Countdown timer for Android, support pause & resume functions.

ResumableTimer.kt

import android.os.Handler
import java.util.concurrent.TimeUnit


class ResumableTimer(
    private var duration: Long, // in milliseconds
    private var delay: Long = TimeUnit.SECONDS.toMillis(1), // in milliseconds
    private val listener: OnCountDownListener?
) {

    interface OnCountDownListener {

        fun onCountDownFinished()

        fun onCountDownTick(elapsed: Long, // in milliseconds
                            remaining: Long, // in milliseconds
                            total: Long) // in milliseconds
    }

    private var handler = Handler()
    var remaining = duration
    private lateinit var runnable: Runnable

    init {
        runnable = Runnable {
            remaining -= delay
            listener?.onCountDownTick(duration - remaining, remaining, duration)
            if (remaining > 0) {
                handler.postDelayed(runnable, delay)
            } else {
                stop()
                listener?.onCountDownFinished()
            }
        }
    }

    fun start(restart: Boolean = false) {
        if (restart) {
            handler.removeCallbacks(runnable)
            remaining = duration
        }

        handler.postDelayed(runnable, delay)
    }

    fun stop() {
        handler.removeCallbacks(runnable)
    }
}