Jetpack Compose - 导航 - 脚手架 + NavHost 不工作

问题描述

所以我正在尝试使用 Jetpack Compose 创建一个应用程序。我有一个 Screen 函数，它包含一个没有顶部应用栏的 Scaffold、一个用于导航的底部栏和一个设置在底部栏中的浮动操作按钮。这一切正常。

但是，当我将 NavHost 添加到脚手架的内容时，整个事情就停止工作了。在没有 NavHost 的情况下，一切都可以正常工作，只是内容是屏幕的可组合功能。我已经尝试过为 NavHost 使用不同数量的可组合位置，不同的填充值都无济于事。

What it looks like without a NavHost (i.e. how I want it to look)

代码：

sealed class Screen(val route: String,@DrawableRes val iconId: Int){
    object Home         : Screen("home",R.drawable.ic_home_24px)
    object Stats        : Screen("stats",R.drawable.ic_stats_icon)
    object Add          : Screen("add",R.drawable.ic_add_24px)
    object Programs     : Screen("programs",R.drawable.ic_programs_icon)
    object Exercises    : Screen("exercises",R.drawable.ic_exercises_icon)
}

@ExperimentalFoundationApi
@Preview
@Composable
fun Screen(){
    val navController = rememberNavController()
    Scaffold(
        backgroundColor = OffWhite,bottomBar = {
            BottomBar(navController = navController)
        },floatingActionButton = {
            FloatingActionButton(
                onClick = {},shape = CircleShape,backgroundColor = Blue
            ) {
                Icon(
                    painter = painterResource(id = R.drawable.ic_add_24px),contentDescription = "Add",tint = OffWhite,modifier = Modifier
                        .padding(12.dp)
                        .size(32.dp)
                )
            }
        },isFloatingActionButtonDocked = true,floatingActionButtonPosition = FabPosition.Center,) {
        HomeScreen()
//        NavHost(
//            navController = navController,//            startDestination = Screen.Home.route
//        ){
//            composable(Screen.Home.route){ HomeScreen() }
//            composable(Screen.Stats.route){ HomeScreen() }
//            composable(Screen.Programs.route){ HomeScreen() }
//            composable(Screen.Exercises.route){ HomeScreen() }
//        }
    }
}


@Composable
fun BottomBar(
    navController : NavController
){
    val items = listof(
        Screen.Home,Screen.Stats,Screen.Add,Screen.Programs,Screen.Exercises
    )
    BottomAppBar(
        backgroundColor = OffWhite,cutoutShape = CircleShape,content = {
            BottomNavigation(
                backgroundColor = OffWhite,contentColor = OffWhite,modifier = Modifier
                    .height(100.dp)
            ) {
                val navBackStackEntry by navController.currentBackStackEntryAsstate()
                val currentDestination = navBackStackEntry?.destination
                items.forEach { screen ->
                    val selected = currentDestination?.hierarchy?.any { it.route == screen.route } == true
                    BottomNavigationItem(
                        icon = {
                            val iconSize = if (selected) 32.dp else 20.dp
                            Icon(
                                painter = painterResource(id = screen.iconId),contentDescription = screen.route,tint = Blue,modifier = Modifier
                                    .padding(12.dp)
                                    .size(iconSize)
                            )
                        },selected = selected,onClick = {
                            //Navigate to selected screen
                            navController.navigate(screen.route) {
                                //Pop all from stack
                                popUpTo(navController.graph.findStartDestination().id){
                                    saveState = true
                                }
                                //Avoid multiple copies of same screen on stack
                                launchSingletop = true
                                //Restore state when reselecting a prevIoUsly selected item
                                restoreState = true
                            }
                        },alwaysShowLabel = false
                    )
                }
            }
        }
    )
}

What it looks like with the NavHost. 该图像中的框是无法绘制的BottomBar，因为可以单击每个框，将我带到BottomBar、BottomNavigationItem、Icon 等。任何人都知道这里发生了什么，以及我可以做些什么来修复它？谢谢

编辑：我想到的一件事是将 fun BottomBar -> BottomNavigationItem 中的“selected”布尔值更改为始终为真，只是为了查看空值是否影响它，但这并没有改变任何东西。

解决方法

原来我的工作室主题隐藏了错误通知。更改主题后，它说

java.lang.IllegalStateException：预览版不支持创建 ViewModel

所以我想我需要在我的手机上敲一下它来测试。

,

错误消息说预览版不支持创建 ViewModel。因为 NavHost 创建视图模型，所以错误。所以我做了一些预览脚手架和一些屏幕的事情，我把脚手架的内容分开了。

示例：

@Composable
fun MainApp(
    navController: NavController,content: @Composable (PaddingValues) -> Unit
) {
    StoreTheme {
        Scaffold(
            bottomBar = { BottomAppNavigationBar(navController) },content = content
        )
    }
}

在真实应用上：

...

val navController = rememberNavController()
MainApp(navController) { innerPadding ->
    NavHost(
        navController = navController,startDestination = BottomNavMenu.Screen1.route,modifier = Modifier.padding(innerPadding)
    ) {
        composable(...) { Screen1... }
        composable(...) { Screen2... }
    }
}
...

预览时：

@Preview(showBackground = true)
@Composable
fun MainAppPreview() {
    val navController = rememberNavController()
    
    MainApp(navController) {
        Screen1(navController)
    }
}