src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable subscribeOn(final Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");

        return RxJavaPlugins.onAssembly(new CompletableSubscribeOn(this, scheduler));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable subscribeOn(final Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");

        return RxJavaPlugins.onAssembly(new CompletableSubscribeOn(this, scheduler));
    }
Returns a Completable which subscribes the child subscriber on the specified scheduler, making
sure the subscription side-effects happen on that specific thread of the scheduler.
<p>
<img width="640" height="686" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.subscribeOn.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code subscribeOn} operates on a {@link Scheduler} you specify.</dd>
</dl>
@param scheduler the Scheduler to subscribe on
@return the new Completable instance
@throws NullPointerException if scheduler is null