src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable doOnSubscribe(Consumer<? super Disposable> onSubscribe) {
        return doOnLifecycle(onSubscribe, Functions.emptyConsumer(),
                Functions.EMPTY_ACTION, Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION, Functions.EMPTY_ACTION);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable doOnSubscribe(Consumer<? super Disposable> onSubscribe) {
        return doOnLifecycle(onSubscribe, Functions.emptyConsumer(),
                Functions.EMPTY_ACTION, Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION, Functions.EMPTY_ACTION);
    }
Returns a Completable instance that calls the given onSubscribe callback with the disposable
that child subscribers receive on subscription.
<p>
<img width="640" height="304" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.doOnSubscribe.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code doOnSubscribe} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param onSubscribe the callback called when a child subscriber subscribes
@return the new Completable instance
@throws NullPointerException if onSubscribe is null