *      Completable</a>.
+     * @since (if this graduates from Experimental/Beta to supported, replace this parenthetical
+     *        with the release number).
+     */
+    @Experimental
+    public final Completable toCompletable() {
+        return Completable.fromSingle(this);
+    }
+
     /**
      * Returns a Single that mirrors the source Single but applies a timeout policy for its emitted item. If it
      * is not emitted within the specified timeout duration, the resulting Single terminates and notifies

src/test/java/rx/SingleTest.java

@@ -812,6 +812,29 @@ public void testToObservable() {
         ts.assertCompleted();
     }
 
+    @Test
+    public void toCompletableSuccess() {
+        Completable completable = Single.just("value").toCompletable();
+        TestSubscriber<Object> testSubscriber = new TestSubscriber<Object>();
+        completable.subscribe(testSubscriber);
+
+        testSubscriber.assertCompleted();
+        testSubscriber.assertNoValues();
+        testSubscriber.assertNoErrors();
+    }
+
+    @Test
+    public void toCompletableError() {
+        TestException exception = new TestException();
+        Completable completable = Single.error(exception).toCompletable();
+        TestSubscriber<Object> testSubscriber = new TestSubscriber<Object>();
+        completable.subscribe(testSubscriber);
+
+        testSubscriber.assertError(exception);
+        testSubscriber.assertNoValues();
+        testSubscriber.assertNotCompleted();
+    }
+
     @Test
     public void doOnErrorShouldNotCallActionIfNoErrorHasOccurred() {
         Action1<Throwable> action = mock(Action1.class);