我需要在应用程序返回前台时自动刷新 Android Compose 屏幕。
我有一个需要权限和定位服务。
如果用户关闭了其中的任何一个，则会绘制一个列表，列出需要更改的项目。当用户转到“设置”并且应用程序返回到前台时，我希望列表能够刷新以反映更改。
我正在使用 Compose 和 Compose 导航。我已经看过，但我无法弄清楚可用于触发刷新的 onResume 生命周期事件的等价物。
任何想法都将不胜感激，因为我不知所措。
我想出了这个：
@Composable
fun OnLifecycleEvent(onEvent: (owner: LifecycleOwner, event: Lifecycle.Event) -> Unit) {
val eventHandler = rememberUpdatedState(onEvent)
val lifecycleOwner = rememberUpdatedState(LocalLifecycleOwner.current)
DisposableEffect(lifecycleOwner.value) {
val lifecycle = lifecycleOwner.value.lifecycle
val observer = LifecycleEventObserver { owner, event ->
eventHandler.value(owner, event)
}
lifecycle.addObserver(observer)
onDispose {
lifecycle.removeObserver(observer)
}
}
}
似乎工作得很好。但在某些情况下可能会出现一些问题，所以要小心。
也有可能是有些多余的代码。
用法：
OnLifecycleEvent { owner, event ->
// do stuff on event
when (event) {
Lifecycle.Event.ON_RESUME -> { /* stuff */ }
else -> { /* other stuff */ }
}
}
我稍微改进了 @JojoIV 答案并使它在没有回调的情况下使用平坦，就像您观察到的
LiveData@Composable
fun Lifecycle.observeAsState(): State<Lifecycle.Event> {
val state = remember { mutableStateOf(Lifecycle.Event.ON_ANY) }
DisposableEffect(this) {
val observer = LifecycleEventObserver { _, event ->
state.value = event
}
[email protected](observer)
onDispose {
[email protected](observer)
}
}
return state
}
然后使用
@Composable
fun SomeComposable() {
val lifecycleState = LocalLifecycleOwner.current.lifecycle.observeAsState()
val state = lifecycleState.value
// or val lifecycleState by LocalLifecycleOwner.current.lifecycle.observeAsState()
// will re-render someComposable each time lifecycleState will change
}
来自谷歌网站的例子
@Composable
fun HomeScreen(
lifecycleOwner: LifecycleOwner = LocalLifecycleOwner.current,
onStart: () -> Unit, // Send the 'started' analytics event
onStop: () -> Unit // Send the 'stopped' analytics event
) {
// Safely update the current lambdas when a new one is provided
val currentOnStart by rememberUpdatedState(onStart)
val currentOnStop by rememberUpdatedState(onStop)
// If `lifecycleOwner` changes, dispose and reset the effect
DisposableEffect(lifecycleOwner) {
// Create an observer that triggers our remembered callbacks
// for sending analytics events
val observer = LifecycleEventObserver { _, event ->
if (event == Lifecycle.Event.ON_START) {
currentOnStart()
} else if (event == Lifecycle.Event.ON_STOP) {
currentOnStop()
}
}
// Add the observer to the lifecycle
lifecycleOwner.lifecycle.addObserver(observer)
// When the effect leaves the Composition, remove the observer
onDispose {
lifecycleOwner.lifecycle.removeObserver(observer)
}
}
/* Home screen content */
}
在谷歌网站上的完整描述 https://developer.android.com/jetpack/compose/side-effects#disposableeffect
使用官方 API（需要 Lifecycle Runtime Compose 2.7.0）：
警告： 在撰写本文时（2023 年 3 月 16 日）Lifecycle Runtime Compose 2.7.0 尚未发布，但更改已在源代码中。
检测
Lifecycle.Stateval lifecycleOwner = LocalLifecycleOwner.current
val state by lifecycleOwner.lifecycle.collectStateAsState()
LaunchedEffect(state) {
// Do something with your state
// You may want to use DisposableEffect or other alternatives
// instead of LaunchedEffect
}
collectStateAsStatecurrentStateFlowval lifecycleOwner = LocalLifecycleOwner.current
val state by lifecycleOwner.lifecycle.currentStateFlow.collectAsState()
LaunchedEffect(state) {
// Do something with your state
// You may want to use DisposableEffect or other alternatives
// instead of LaunchedEffect
}
记得导入
androidx.compose.runtime.getValueLifecycle.State旧答案：
Compose 不知道像
onPauseonResume一个例子是您的 Activity 中的一个
LiveDataonResume我们来看下面的例子：
class MainActivity : AppCompatActivity() {
// Use whatever type your prefer/require, this is just an example
private val exampleLiveData = MutableLiveData("")
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContent {
// Your main composable
MyApplicationTheme {
// Save the state into a variable otherwise it won't work
val state = exampleLiveData.observeAsState()
Log.d("EXAMPLE", "Recomposing screen - ${state.value}")
Surface(color = MaterialTheme.colors.background) {
Greeting("Android")
}
}
}
}
override fun onResume() {
super.onResume()
// Save whatever you want in your live data, this is just an example
exampleLiveData.value = DateTimeFormatter.ISO_INSTANT.format(Instant.now())
}
}
@Composable
fun Greeting(name: String) {
Text(text = "Hello $name!")
}
@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
MyApplicationTheme {
Greeting("Android")
}
}
如您在此示例中所见，我的活动中有一个包含字符串的
LiveDataonResume我把@ojoIV代码改成了这个 （如果您的可组合代码在 Activity 中）
@Composable
fun ComponentActivity.LifecycleEventListener(event: (Lifecycle.Event) -> Unit) {
val eventHandler by rememberUpdatedState(newValue = event)
val lifecycle = [email protected]
DisposableEffect(lifecycle) {
val observer = LifecycleEventObserver { _, event ->
eventHandler(event)
}
lifecycle.addObserver(observer)
onDispose {
lifecycle.removeObserver(observer)
}
}
}
用法
LifecycleEventListener(event = { lifecycleEvent ->
when (lifecycleEvent ) {
Lifecycle.Event.ON_CREATE -> {}
Lifecycle.Event.ON_START -> {}
Lifecycle.Event.ON_RESUME -> {}
Lifecycle.Event.ON_PAUSE -> {}
Lifecycle.Event.ON_STOP -> {}
Lifecycle.Event.ON_DESTROY -> {}
else -> return@LifecycleEventListener
}
})