src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Single<T> observeOn(final Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new SingleObserveOn<T>(this, scheduler));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Single<T> observeOn(final Scheduler scheduler) {
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new SingleObserveOn<T>(this, scheduler));
    }
Modifies a Single to emit its item (or notify of its error) on a specified {@link Scheduler},
asynchronously.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.observeOn.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 notify subscribers on
@return the source Single modified so that its subscribers are notified on the specified
{@link Scheduler}
@throws NullPointerException if scheduler is null
@see <a href="http://reactivex.io/documentation/operators/observeon.html">ReactiveX operators documentation: ObserveOn</a>
@see <a href="http://www.grahamlea.com/2014/07/rxjava-threading-examples/">RxJava Threading Examples</a>
@see #subscribeOn