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kotlinlang

How would you test this countdown timer based on coroutines? What can I use as the time source because I can't use `SystemClock.elapsedRealtime` in a test.

```fun startTimer(
    millisInFuture: Long,
    countDownInterval: Long,
    onTick: (millisUntilFinished: Long) -&gt; Unit,
    onFinish: () -&gt; Unit,
) {
    millisUntilFinished = millisInFuture
    timerJob = scope.launch {

        val startTime = timeSource.elapsedRealTime
        val targetTime = startTime + millisInFuture
        while (true) {
            if (timeSource.elapsedRealTime &lt; targetTime) {
                delay(countDownInterval)
                millisUntilFinished = targetTime - timeSource.elapsedRealTime
                onTick(millisUntilFinished)
            } else {
                onFinish()
            }
        }
    }
}```


SystemClock time is needed because `delay` is not exact

I can create a fake time source that uses a Long field, but I don't know how to increment it in my test

Inject the time source so you can control it from the test

that's what we are doing but I don't know how to  use that in the test properly

we always end up stuck in some loop or don't increment the time at all

Maybe raceOf can help you, where one coroutine is the test, and the other is the time control: <https://blog.louiscad.com/coroutines-racing-why-and-how|https://blog.louiscad.com/coroutines-racing-why-and-how>