src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> repeatUntil(BooleanSupplier stop) {
        ObjectHelper.requireNonNull(stop, "stop is null");
        return RxJavaPlugins.onAssembly(new ObservableRepeatUntil<T>(this, stop));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> repeatUntil(BooleanSupplier stop) {
        ObjectHelper.requireNonNull(stop, "stop is null");
        return RxJavaPlugins.onAssembly(new ObservableRepeatUntil<T>(this, stop));
    }
Returns an Observable that repeats the sequence of items emitted by the source ObservableSource until
the provided stop function returns true.
<p>
<img width="640" height="262" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/repeatUntil.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code repeatUntil} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param stop
a boolean supplier that is called when the current Observable completes;
if it returns true, the returned Observable completes; if it returns false,
the upstream Observable is resubscribed.
@return the new Observable instance
@throws NullPointerException
if {@code stop} is null
@see <a href="http://reactivex.io/documentation/operators/repeat.html">ReactiveX operators documentation: Repeat</a>