rememberUpdatedState 用法

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在Android Compose中， 如果需要在协程LaunchedEffect中读取某个状态的最新状态值，而不需要重启协程需要使用到rememberUpdatedState。

下面例子在LaunchedEffect下，对delay期间状态a的值变化进行观测。
1. 使用 rememberUpdatedState
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class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            var a by remember { mutableStateOf(1) }
            Button(onClick = { a = 2 }) {
                LandingScreen(a)
            }
        }
    }
}

@Composable
fun LandingScreen(a:Int) {
    val current by rememberUpdatedState(a)
    Log.i("LandingScreen","a = $a")
    LaunchedEffect(Unit) {
        Log.i("LandingScreen","LaunchedEffect start")
        delay(3000)
        Log.i("LandingScreen","LaunchedEffect current=$current")
    }
    Text(text = "a=$a")
}

//应用启动后，3秒内点击Button，log输出结果：
a = 1
LaunchedEffect start
a = 2
LaunchedEffect current=2

从结果中可以看到当前状态值，显示为最新的状态值2，并且不重启LaunchedEffect。

2. 如果不使用rememberUpdatedState，使用remember
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@Composable
fun LandingScreen(a:Int) {
    val current by remember{ mutableStateOf(a) }
    Log.i("LandingScreen","a = $a")
    LaunchedEffect(Unit) {
        Log.i("LandingScreen","LaunchedEffect start")
        delay(3000)
        Log.i("LandingScreen","LaunchedEffect current=$current")
    }
    Text(text = "a=$a")
}

//应用启动后，3秒内点击Button，log输出结果：
a = 1
LaunchedEffect start
a = 2
LaunchedEffect current=1 //current状态对象还是之前的值没有更新

从结果中可以看到当前状态值，还是显示旧的值1，并且不重启LaunchedEffect。

3. 如果不使用rememberUpdatedState，使用key标识remember和LaunchedEffect
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@Composable
fun LandingScreen(a:Int) {
    val current by remember(key1 = a){ mutableStateOf(a) }
    Log.i("LandingScreen","a = $a")
    LaunchedEffect(key1 = a) {
        Log.i("LandingScreen","LaunchedEffect start")
        delay(3000)
        Log.i("LandingScreen","LaunchedEffect current=$current")
    }
    Text(text = "a=$a")
}

//应用启动后，3秒内点击Button，log输出结果：
a = 1
LaunchedEffect start
a = 2
LaunchedEffect start
LaunchedEffect current=2

从结果中可以看到当前状态值，显示为最新的状态值2，并且重启了LaunchedEffect。使用key，当状态值变更重组时，会取消当前LaunchedEffect，重启LaunchedEffect读取变更后的状态值。

rememberUpdatedState实现

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@Composable  
fun <T> rememberUpdatedState(newValue: T): State<T> = remember {  
mutableStateOf(newValue)  
}.apply { value = newValue }

可以看到，它只是对状态值进行重新赋值了。当状态值变更重组时，会重新调用可组合项函数，并且调用一次apply。remember保存的状态对象不会改变，但它的值更新了。因此，LaunchedEffect(Unit){}会读取到最新的值。