How to create a simple countdown timer in Kotlin?

You can use Kotlin objects:

val timer = object: CountDownTimer(20000, 1000) {
    override fun onTick(millisUntilFinished: Long) {...}

    override fun onFinish() {...}
}
timer.start()

I've solved my problem with timer in Kotlin like this:

class Timer {

    private val job = SupervisorJob()
    private val scope = CoroutineScope(Dispatchers.Default + job)

    private fun startCoroutineTimer(delayMillis: Long = 0, repeatMillis: Long = 0, action: () -> Unit) = scope.launch(Dispatchers.IO) {
        delay(delayMillis)
        if (repeatMillis > 0) {
            while (true) {
                action()
                delay(repeatMillis)
            }
        } else {
            action()
        }
    }

    private val timer: Job = startCoroutineTimer(delayMillis = 0, repeatMillis = 20000) {
        Log.d(TAG, "Background - tick")
        doSomethingBackground()
        scope.launch(Dispatchers.Main) {
            Log.d(TAG, "Main thread - tick")
            doSomethingMainThread()
        }
    }

    fun startTimer() {
        timer.start()
    }

    fun cancelTimer() {
        timer.cancel()
    }
//...
}

I've used Coroutines for a timer.

Chronometer can be set to count down and it seems to me the easiest way.

Add the Chronometer view in your layout xml, example

<Chronometer  
 android:id="@+id/view_timer"   
 tools:targetApi="24"  
 android:layout_width="wrap_content"  
 android:layout_height="wrap_content"/>

Then in your activity or fragment:

   view_timer.isCountDown = true
   view_timer.base = SystemClock.elapsedRealtime() + 20000
   view_timer.start()