timer_observable_rxjava2.java

int delay = 25;
observableRemainingTime.setValue(TIME_FOR_ANSWER);
timerDisposable = Observable.timer(delay, TimeUnit.MILLISECONDS, Schedulers.computation())
        .observeOn(AndroidSchedulers.mainThread())
        .repeatUntil(() -> time <= 0)
        .subscribe(tick -> {
                    time = time - delay;
                    observableRemainingTime.setValue(time);
                },
                Throwable::printStackTrace,
                () -> onOptionSelected(null)
        );