Android ViewModel中CountDownTimer的onFinish重复触发问题排查

我尝试在ViewModel中实现一个10秒倒计时器，到期后返回上一视图，但出现多个异常：

• onFinish方法会触发两次返回操作
• 调用counter?.cancel()无法取消计时器
• ViewModel的onCleared方法似乎从未执行
• 同时疑惑CountDownTimer是否可以在ViewModel中使用

相关ViewModel代码如下：

class PairingScreenViewModel(
    private val routing: NavController,
) : ViewModel() {
    var content = mutableStateOf(10)
    var counter: CountDownTimer? = null

    init {
        viewModelScope.launch {
            delay(1000L)
            counter = object : CountDownTimer(9000L, 1000L) {
                override fun onTick(millisUntilFinished: Long) {
                    content.value = content.value.dec()
                }

                override fun onFinish() {
                    routing.popBackStack() // 会执行两次返回，原因不明？
                }
            }.start()
        }
    }

    override fun onCleared() {
        super.onCleared() // 似乎从未执行
        counter?.cancel()
    }

    fun onButtonClicked() = viewModelScope.launch {
        counter?.cancel() // 无法取消计时器
        routing.navigate("next-route")
    }
}

编辑补充：我怀疑问题出在NavHostController的使用上，这是我首次使用androidx.navigation:navigation-compose，MainActivity相关代码如下：

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            val routing = rememberNavController()
            val heading = "MyApplication"
            AndroidNavigationDemoTheme {
                Surface(modifier = Modifier.fillMaxSize(), color = MaterialTheme.colors.background) {
                    Scaffold(
                        topBar = {
                            TopAppBar(
                                title = { Text(heading, textAlign = TextAlign.Center, modifier = Modifier.fillMaxWidth()) }
                            )
                        },
                        content = {
                            Router(routing)
                        }
                    )
                }
            }
        }
    }
}

@Composable
fun Router(routing: NavHostController) {
    NavHost(
        navController = routing,
        startDestination = "welcome"
    ) {
        composable("welcome") {
            WelcomeScreen(routing)
        }
        composable("hisense") {
            HisenseScreen(HisenseScreenViewModel(routing))
        }
        composable("pairing") {
            PairingScreen(PairingScreenViewModel(routing))
        }
        // ...
    }
}

内容的提问来源于stack exchange，提问作者0lan