src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable observeOn(final Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new CompletableObserveOn(this, scheduler));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable observeOn(final Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new CompletableObserveOn(this, scheduler));
    }
Returns a Completable which emits the terminal events from the thread of the specified scheduler.
<p>
<img width="640" height="523" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.observeOn.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code observeOn} operates on a {@link Scheduler} you specify.</dd>
</dl>
@param scheduler the scheduler to emit terminal events on
@return the new Completable instance
@throws NullPointerException if scheduler is null