src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> unsubscribeOn(Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new ObservableUnsubscribeOn<T>(this, scheduler));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> unsubscribeOn(Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new ObservableUnsubscribeOn<T>(this, scheduler));
    }
Modifies the source ObservableSource so that subscribers will dispose it on a specified
{@link Scheduler}.
<p>
<img width="640" height="452" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/unsubscribeOn.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param scheduler
the {@link Scheduler} to perform the call to dispose() of the upstream Disposable
@return the source ObservableSource modified so that its dispose() calls happen on the specified
{@link Scheduler}
@see <a href="http://reactivex.io/documentation/operators/subscribeon.html">ReactiveX operators documentation: SubscribeOn</a>