做主页导航时会用到底部导航栏，Jetpack Compose提供了基础槽位的布局Scaffold，使用Scaffold可以构建底部导航栏，例如：
@Composable
fun Greeting(vm: VM) {
val list = listOf("One", "Two", "Three")
var selectedItem = remember {
mutableStateOf(0)
}
val navController = rememberNavController()
Scaffold(bottomBar = {
state.takeIf { it.value }?.let {
BottomNavigation {
list.forEachIndexed { index, label ->
BottomNavigationItem(
label = { Text(text = label) },
selected = index == selectedItem.value,
onClick = { selectedItem.value = index },
icon = {
Icon(
painter = painterResource(id = R.drawable.ic_launcher_foreground),
contentDescription = null
)
})
}
}
}
}) {
NavHost(navController = navController, startDestination = "one") {
composable(route = "one") { PageList(navController, vm) }
composable(route = "detail") { PageDetail(vm) }
}
}
}
这是一个最简单的Scaffold，其主页时PageList，显示一列数字，点击数字后会跳转到PageDetail页面。
但是有个很大的问题，就是在跳转到PageDetail页面之后，BottomNavigation并没有随之消失，于是乎出现了这样一个奇怪的现象：
二更。
其实核心思路是判断最新的页面是不是包含Scaffold的页面（即MainPage），用那种方式判断并不重要。
今天又发现一种新的判断思路，不用ViewModel，直接用全局栈保存页面跳转即可，而且只需让栈底保存MainPage的标志位即可，跳转目的地的标志位不重要。
```
object NavUtil {
private val routeStack: Stack<String> = Stack()
private var mainRoutes = listOf<String>("Main")
fun setMainRoutes(list: List<String>) {
mainRoutes = list
}
init {
routeStack.push("Main")
}
fun isMain(): Boolean {
Log.d("TEST", "routeStack: ${routeStack.empty()}")
val result = if (routeStack.empty()) {
true
} else {
Log.d("TEST", "peek: ${routeStack.peek()}")
mainRoutes.contains(routeStack.peek())
}
return result
}
fun pushRoute() {
routeStack.push("destination")
}
fun popRoute() {
if (!routeStack.empty()) {
routeStack.pop()
}
}
}
@ExperimentalPagerApi
@Composable
fun MainView(navController: NavHostController) {
LaunchedEffect(key1 = true) {
NavUtil.popRoute()
}
val data = listOf("One", "Two", "Three")
val scope = rememberCoroutineScope()
val pagerState = rememberPagerState(pageCount = 3)
val bottomNavSelectedState = remember { mutableStateOf(0) }
Scaffold(bottomBar = {
var status = NavUtil.isMain()
Log.d("TEST", "status: ${status}")
if (status) {
BottomNavigation { }
}
}) {
HorizontalPager(state = pagerState) {
PageNav(navController = navController, title = data[pagerState.currentPage])
}
}
DisposableEffect(key1 = true) {
onDispose {
NavUtil.pushRoute()
}
}
}
```
为了解决这个问题，可以采用State去控制BottomNavigation的可见性，并将其保存在ViewModel中。
具体做法是：
1.在ViewModel中创建一个包含Boolean值的LiveData变量state。当state为true时绘制BottomNavigation，为false时不绘制
2.在包含Scaffold页面中监听state，并控制BottomNavigation的可见性。
3.在PageList（也就是Scaffold导航的主页）进入时设置state为true、退出时设置state为false
// ViewModel
class VM: ViewModel() {
private val _state: MutableLiveData<Boolean> = MutableLiveData(true)
val state: LiveData<Boolean> get() = _state
fun setState(status: Boolean) {
_state.postValue(status)
}
}
// MainPage
@Compose MainPage(vm: VM) {
LaunchedEffect(key1 = true) {
vm.setState(true)
}
DisposableEffect(key1 = true) {
onDispose {
vm.setState(false)
}
}
}
// page contains Scaffold
@Composable
fun Greeting(vm: VM) {
// State of BottomNavigation`s visibility
val state = remember { mutableStateOf<Boolean>(true) }
// read the BottomNavigation`s visibility from ViewModel and send to State
vm.state.observeAsState().value?.let { state.value = it }
Scaffold(bottomBar = {
// show / hide BottomNavigation controlled by State
state.takeIf { it.value }?.let {
BottomNavigation {
list.forEachIndexed { index, label ->
BottomNavigationItem(
label = { Text(text = label) },
selected = index == selectedItem.value,
onClick = { selectedItem.value = index },
icon = {
Icon(
painter = painterResource(id = R.drawable.ic_launcher_foreground),
contentDescription = null
)
})
}
}
}
}) {
NavHost(navController = navController, startDestination = "one") {
composable(route = "one") { PageList(navController, vm) }
composable(route = "detail") { PageDetail(vm) }
}
}
}
这种做法的好处是简单，侵入性低，无需修改系统api也无需自定义view。缺点就是麻烦，需要在导航中的每个主页都进行设置。
我在StackOverflow上提问时有人回答了另一个办法。这个办法是给每个屏幕添加标志位，来区分是否是导航的主页，之后再创建BottomNavigation时进行判断。贴一下：
You need to specify which screens you want to show and which screens you dont want; Otherwise it will show to all the screens inside Scaffold's body (which you have bottomBar). The code below was from my app.
Create a state which observes any destination changes on the navController
Inside when you can put any screens that you want to show navigationBar else just set currentScreen to NoBottomBar
@Composable
private fun NavController.currentScreen(): State<MainSubScreen> {
val currentScreen = remember { mutableStateOf<MainSubScreen>(MainSubScreen.Home) }
DisposableEffect(key1 = this) {
val listener = NavController.OnDestinationChangedListener { _, destination, _ ->
when {
destination.hierarchy.any { it.route == MainSubScreen.Home.route } -> {
currentScreen.value = MainSubScreen.Home
} else -> currentScreen.value = MainSubScreen.NoBottomBar
}
}
addOnDestinationChangedListener(listener)
}
return currentScreen
}
On the Scaffold where you put ur bottomBar
so you can check if currentScreen was NoBottomBar if it was, don't show it
// initialized currentScreeen above
val currentScreen by navController.currentScreen()
Scaffold(
bottomBar = {
if (currentScreen != MainSubScreen.NoBottomBar) {
MainBottomNavigation()
} else Unit
}
) {
// Your screen
}