为Android构建现代应用——应用导航设计

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 在前一章节的实现中，Skeleton: Main structure，我们留下了几个 Jetpack 架构组件，这些组件将在本章中使用，例如 Composables、ViewModels、Navigation 和 Hilt。此外，我们还通过 Scaffold 集成了 TopAppBar 和 BottomAppBar UI 模式的基本结构。为了继续改进这个实现，我们需要添加一个新的关键选项卡，即“应用程序的通用状态”。

App程序的状态：一般状态

定义导航地图

导航从其他UI元素

摘要

应用程序状态：一个通用状态

 “设计原则”中我们讨论了状态在现代 Android 应用程序中的重要作用。
设计中可能存在三种类型的状态：属性 UI 状态、组件 UI 状态和屏幕 UI 状态。
除了这些状态之外，我们还可以定义一种新类型的状态，即应用程序状态。
这个新状态将定义应用程序的通用状态。它将用于屏幕之间的导航、自发消息（snackbars）的呈现以及应用程序中其他可用的进程。
在主目录中，我们将定义一个名为 OrderNowState 的类，它将是我们的状态持有者，代表这种类型的状态。

​编辑

接下来，我们像这样执行OrderNowState的初始实现:

@SuppressLint("RememberReturnType")
@Composable
fun rememberAppState(scaffoldState:ScaffoldState=rememberScaffoldState(),
                     navController: NavHostController =rememberNavController(),
                     resources1: Resources = resources(),
                     coroutineScope: CoroutineScope=rememberCoroutineScope()
)=remember(scaffoldState,navController,resources1,coroutineScope){
   OrderNowState(scaffoldState,navController,resources1,coroutineScope)
}
class OrderNowState(val scaffoldState:ScaffoldState,val navController:NavHostController,private val resources:Resources,coroutineScope:CoroutineScope)
@Composable
@ReadOnlyComposable
fun resources():Resources{
    LocalConfiguration.current
    return LocalContext.current.resources
}
@SuppressLint("RememberReturnType")
@Composable

fun rememberAppState(scaffoldState:ScaffoldState=rememberScaffoldState(),
                     navController: NavHostController =rememberNavController(),
                     resources1: Resources = resources(),
                     coroutineScope: CoroutineScope=rememberCoroutineScope()
)=remember(scaffoldState,navController,resources1,coroutineScope){
   OrderNowState(scaffoldState,navController,resources1,coroutineScope)
}

class OrderNowState(val scaffoldState:ScaffoldState,val navController:NavHostController,private val resources:Resources,coroutineScope:CoroutineScope)


@Composable
@ReadOnlyComposable
fun resources():Resources{
    LocalConfiguration.current
    return LocalContext.current.resources
}
@SuppressLint("RememberReturnType")
@Composable

fun rememberAppState(scaffoldState:ScaffoldState=rememberScaffoldState(),
                     navController: NavHostController =rememberNavController(),
                     resources1: Resources = resources(),
                     coroutineScope: CoroutineScope=rememberCoroutineScope()
)=remember(scaffoldState,navController,resources1,coroutineScope){
   OrderNowState(scaffoldState,navController,resources1,coroutineScope)
}

class OrderNowState(val scaffoldState:ScaffoldState,val navController:NavHostController,private val resources:Resources,coroutineScope:CoroutineScope)


@Composable
@ReadOnlyComposable
fun resources():Resources{
    LocalConfiguration.current
    return LocalContext.current.resources
}



然后，我们修改我们的OrderNowScreen视图以包含之前定义的状态，如下所示:
然后，我们修改我们的OrderNowScreen视图以包含之前定义的状态，如下所示:
然后，我们修改我们的OrderNowScreen视图以包含之前定义的状态，如下所示:

@Preview
@Composable
fun OrderNowScreen(){
    MyTestTheme {
        Surface(modifier = Modifier.fillMaxSize(),color=MaterialTheme.colors.background) {
            val appState =rememberAppState()
            Scaffold(scaffoldState = appState.scaffoldState, topBar = { OrderNowTopBar()}, bottomBar = {OrderNowBottomBar()}){contentPadding->
                println(contentPadding)
            }
        }
    }
}
@Preview
@Composable
fun OrderNowScreen(){
    MyTestTheme {
        Surface(modifier = Modifier.fillMaxSize(),color=MaterialTheme.colors.background) {
            val appState =rememberAppState()
            Scaffold(scaffoldState = appState.scaffoldState, topBar = { OrderNowTopBar()}, bottomBar = {OrderNowBottomBar()}){contentPadding->
                println(contentPadding)


            }
        }
    }

}
@Preview
@Composable
fun OrderNowScreen(){
    MyTestTheme {
        Surface(modifier = Modifier.fillMaxSize(),color=MaterialTheme.colors.background) {
            val appState =rememberAppState()
            Scaffold(scaffoldState = appState.scaffoldState, topBar = { OrderNowTopBar()}, bottomBar = {OrderNowBottomBar()}){contentPadding->
                println(contentPadding)


            }
        }
    }

}

注意:

还需要向OrderNowScreen添加资源函数，它将使用该函数访问App资源。

突出显示更改的代码行如下:

  Scaffold(
                scaffoldState = appState.scaffoldState,
                topBar = { OrderNowTopBar() },
                bottomBar = { OrderNowBottomBar() }) { contentPadding ->
                println(contentPadding)
            }
  Scaffold(
                scaffoldState = appState.scaffoldState,
                topBar = { OrderNowTopBar() },
                bottomBar = { OrderNowBottomBar() }) { contentPadding ->
                println(contentPadding)

            }
  Scaffold(
                scaffoldState = appState.scaffoldState,
                topBar = { OrderNowTopBar() },
                bottomBar = { OrderNowBottomBar() }) { contentPadding ->
                println(contentPadding)

            }

有了上面的代码，我们现在可以告诉Scaffold它应该把哪个状态作为引用:应用程序的状态。之后，视图之间的导航操作、自发消息的呈现，以及视图只把AppState作为真实源的其他独占任务都将被允许。

既然已经为APP定义了状态，我们就可以继续实现APP的导航了。

定义导航地图

我们在应用程序中使用的导航策略由以下元素组成:

NavHost:它是负责在视图中显示导航结果的组件。导航的结果由NavigationController和导航图中给出的定义决定。

AppSoGraph:它是导航图的实现。它应该根据指定的路由将导航指向哪个视图或可组合对象。

屏幕路由: 它们是可以通过导航到达的应用程序的不同屏幕。无论导航是从选项菜单、链接、按钮还是任何其他活动代理激活的，都无关紧要。每个屏幕都有一个与之关联的唯一路由。

一般导航图

​编辑

 我们将继续在OrderNow中包含这些元素

OrderNowScreenRoute

首先，创建一个名为common -> navigation的新遍历目录。在这个包中，我们像这样添加了一个名为OrderNowScreenRoute的类:

​编辑

 在这个类中，可以导航到的屏幕定义如下:

sealed class OrderNowScreenRoute(val route:String){
    object Home :OrderNowScreenRoute("home")
    object Cart:OrderNowScreenRoute("cart")
    object ProductList:OrderNowScreenRoute("product_list")
    object ProductDetail:OrderNowScreenRoute("product_detail")
}
sealed class OrderNowScreenRoute(val route:String){

    object Home :OrderNowScreenRoute("home")

    object Cart:OrderNowScreenRoute("cart")

    object ProductList:OrderNowScreenRoute("product_list")

    object ProductDetail:OrderNowScreenRoute("product_detail")

}
sealed class OrderNowScreenRoute(val route:String){

    object Home :OrderNowScreenRoute("home")

    object Cart:OrderNowScreenRoute("cart")

    object ProductList:OrderNowScreenRoute("product_list")

    object ProductDetail:OrderNowScreenRoute("product_detail")

}

OrderNowNavHost 和 AppSoGraph

现在，我们创建OrderNowNavHost类，它将像这样表示应用程序的NavHost:

​编辑

@Composable
fun OrderNowNavHost(appState:OrderNowState){
    NavHost(navController = appState.navController,5,startDestination=1){
        appSoGraph(appState)
    }
}
fun NavGraphBuilder.appSoGraph(appState: OrderNowState) {
}
@Composable
fun OrderNowNavHost(appState:OrderNowState){
    NavHost(navController = appState.navController,5,startDestination=1){
        appSoGraph(appState)
    }
}





fun NavGraphBuilder.appSoGraph(appState: OrderNowState) {


}
@Composable
fun OrderNowNavHost(appState:OrderNowState){
    NavHost(navController = appState.navController,5,startDestination=1){
        appSoGraph(appState)
    }
}





fun NavGraphBuilder.appSoGraph(appState: OrderNowState) {


}

从之前的代码片段中，我们应该强调以下定义：
• OrderNowNavHost 需要知道 APP 的状态。
• NavController 是从 APP 状态中托管和获取的。
• Navigation map（appSoGraph）将基于 APP 的状态创建，并且是在 OrderNowNavHost 内定义的扩展。
为了继续完成 OrderNow 中导航的实现，我们必须添加一个描述如下的辅助类。
NavigationBarSection
NavigationBarSection 是一个辅助类，代表应用程序底部菜单中组成筛选组的部分。
请记住，我们可以从选项菜单或其他 UI 组件（如链接、按钮或内部重定向）中的操作开始导航。
在下一节中，我们将对内部重定向（从按钮、链接等）进行更改；现在，让我们专注于从 BottomBar 导航。

​编辑

我们看到NavigationBarSection帮助器类将如何只对Home和Cart屏幕进行分组，这是我们希望从BottomBar菜单启用的选项。这个类将像这样放置在导航目录中:

​编辑

它的实现是这样的:

sealed class OrderNowScreenRoute(val route:String){
    object Home :OrderNowScreenRoute("home")
    object Cart:OrderNowScreenRoute("cart")
    object ProductList:OrderNowScreenRoute("product_list")
    object ProductDetail:OrderNowScreenRoute("product_detail")
}
sealed class NavigationBarSection(val title: String, val icon: ImageVector, val route:String){
    companion object{
        val sections = listOf(OrderNowScreenRoute.Home,OrderNowScreenRoute.Cart)
    }
    object Home:NavigationBarSection(title ="Home" , icon = Icons.Default.Home, route =OrderNowScreenRoute.Home.route)
    object Cart:NavigationBarSection(title = "Cart", icon =Icons.Default.ShoppingCart, route = OrderNowScreenRoute.Cart.route)
}
sealed class OrderNowScreenRoute(val route:String){

    object Home :OrderNowScreenRoute("home")

    object Cart:OrderNowScreenRoute("cart")

    object ProductList:OrderNowScreenRoute("product_list")

    object ProductDetail:OrderNowScreenRoute("product_detail")

}






sealed class NavigationBarSection(val title: String, val icon: ImageVector, val route:String){
    companion object{
        val sections = listOf(OrderNowScreenRoute.Home,OrderNowScreenRoute.Cart)
    }

    object Home:NavigationBarSection(title ="Home" , icon = Icons.Default.Home, route =OrderNowScreenRoute.Home.route)

    object Cart:NavigationBarSection(title = "Cart", icon =Icons.Default.ShoppingCart, route = OrderNowScreenRoute.Cart.route)
}
sealed class OrderNowScreenRoute(val route:String){

    object Home :OrderNowScreenRoute("home")

    object Cart:OrderNowScreenRoute("cart")

    object ProductList:OrderNowScreenRoute("product_list")

    object ProductDetail:OrderNowScreenRoute("product_detail")

}






sealed class NavigationBarSection(val title: String, val icon: ImageVector, val route:String){
    companion object{
        val sections = listOf(OrderNowScreenRoute.Home,OrderNowScreenRoute.Cart)
    }

    object Home:NavigationBarSection(title ="Home" , icon = Icons.Default.Home, route =OrderNowScreenRoute.Home.route)

    object Cart:NavigationBarSection(title = "Cart", icon =Icons.Default.ShoppingCart, route = OrderNowScreenRoute.Cart.route)
}

将helper类添加到项目中后，我们继续更新OrderNowNavHost类，如下所示:

@Composable
fun OrderNowNavHost(appState: OrderNowState, paddingValues: PaddingValues) {
    NavHost(
        navController = appState.navController,
        startDestination = NavigationBarSection.Home.route,
        modifier = Modifier.padding(paddingValues)
    ) {
        appSoGraph(appState)
    }
}
fun NavGraphBuilder.appSoGraph(appState: OrderNowState) {
    composable(NavigationBarSection.Home.route) {
       // HomeScreen()
    }
    composable(NavigationBarSection.Cart.route) {
       // CartScreen()
    }
    
    composable(OrderNowScreenRoute.ProductList.route){
        //ProductListScreen()
    }
    
    composable(OrderNowScreenRoute.ProductDetail.route){
        //ProductDetailScreen()
    }
}
@Composable
fun OrderNowNavHost(appState: OrderNowState, paddingValues: PaddingValues) {
    NavHost(
        navController = appState.navController,
        startDestination = NavigationBarSection.Home.route,
        modifier = Modifier.padding(paddingValues)
    ) {
        appSoGraph(appState)
    }


}





fun NavGraphBuilder.appSoGraph(appState: OrderNowState) {
    composable(NavigationBarSection.Home.route) {
       // HomeScreen()
    }
    composable(NavigationBarSection.Cart.route) {
       // CartScreen()
    }

    
    composable(OrderNowScreenRoute.ProductList.route){
        //ProductListScreen()
    }
    
    composable(OrderNowScreenRoute.ProductDetail.route){
        //ProductDetailScreen()
    }

}
@Composable
fun OrderNowNavHost(appState: OrderNowState, paddingValues: PaddingValues) {
    NavHost(
        navController = appState.navController,
        startDestination = NavigationBarSection.Home.route,
        modifier = Modifier.padding(paddingValues)
    ) {
        appSoGraph(appState)
    }


}





fun NavGraphBuilder.appSoGraph(appState: OrderNowState) {
    composable(NavigationBarSection.Home.route) {
       // HomeScreen()
    }
    composable(NavigationBarSection.Cart.route) {
       // CartScreen()
    }

    
    composable(OrderNowScreenRoute.ProductList.route){
        //ProductListScreen()
    }
    
    composable(OrderNowScreenRoute.ProductDetail.route){
        //ProductDetailScreen()
    }

}

appSoGraph函数的实现是NavGraphBuilder的扩展，在那里，我们为App的每个屏幕指定导航地图。另外，通过startDestination参数，指定将首先呈现的默认屏幕，即Home屏幕。采用更改的下一步是更新名为 OrderNowBottomBar 的类，如下所示：

@Composable
fun OrderNowBottomBar(navController: NavHostController) {
    val navBackStackEntry by navController.currentBackStackEntryAsState()
    val currentDestination = navBackStackEntry?.destination
    BottomNavigation(
        backgroundColor = MaterialTheme.colors.background,
        contentColor = contentColorFor(MaterialTheme.colors.background),
        elevation = 10.dp
    ) {
        NavigationBarSection.sections.forEach { section ->
            val selected = currentDestination?.hierarchy?.any {
                it.route == section.route
            } == true
            BottomNavigationItem(icon = {
                Icon(
                    imageVector =  ImageVector.vectorResource(R.drawable.ic_launcher_foreground),
                    contentDescription = stringResource(id = R.string.app_name)
                )
            },
                label = { Text(text = stringResource(id =R.string.app_name)) },
                selected = selected,
                unselectedContentColor = Color.Gray,
                selectedContentColor = Color.Red,
                onClick = {
                    navController.navigate(section.route) {
                        popUpTo(navController.graph.findStartDestination().id) {
                            saveState = true
                        }
                        launchSingleTop = true
                        restoreState = true
                    }
                })
        }
    }
}
@Composable
fun OrderNowBottomBar(navController: NavHostController) {
    val navBackStackEntry by navController.currentBackStackEntryAsState()
    val currentDestination = navBackStackEntry?.destination
    BottomNavigation(
        backgroundColor = MaterialTheme.colors.background,
        contentColor = contentColorFor(MaterialTheme.colors.background),
        elevation = 10.dp
    ) {
        NavigationBarSection.sections.forEach { section ->
            val selected = currentDestination?.hierarchy?.any {
                it.route == section.route
            } == true
            BottomNavigationItem(icon = {
                Icon(
                    imageVector =  ImageVector.vectorResource(R.drawable.ic_launcher_foreground),
                    contentDescription = stringResource(id = R.string.app_name)
                )
            },
                label = { Text(text = stringResource(id =R.string.app_name)) },
                selected = selected,
                unselectedContentColor = Color.Gray,
                selectedContentColor = Color.Red,
                onClick = {
                    navController.navigate(section.route) {
                        popUpTo(navController.graph.findStartDestination().id) {
                            saveState = true
                        }
                        launchSingleTop = true
                        restoreState = true

                    }
                })
        }
    }

}
@Composable
fun OrderNowBottomBar(navController: NavHostController) {
    val navBackStackEntry by navController.currentBackStackEntryAsState()
    val currentDestination = navBackStackEntry?.destination
    BottomNavigation(
        backgroundColor = MaterialTheme.colors.background,
        contentColor = contentColorFor(MaterialTheme.colors.background),
        elevation = 10.dp
    ) {
        NavigationBarSection.sections.forEach { section ->
            val selected = currentDestination?.hierarchy?.any {
                it.route == section.route
            } == true
            BottomNavigationItem(icon = {
                Icon(
                    imageVector =  ImageVector.vectorResource(R.drawable.ic_launcher_foreground),
                    contentDescription = stringResource(id = R.string.app_name)
                )
            },
                label = { Text(text = stringResource(id =R.string.app_name)) },
                selected = selected,
                unselectedContentColor = Color.Gray,
                selectedContentColor = Color.Red,
                onClick = {
                    navController.navigate(section.route) {
                        popUpTo(navController.graph.findStartDestination().id) {
                            saveState = true
                        }
                        launchSingleTop = true
                        restoreState = true

                    }
                })
        }
    }

}

对于添加到 NavigationBarSection 中的每个项目，BottomBar 中都会显示一个选项。OrderNowBottomBar 的这个实现比之前的架构：主要结构中的实现更清晰。

然后，我们再次更新 OrderNowScreen 视图，如下所示：

@Preview
@Composable
fun OrderNowScreeen(){
    MyTestTheme(){
        Surface {
            val appState =rememberAppState()
            Scaffold(
                scaffoldState = appState.scaffoldState,
                topBar = {OrderNowTopBar()},
                bottomBar = {OrderNowBottomBar(appState.navController)}
            ){contentPadding ->
                OrderNowNavHost(appState,contentPadding)
            }
        }
    }
}
@Preview
@Composable
fun OrderNowScreeen(){
    MyTestTheme(){
        Surface {
            val appState =rememberAppState()
            Scaffold(
                scaffoldState = appState.scaffoldState,
                topBar = {OrderNowTopBar()},
                bottomBar = {OrderNowBottomBar(appState.navController)}
            ){contentPadding ->
                OrderNowNavHost(appState,contentPadding)
            }
        }
    }
}



@Preview
@Composable
fun OrderNowScreeen(){
    MyTestTheme(){
        Surface {
            val appState =rememberAppState()
            Scaffold(
                scaffoldState = appState.scaffoldState,
                topBar = {OrderNowTopBar()},
                bottomBar = {OrderNowBottomBar(appState.navController)}
            ){contentPadding ->
                OrderNowNavHost(appState,contentPadding)
            }
        }
    }
}

现在 OrderNowBottomBar 将需要引用负责导航的 navController。

在Scaffold的内容部分，添加了OrderNowNavHost的实例，该实例接收APP的一般状态作为参数。Order Now with simple navigation：

​编辑

从其他UI元素进行导航

 现在我们已经准备好从 BottomBar 选项导航的实现，我们需要定义从其他 UI 元素（例如按钮、链接、DeepLinks）甚至根据应用程序的其他内部组件的请求以编程方式导航应用程序。我们要做的第一个更改是添加一个名为 OrderNowNavigationState 的结构，它允许我们扩展应用程序的一般状态。

​编辑

OrderNowNavigationState，是APP通用状态的扩展，即一组用于导航目的的OrderNowState状态的扩展。我们还将使用此结构来集中取决于应用程序状态的导航逻辑。

​编辑

OrderNowNavigationState的实现如下:

fun OrderNowState.popUp(){
    navController.popBackStack()
}
fun OrderNowState.navigate(route:String){
    navController.navigate(route){
        launchSingleTop  = true
    }
}
fun OrderNowState.navigateAndPopUp(route:String,popUp:String){
    navController.navigate(route){
        launchSingleTop =true
        popUpTo(popUp){
            inclusive =true
        }
    }
}
fun OrderNowState.navigateSaved(route:String,popUp:String){
    navController.navigate(route){
        launchSingleTop =true
        restoreState =true
        popUpTo(popUp){
            saveState =true
        }
    }
}
fun OrderNowState.clearAndNavigate(route:String){
    navController.navigate(route){
        launchSingleTop =true
        popUpTo(0){ inclusive =true}
    }
}
fun OrderNowState.popUp(){
    navController.popBackStack()
}

fun OrderNowState.navigate(route:String){
    navController.navigate(route){
        launchSingleTop  = true
    }


}





fun OrderNowState.navigateAndPopUp(route:String,popUp:String){
    navController.navigate(route){
        launchSingleTop =true
        popUpTo(popUp){
            inclusive =true
        }
    }

}


fun OrderNowState.navigateSaved(route:String,popUp:String){
    navController.navigate(route){
        launchSingleTop =true
        restoreState =true
        popUpTo(popUp){
            saveState =true
        }
    }
}

fun OrderNowState.clearAndNavigate(route:String){
    navController.navigate(route){
        launchSingleTop =true
        popUpTo(0){ inclusive =true}
    }

}



fun OrderNowState.popUp(){
    navController.popBackStack()
}

fun OrderNowState.navigate(route:String){
    navController.navigate(route){
        launchSingleTop  = true
    }


}





fun OrderNowState.navigateAndPopUp(route:String,popUp:String){
    navController.navigate(route){
        launchSingleTop =true
        popUpTo(popUp){
            inclusive =true
        }
    }

}


fun OrderNowState.navigateSaved(route:String,popUp:String){
    navController.navigate(route){
        launchSingleTop =true
        restoreState =true
        popUpTo(popUp){
            saveState =true
        }
    }
}

fun OrderNowState.clearAndNavigate(route:String){
    navController.navigate(route){
        launchSingleTop =true
        popUpTo(0){ inclusive =true}
    }

}

对Home、ProductList和ProductDetail屏幕做了一些更改，如下图所示:

​编辑

 以in – home视图为例，导航动作通过Button执行，方式如下:

@Composable
fun HomeScreen(goToProductList:()->Unit,modifier:Modifier=Modifier,viewModel2:HomeViewModel =hiltViewModel()){
    Column(){
        Button(onClick = goToProductList,){
            Text(text = stringResource(id = R.string.app_name))
        }
    }
}
@Composable

fun HomeScreen(goToProductList:()->Unit,modifier:Modifier=Modifier,viewModel2:HomeViewModel =hiltViewModel()){
    Column(){
        Button(onClick = goToProductList,){
            Text(text = stringResource(id = R.string.app_name))



        }
    }


}



@Composable

fun HomeScreen(goToProductList:()->Unit,modifier:Modifier=Modifier,viewModel2:HomeViewModel =hiltViewModel()){
    Column(){
        Button(onClick = goToProductList,){
            Text(text = stringResource(id = R.string.app_name))



        }
    }


}

代码的重要部分是“Go to —> ProductList Screen”按钮的动作 onClick = goToProductList 的定义，其中通过设计原理章节中解释的状态提升技术，我们将动作 goToProductList 委托给在 OrderNowNavHost 中定义的 appSoGraph 导航图如下：

un NavGraphBuilder.appSoGraph1(appState:OrderNowState){
    val goToListFromHome:()->Unit ={
        appState.navigateSaved(OrderNowScreenRoute.ProductList.route,OrderNowScreenRoute.Home.route)
    }
    composable(NavigationBarSection.Home.route){
        HomeScreen(goToProductList =goToListFromHome)
    }
}
un NavGraphBuilder.appSoGraph1(appState:OrderNowState){
    val goToListFromHome:()->Unit ={
        appState.navigateSaved(OrderNowScreenRoute.ProductList.route,OrderNowScreenRoute.Home.route)
    }
    composable(NavigationBarSection.Home.route){
        HomeScreen(goToProductList =goToListFromHome)
    }
}
un NavGraphBuilder.appSoGraph1(appState:OrderNowState){
    val goToListFromHome:()->Unit ={
        appState.navigateSaved(OrderNowScreenRoute.ProductList.route,OrderNowScreenRoute.Home.route)
    }
    composable(NavigationBarSection.Home.route){
        HomeScreen(goToProductList =goToListFromHome)
    }
}

回想一下，navigatesave函数是OrderNowNavigationState结构中定义的扩展的一部分。同样的实现应用于导航到其他ProductList和ProductDetail屏幕，这样在OrderNowNavHost中的性能如下所示:

fun  NavGraphBuilder.appSoGraph2(appState:OrderNowState){
    val homeRoute= OrderNowScreenRoute.Home.route
    val listRoute= OrderNowScreenRoute.ProductList.route
    val detailRoute =OrderNowScreenRoute.ProductDetail.route
    
    val goToListFromHome:()->Unit ={
        appState.navigateSaved(listRoute,homeRoute)
    }
    val goToDetailFromList:()->Unit ={
        appState.navigateSaved(detailRoute,listRoute)
    }
    val goBack:()->Unit ={
        appState.popUp()
    }
    composable(NavigationBarSection.Home.route){
        HomeScreen(goToProductList =goToListFromHome)
    }
    composable(NavigationBarSection.ProductList.route){
        ProductListScreen(goToProductDetail =goToDetailFromList,goBack =goBack)
    }
    composable(NavigationBarSection.ProductDetail.route){
        ProductDetailScreen(goBack =goBack)
    }
    
}
fun  NavGraphBuilder.appSoGraph2(appState:OrderNowState){
    val homeRoute= OrderNowScreenRoute.Home.route
    val listRoute= OrderNowScreenRoute.ProductList.route
    val detailRoute =OrderNowScreenRoute.ProductDetail.route
    
    val goToListFromHome:()->Unit ={
        appState.navigateSaved(listRoute,homeRoute)
    }
    val goToDetailFromList:()->Unit ={
        appState.navigateSaved(detailRoute,listRoute)
    }
    val goBack:()->Unit ={
        appState.popUp()
    }
    composable(NavigationBarSection.Home.route){
        HomeScreen(goToProductList =goToListFromHome)
    }
    composable(NavigationBarSection.ProductList.route){
        ProductListScreen(goToProductDetail =goToDetailFromList,goBack =goBack)
    }
    composable(NavigationBarSection.ProductDetail.route){
        ProductDetailScreen(goBack =goBack)
    }
    
}
fun  NavGraphBuilder.appSoGraph2(appState:OrderNowState){
    val homeRoute= OrderNowScreenRoute.Home.route
    val listRoute= OrderNowScreenRoute.ProductList.route
    val detailRoute =OrderNowScreenRoute.ProductDetail.route
    
    val goToListFromHome:()->Unit ={
        appState.navigateSaved(listRoute,homeRoute)
    }
    val goToDetailFromList:()->Unit ={
        appState.navigateSaved(detailRoute,listRoute)
    }
    val goBack:()->Unit ={
        appState.popUp()
    }
    composable(NavigationBarSection.Home.route){
        HomeScreen(goToProductList =goToListFromHome)
    }
    composable(NavigationBarSection.ProductList.route){
        ProductListScreen(goToProductDetail =goToDetailFromList,goBack =goBack)
    }
    composable(NavigationBarSection.ProductDetail.route){
        ProductDetailScreen(goBack =goBack)
    }
    
}

总结起来，前面的代码段中配置了以下导航定义（导航图）：

  1. 从主页（Home screen）导航到产品列表（ProductList screen）。
  2. 从产品列表（ProductList screen）导航到产品详情（ProductDetail screen）。
  3. 从产品详情（ProductDetail screen）导航回到上一个呈现的屏幕。
  4. 从底部导航栏菜单导航到主页（Home screen）。
  5. 从底部导航栏菜单导航到购物车（Cart screen）。

到此为止，我们在 OrderNow 应用中已经实现了一个很好的基本导航。然而，还有一些东西缺失。 您有什么想法可能是缺失的吗？ 我们需要在导航中包含数据或信息的传递。 由于在本章中设计了相关的实现，因此很容易实现数据传递。我们将在最后“实现功能”中详细探讨。

总结

本章中，我们完成了应用程序的主要部分的设计，这些将是以后添加功能的基础。 我们知道导航是应用程序的重要组成部分，必须从应用程序设计的开始就考虑它。 在本章中，我们采用了谷歌的建议，将状态纳入导航逻辑中。 同时，读者可以注意到我们的策略中没有直接涉及 ViewModel 或 View，这使得它在测试时更加灵活，可以检查导航。

​

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# Android
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