在Jetpack编写中按下后退按钮时，滚动位置丢失

Question

我使用的是底部导航栏，每个菜单将用户导航到特定的可组合屏幕。我使用了导航库来管理它们之间的导航。

我正在为所有可组合使用通用的ViewModel。我正在其中一个可组合中使用懒惰列，当我在底部导航栏中单击菜单项在菜单项之间导航时，它会按预期工作，在懒惰列的滚动位置保存的位置。

当我在具有一个lazyColumn的可组合屏幕中单击按钮(我已经将它编程为导航到导航图中的起始目的地)并导航回该按钮时，问题出现了，然后滚动位置被刷新为0。这是一个错误还是我做错了什么

以下是我的代码:

导航

@Composable
fun PopulateHomeComposables(navController: NavHostController,             
homeViewModel: HomeViewModel, lifecycleScope : LifecycleCoroutineScope) {
NavHost(navController = navController, startDestination = 
WHATS_NEW_COMPOSABLE) {

    composable(WHATS_NEW_COMPOSABLE) {
        WhatsNewComposable(navController)
    }
    composable(COMPLIMENTS_COMPOSABLE) {
        ComplimentsComposable(navController)
    }
    composable(INSIGHTS_COMPOSABLE) {
        InsightsComposable(navController)
    }
    composable(NOTIFICATIONS_COMPOSABLE) {

        NotificationsComposable(homeViewModel, lifecycleScope)
    }
    }
}

我的脚手架看起来像这样

Scaffold(
    topBar = {

    },
    content = {

    },
    floatingActionButton = {
    },
    bottomBar = {
        val items = listOf(BottomNavScreens.WhatsNew, 
BottomNavScreens.Compliments, BottomNavScreens.Insights, 
BottomNavScreens.Notifications)
        BottomNavigation {
            val navBackStackEntry by navController.currentBackStackEntryAsState()
            val currentRoute = navBackStackEntry?.destination?.route
            //draw the menu items for each one
            items.forEach {
                BottomNavigationItem(
                    icon = {
                        Icon(it.icon, it.label)
                    },
                    label = {
                        Text(it.label)
                    },
                    alwaysShowLabel = true,
                    selected = currentRoute == it.route,
                    onClick = {
                        navController.navigate(it.route) {
               // Pop up to the start destination of the graph to
               // avoid building up a large stack of destinations
                            // on the back stack as users select items
                            
navController.graph.startDestinationRoute?.let { route ->
                                popUpTo(route) {
                                    saveState = true
                                }
                            }
                     //Avoid multiple copies of the same estination when
                            //re selecting the same item
                            launchSingleTop = true
         //Restore state when re selecting a previously selected item
                            restoreState = true
                        }
                    }
                )
            }
        }
    }
)

Answer 1

所以我解决了这个问题。由于我正在使用导航库，我希望它在使用底部NavBar时单击后退按钮时保存可组合的状态。但我所要做的就是在可组合文件中覆盖后退按钮，并添加一个功能以使用navHostController导航到起始目标。

//override the back button press to navigate back to the 
//whatsNewComposable
BackHandler {
    Timber.i("Back button clicked in notifications composable")

    navController.navigate("myStartDestinationRoute) {
    // Pop up to the start destination of the graph to
    // avoid building up a large stack of destinations
    // on the back stack as users select items
    navController.graph.startDestinationRoute?.let { route ->
        popUpTo(route) {
            saveState = true
        }
    }
    //Avoid multiple copies of the same destination when
    //re selecting the same item
    launchSingleTop = true
    //Restore state when re selecting a previously selected item
    restoreState = true
}