StopWatch using RxJava

StopWatch.kt

class StopWatch {
    private val disposableBag = CompositeDisposable()
    var onTick: (String) -> Unit = {}
    var elapsedTime: Long = 0

    companion object {
        const val INITIAL_VALUE = "00:00:00"
    }

    fun start() {
        disposableBag.add(
            Observable.interval(1000, TimeUnit.MILLISECONDS)
                .subscribeOn(Schedulers.io())
                .subscribe {
                    onTick.invoke(formatElapsedTime(it))
                })
    }

    fun stop() {
        disposableBag.clear()
    }

    private fun formatElapsedTime(elapsed: Long): String {
        val hours = elapsed / 3600
        val minutes = (elapsed % 3600) / 60
        val seconds = elapsed % 60

        return String.format("%02d:%02d:%02d", hours, minutes, seconds)
    }
}