src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> throttleFirst(long skipDuration, TimeUnit unit, Scheduler scheduler) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new ObservableThrottleFirstTimed<T>(this, skipDuration, unit, scheduler));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable<T> throttleFirst(long skipDuration, TimeUnit unit, Scheduler scheduler) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new ObservableThrottleFirstTimed<T>(this, skipDuration, unit, scheduler));
    }
Returns an Observable that emits only the first item emitted by the source ObservableSource during sequential
time windows of a specified duration, where the windows are managed by a specified Scheduler.
<p>
This differs from {@link #throttleLast} in that this only tracks passage of time whereas
{@link #throttleLast} ticks at scheduled intervals.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/throttleFirst.s.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param skipDuration
time to wait before emitting another item after emitting the last item
@param unit
the unit of time of {@code skipDuration}
@param scheduler
the {@link Scheduler} to use internally to manage the timers that handle timeout for each
event
@return an Observable that performs the throttle operation
@see <a href="http://reactivex.io/documentation/operators/sample.html">ReactiveX operators documentation: Sample</a>