代码： 

class FlowTimer(private val duration: Int,private val scope: CoroutineScope,private val onTick: (Int) -> Unit,private val onStart: (() -> Unit)? = null,private val onFinish: (() -> Unit)? = null,private val interval: Int = 1
) {private val id = System.currentTimeMillis()   fun start(): Job {if (duration <= 0 || interval <= 0) {throw IllegalArgumentException("duration or interval cannot less than zero")}return flow {for (i in duration downTo 0) {emit(i)delay((interval * 1000).toLong())}}.onStart { onStart?.invoke() }.onCompletion {if (it == null) {onFinish?.invoke()}}.onEach { onTick.invoke(it) }.flowOn(Dispatchers.Main)   // 确保上游回调，是在主线程回调.launchIn(scope)}override fun toString(): String = "Timer:(id=${id}"
}

用法： 

private var mTimer: Job? = nulloverride fun onCreate(savedInstanceState: Bundle?) {mTimer = FlowTimer(duration, lifecycleScope,onTick = { sec ->Log.d("MainActivity", "sec：${sec}")}, onFinish = { }).start()
}override fun onDestroy() {mTimer?.cancel
}