src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable delay(final long delay, final TimeUnit unit, final Scheduler scheduler, final boolean delayError) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new CompletableDelay(this, delay, unit, scheduler, delayError));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable delay(final long delay, final TimeUnit unit, final Scheduler scheduler, final boolean delayError) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        return RxJavaPlugins.onAssembly(new CompletableDelay(this, delay, unit, scheduler, delayError));
    }
Returns a Completable which delays the emission of the completion event, and optionally the error as well, by the given time while
running on the specified scheduler.
<p>
<img width="640" height="253" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.delay.sb.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code delay} operates on the {@link Scheduler} you specify.</dd>
</dl>
@param delay the delay time
@param unit the delay unit
@param scheduler the scheduler to run the delayed completion on
@param delayError delay the error emission as well?
@return the new Completable instance
@throws NullPointerException if unit or scheduler is null