@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<T> sample(ObservableSource<U> sampler) {
        ObjectHelper.requireNonNull(sampler, "sampler is null");
        return RxJavaPlugins.onAssembly(new ObservableSampleWithObservable<T>(this, sampler, false));
    }