src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public static Flowable<Long> timer(long delay, TimeUnit unit) {
        return timer(delay, unit, Schedulers.computation());
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public static Flowable<Long> timer(long delay, TimeUnit unit) {
        return timer(delay, unit, Schedulers.computation());
    }
Returns a Flowable that emits {@code 0L} after a specified delay, and then completes.
<p>
<img width="640" height="200" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/timer.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>This operator does not support backpressure as it uses time. If the downstream needs a slower rate
it should slow the timer or use something like {@link #onBackpressureDrop}.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code timer} operates by default on the {@code computation} {@link Scheduler}.</dd>
</dl>

@param delay
the initial delay before emitting a single {@code 0L}
@param unit
time units to use for {@code delay}
@return a Flowable that emits {@code 0L} after a specified delay, and then completes
@see <a href="http://reactivex.io/documentation/operators/timer.html">ReactiveX operators documentation: Timer</a>