src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final <R> Observable<R> replay(final Function<? super Observable<T>, ? extends ObservableSource<R>> selector, final Scheduler scheduler) {
        ObjectHelper.requireNonNull(selector, "selector is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return ObservableReplay.multicastSelector(ObservableInternalHelper.replayCallable(this),
                ObservableInternalHelper.replayFunction(selector, scheduler));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final <R> Observable<R> replay(final Function<? super Observable<T>, ? extends ObservableSource<R>> selector, final Scheduler scheduler) {
        ObjectHelper.requireNonNull(selector, "selector is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return ObservableReplay.multicastSelector(ObservableInternalHelper.replayCallable(this),
                ObservableInternalHelper.replayFunction(selector, scheduler));
    }
Returns an Observable that emits items that are the results of invoking a specified selector on items
emitted by a {@link ConnectableObservable} that shares a single subscription to the source ObservableSource.
<p>
<img width="640" height="406" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/replay.o.fs.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param <R>
the type of items emitted by the resulting ObservableSource
@param selector
a selector function, which can use the multicasted sequence as many times as needed, without
causing multiple subscriptions to the ObservableSource
@param scheduler
the Scheduler where the replay is observed
@return an Observable that emits items that are the results of invoking the selector on items emitted by
a {@link ConnectableObservable} that shares a single subscription to the source ObservableSource,
replaying all items
@see <a href="http://reactivex.io/documentation/operators/replay.html">ReactiveX operators documentation: Replay</a>