src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable doOnDispose(Action onDispose) {
        return doOnLifecycle(Functions.emptyConsumer(), Functions.emptyConsumer(),
                Functions.EMPTY_ACTION, Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION, onDispose);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable doOnDispose(Action onDispose) {
        return doOnLifecycle(Functions.emptyConsumer(), Functions.emptyConsumer(),
                Functions.EMPTY_ACTION, Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION, onDispose);
    }
Calls the shared {@code Action} if a CompletableObserver subscribed to the current
Completable disposes the common Disposable it received via onSubscribe.
<p>
<img width="640" height="589" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.doOnDispose.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code doOnDispose} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param onDispose the action to call when the child subscriber disposes the subscription
@return the new Completable instance
@throws NullPointerException if onDispose is null