jetpack compose中的CountDownTimer

Question

我正在使用以下代码在 jetpack 中尝试 CountDownTimer 组合

@Composable
fun Timer() {
    val millisInFuture: Long = 10 * 1000 // TODO: get actual value

    val timeData = remember {
        mutableStateOf(millisInFuture)
    }

    val countDownTimer =
        object : CountDownTimer(millisInFuture, 1000) {
            override fun onTick(millisUntilFinished: Long) {
                Log.d("TAG", "onTick: ")
                timeData.value = millisInFuture
            }

            override fun onFinish() {
               
            }
        }
    
    DisposableEffect(key1 = "key") {
        countDownTimer.start()
        onDispose {
            countDownTimer.cancel()
        }
    }

    Text(
        text = timeData.value.toString()
    )
}

在 logcat 中，我可以看到计时器正在滴答作响，但 UI 没有更新。 请解释为什么改变状态变量的值会发生重组。

Answer 1

简单的倒数计时器只为第二个

var timer by remember { mutableStateOf(durationSeconds) }
LaunchedEffect(key1 = timer) {
    if (timer > 0) {
        delay(1_000)
        timer -= 1
    }
}


Text(
    text = timer.toString()
) 

Answer 2

嗯，在 CountDownTimer 中，您应该设置 millisUntilFinished，而不是设置 millisInFuture。该变量保存更新后的值，millisInFuture 永远不会改变

timeData.value = millisUntilFinished

Answer 3

您可以尝试使用此代码来实现倒计时器：

val time = (timerDate.time).minus(Calendar.getInstance().timeInMillis)
var timer by remember { mutableStateOf(time) }
  LaunchedEffect(key1 = timer) {
      if (timer > 0) {
          delay(1000L)
          timer -= 1000L
       }
     }
  val secMilSec: Long = 1000
  val minMilSec = 60 * secMilSec
  val hourMilSec = 60 * minMilSec
  val dayMilSec = 24 * hourMilSec
  val hours = (time % dayMilSec / hourMilSec).toInt()
  val minutes = (time % dayMilSec % hourMilSec / minMilSec).toInt()
  val seconds = (time % dayMilSec % hourMilSec % minMilSec / secMilSec).toInt()

Text(text = String.format("%02d:%02d:%02d", hours, minutes, seconds))

Answer 4

最简单的解决方案是在几秒钟内进行类似于

countDownTimer

的延迟，如下所示：

@Composable
private fun Timer(
    waitingTime: Long = WAITING_TIME_MILLIS,
    onFinish: () -> Unit
) = LaunchedEffect(waitingTime) {
    delay(waitingTime)
    onFinish()
}

Answer 5

仅当可组合函数参数或可组合本身内部

State<T>

的值更改（如

mutableStateOf()

或

mutableStateListOf()

）发生状态更改时，才可组合。就您而言，您还没有启动

countDownTimer

本身。尝试在

countDownTimer.start()

内调用

DisposableEffect

。其次，您将 timeData 设置为错误的值，尝试使用

millisUntilFinished

设置它