src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<T> takeUntil(ObservableSource<U> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableTakeUntil<T, U>(this, other));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<T> takeUntil(ObservableSource<U> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableTakeUntil<T, U>(this, other));
    }
Returns an Observable that emits the items emitted by the source Observable until a second ObservableSource
emits an item.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/takeUntil.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code takeUntil} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param other
the ObservableSource whose first emitted item will cause {@code takeUntil} to stop emitting items
from the source Observable
@param <U>
the type of items emitted by {@code other}
@return an Observable that emits the items emitted by the source Observable until such time as {@code other} emits its first item
@see <a href="http://reactivex.io/documentation/operators/takeuntil.html">ReactiveX operators documentation: TakeUntil</a>