[Android]アプリ開発でタイマー機能(時間計測)を実装する方法

class MainActivity : AppCompatActivity(), View.OnClickListener {

    val handler = Handler()
    var timeValue = 0
    lateinit var timer: Timer

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_calculation)

        runnable = object: Runnable {
            override fun run() {
                timeValue++
                timeText(timeValue)?.let {
                    textViewCalcTime.text = it
                }
                handler.postDelayed(this, 1000)
            }
        }

        startButton.setOnClickListener {
            handler.post(runnable)
        }

        stopButton.setOnClickListener {
            handler.removeCallbacks(runnable)
        }
    }

    override fun onResume() {
        super.onResume()
        timer = Timer()
    }

    override fun onPause() {
        super.onPause()
        timer.cancel()
    }

    private fun timeText(time: Int = 0): String? {
        return if (time < 0) {
            null
        } else if (time == 0) {
            "00:00:00"
        } else {
            val h = time / 3600
            val m = time % 3600 / 60
            val s = time % 60
            "%1$02d:%2$02d:%3$02d".format(h, m, s)
        }
    }
}