src/main/java/io/reactivex/Completable.java
@SchedulerSupport(SchedulerSupport.NONE)
    public final Disposable subscribe() {
        EmptyCompletableObserver observer = new EmptyCompletableObserver();
        subscribe(observer);
        return observer;
    }
@SchedulerSupport(SchedulerSupport.NONE)
    public final Disposable subscribe() {
        EmptyCompletableObserver observer = new EmptyCompletableObserver();
        subscribe(observer);
        return observer;
    }
Subscribes to this CompletableConsumable and returns a Disposable which can be used to dispose
the subscription.
<p>
<img width="640" height="352" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.subscribe.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code subscribe} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@return the Disposable that allows disposing the subscription