拥有一个保持状态的适当回栈的关键是拥有NavHostFragment具有 childFragmentManager 和它们自己的回栈的 's。Navigation 组件的高级示例的扩展文件实际上就是这样做的。
/**
* Manages the various graphs needed for a [BottomNavigationView].
*
* This sample is a workaround until the Navigation Component supports multiple back stacks.
*/
fun BottomNavigationView.setupWithNavController(
navGraphIds: List<Int>,
fragmentManager: FragmentManager,
containerId: Int,
intent: Intent
): LiveData<NavController> {
// Map of tags
val graphIdToTagMap = SparseArray<String>()
// Result. Mutable live data with the selected controlled
val selectedNavController = MutableLiveData<NavController>()
var firstFragmentGraphId = 0
// First create a NavHostFragment for each NavGraph ID
navGraphIds.forEachIndexed { index, navGraphId ->
val fragmentTag = getFragmentTag(index)
// Find or create the Navigation host fragment
val navHostFragment = obtainNavHostFragment(
fragmentManager,
fragmentTag,
navGraphId,
containerId
)
// Obtain its id
val graphId = navHostFragment.navController.graph.id
if (index == 0) {
firstFragmentGraphId = graphId
}
// Save to the map
graphIdToTagMap[graphId] = fragmentTag
// Attach or detach nav host fragment depending on whether it's the selected item.
if (this.selectedItemId == graphId) {
// Update livedata with the selected graph
selectedNavController.value = navHostFragment.navController
attachNavHostFragment(fragmentManager, navHostFragment, index == 0)
} else {
detachNavHostFragment(fragmentManager, navHostFragment)
}
}
// Now connect selecting an item with swapping Fragments
var selectedItemTag = graphIdToTagMap[this.selectedItemId]
val firstFragmentTag = graphIdToTagMap[firstFragmentGraphId]
var isOnFirstFragment = selectedItemTag == firstFragmentTag
// When a navigation item is selected
setOnNavigationItemSelectedListener { item ->
// Don't do anything if the state is state has already been saved.
if (fragmentManager.isStateSaved) {
false
} else {
val newlySelectedItemTag = graphIdToTagMap[item.itemId]
if (selectedItemTag != newlySelectedItemTag) {
// Pop everything above the first fragment (the "fixed start destination")
fragmentManager.popBackStack(
firstFragmentTag,
FragmentManager.POP_BACK_STACK_INCLUSIVE
)
val selectedFragment = fragmentManager.findFragmentByTag(newlySelectedItemTag)
as NavHostFragment
// Exclude the first fragment tag because it's always in the back stack.
if (firstFragmentTag != newlySelectedItemTag) {
// Commit a transaction that cleans the back stack and adds the first fragment
// to it, creating the fixed started destination.
fragmentManager.beginTransaction()
.attach(selectedFragment)
.setPrimaryNavigationFragment(selectedFragment)
.apply {
// Detach all other Fragments
graphIdToTagMap.forEach { _, fragmentTagIter ->
if (fragmentTagIter != newlySelectedItemTag) {
detach(fragmentManager.findFragmentByTag(firstFragmentTag)!!)
}
}
}
.addToBackStack(firstFragmentTag)
.setCustomAnimations(
R.anim.nav_default_enter_anim,
R.anim.nav_default_exit_anim,
R.anim.nav_default_pop_enter_anim,
R.anim.nav_default_pop_exit_anim
)
.setReorderingAllowed(true)
.commit()
}
selectedItemTag = newlySelectedItemTag
isOnFirstFragment = selectedItemTag == firstFragmentTag
selectedNavController.value = selectedFragment.navController
true
} else {
false
}
}
}
// Optional: on item reselected, pop back stack to the destination of the graph
setupItemReselected(graphIdToTagMap, fragmentManager)
// Handle deep link
setupDeepLinks(navGraphIds, fragmentManager, containerId, intent)
// Finally, ensure that we update our BottomNavigationView when the back stack changes
fragmentManager.addOnBackStackChangedListener {
if (!isOnFirstFragment && !fragmentManager.isOnBackStack(firstFragmentTag)) {
this.selectedItemId = firstFragmentGraphId
}
// Reset the graph if the currentDestination is not valid (happens when the back
// stack is popped after using the back button).
selectedNavController.value?.let { controller ->
if (controller.currentDestination == null) {
controller.navigate(controller.graph.id)
}
}
}
return selectedNavController
}
private fun BottomNavigationView.setupDeepLinks(
navGraphIds: List<Int>,
fragmentManager: FragmentManager,
containerId: Int,
intent: Intent
) {
navGraphIds.forEachIndexed { index, navGraphId ->
val fragmentTag = getFragmentTag(index)
// Find or create the Navigation host fragment
val navHostFragment = obtainNavHostFragment(
fragmentManager,
fragmentTag,
navGraphId,
containerId
)
// Handle Intent
if (navHostFragment.navController.handleDeepLink(intent)
&& selectedItemId != navHostFragment.navController.graph.id
) {
this.selectedItemId = navHostFragment.navController.graph.id
}
}
}
private fun BottomNavigationView.setupItemReselected(
graphIdToTagMap: SparseArray<String>,
fragmentManager: FragmentManager
) {
setOnNavigationItemReselectedListener { item ->
val newlySelectedItemTag = graphIdToTagMap[item.itemId]
val selectedFragment = fragmentManager.findFragmentByTag(newlySelectedItemTag)
as NavHostFragment
val navController = selectedFragment.navController
// Pop the back stack to the start destination of the current navController graph
navController.popBackStack(
navController.graph.startDestination, false
)
}
}
private fun detachNavHostFragment(
fragmentManager: FragmentManager,
navHostFragment: NavHostFragment
) {
fragmentManager.beginTransaction()
.detach(navHostFragment)
.commitNow()
}
private fun attachNavHostFragment(
fragmentManager: FragmentManager,
navHostFragment: NavHostFragment,
isPrimaryNavFragment: Boolean
) {
fragmentManager.beginTransaction()
.attach(navHostFragment)
.apply {
if (isPrimaryNavFragment) {
setPrimaryNavigationFragment(navHostFragment)
}
}
.commitNow()
}
private fun obtainNavHostFragment(
fragmentManager: FragmentManager,
fragmentTag: String,
navGraphId: Int,
containerId: Int
): NavHostFragment {
// If the Nav Host fragment exists, return it
val existingFragment = fragmentManager.findFragmentByTag(fragmentTag) as NavHostFragment?
existingFragment?.let { return it }
// Otherwise, create it and return it.
val navHostFragment = NavHostFragment.create(navGraphId)
fragmentManager.beginTransaction()
.add(containerId, navHostFragment, fragmentTag)
.commitNow()
return navHostFragment
}
private fun FragmentManager.isOnBackStack(backStackName: String): Boolean {
val backStackCount = backStackEntryCount
for (index in 0 until backStackCount) {
if (getBackStackEntryAt(index).name == backStackName) {
return true
}
}
return false
}
private fun getFragmentTag(index: Int) = "bottomNavigation#$index"
这里的重要部分是如果NavHostFragment 在后台堆栈中不存在，则使用上述函数获取并将其添加到后台堆栈。commitNow是同步的，不像commit
private fun acquireNavHostFragment( fragmentManager: FragmentManager, fragmentTag: String, navGraphId: Int, containerId: Int ): NavHostFragment { // 如果导航主机片段存在，则返回 val existingFragment = fragmentManager.findFragmentByTag(fragmentTag) as NavHostFragment? existingFragment?.let { return it }
// Otherwise, create it and return it.
val navHostFragment = NavHostFragment.create(navGraphId)
fragmentManager.beginTransaction()
.add(containerId, navHostFragment, fragmentTag)
.commitNow()
return navHostFragment
}
我使用上面的 NavigationExtension 构建了一个，看起来像这样
带有嵌套导航。
导航图类似，所以我只添加一个
nav_graph_home.xml
<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/nav_graph_home"
app:startDestination="@id/homeFragment1">
<fragment
android:id="@+id/homeFragment1"
android:name="com.smarttoolfactory.tutorial5_3navigationui_bottomnavigation_nestednavigation.blankfragment.HomeFragment1"
android:label="HomeFragment1"
tools:layout="@layout/fragment_home1">
<action
android:id="@+id/action_homeFragment1_to_homeFragment2"
app:destination="@id/homeFragment2" />
</fragment>
<fragment
android:id="@+id/homeFragment2"
android:name="com.smarttoolfactory.tutorial5_3navigationui_bottomnavigation_nestednavigation.blankfragment.HomeFragment2"
android:label="HomeFragment2"
tools:layout="@layout/fragment_home2">
<action
android:id="@+id/action_homeFragment2_to_homeFragment3"
app:destination="@id/homeFragment3" />
</fragment>
<fragment
android:id="@+id/homeFragment3"
android:name="com.smarttoolfactory.tutorial5_3navigationui_bottomnavigation_nestednavigation.blankfragment.HomeFragment3"
android:label="HomeFragment3"
tools:layout="@layout/fragment_home3" >
<action
android:id="@+id/action_homeFragment3_to_homeFragment1"
app:destination="@id/homeFragment1"
app:popUpTo="@id/homeFragment1"
app:popUpToInclusive="true" />
</fragment>
</navigation>
底部导航菜单
menu_bottom_nav.xml
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item
android:id="@+id/nav_graph_home"
android:icon="@drawable/ic_baseline_home_24"
android:title="Home"/>
<item
android:id="@+id/nav_graph_dashboard"
android:icon="@drawable/ic_baseline_dashboard_24"
android:title="Dashboard"/>
<item
android:id="@+id/nav_graph_notification"
android:icon="@drawable/ic_baseline_notifications_24"
android:title="Notification"/>
</menu>
MainActivity 的布局，包含FragmentContainerView和BottomNavigationView
active_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<androidx.fragment.app.FragmentContainerView
android:id="@+id/nav_host_container"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toTopOf="@+id/bottom_nav"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/bottom_nav"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:menu="@menu/menu_bottom_nav" />
</androidx.constraintlayout.widget.ConstraintLayout>
MainActivity.kt
class MainActivity : AppCompatActivity() {
private var currentNavController: LiveData<NavController>? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
supportFragmentManager.addOnBackStackChangedListener {
val backStackEntryCount = supportFragmentManager.backStackEntryCount
val fragments = supportFragmentManager.fragments
val fragmentCount = fragments.size
Toast.makeText(
this,
"MainActivity backStackEntryCount: $backStackEntryCount, fragmentCount: $fragmentCount, fragments: $fragments",
Toast.LENGTH_SHORT
).show()
}
if (savedInstanceState == null) {
setupBottomNavigationBar()
} // Else, need to wait for onRestoreInstanceState
}
override fun onRestoreInstanceState(savedInstanceState: Bundle?) {
super.onRestoreInstanceState(savedInstanceState)
// Now that BottomNavigationBar has restored its instance state
// and its selectedItemId, we can proceed with setting up the
// BottomNavigationBar with Navigation
setupBottomNavigationBar()
}
/**
* Called on first creation and when restoring state.
*/
private fun setupBottomNavigationBar() {
val bottomNavigationView = findViewById<BottomNavigationView>(R.id.bottom_nav)
val navGraphIds = listOf(
R.navigation.nav_graph_home,
R.navigation.nav_graph_dashboard,
R.navigation.nav_graph_notification
)
// Setup the bottom navigation view with a list of navigation graphs
val controller = bottomNavigationView.setupWithNavController(
navGraphIds = navGraphIds,
fragmentManager = supportFragmentManager,
containerId = R.id.nav_host_container,
intent = intent
)
// Whenever the selected controller changes, setup the action bar.
controller.observe(this, Observer { navController ->
setupActionBarWithNavController(navController)
})
currentNavController = controller
}
override fun onSupportNavigateUp(): Boolean {
return currentNavController?.value?.navigateUp() ?: false
}
}
片段布局和类是简单的类，所以我跳过了它们。您可以查看我构建的完整示例，或Google 的存储库来检查高级导航或其他示例的扩展。
