Android Jetpack导航,使用Youtube或Instagram的BottomNavigationView,如正确的后退导航(片段后退堆栈)？

Question

Android Jetpack导航,BottomNavigationView在后退按钮上点击自动片段后栈？

我想要的是,在用户一个接一个地选择多个标签后,用户点击后退按钮应用必须重定向到他/她打开的最后一页.

通过在ArrayList中保存当前选定的项目,我使用Android ViewPager实现了相同的功能.Android Jetpack导航发布后是否有任何自动后退堆栈？我想用导航图实现它

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".main.MainActivity">

    <fragment
        android:id="@+id/my_nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@+id/navigation"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation" />

</android.support.constraint.ConstraintLayout>

navigation.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <item
        android:id="@+id/navigation_home"
        android:icon="@drawable/ic_home"
        android:title="@string/title_home" />

    <item
        android:id="@+id/navigation_people"
        android:icon="@drawable/ic_group"
        android:title="@string/title_people" />

    <item
        android:id="@+id/navigation_organization"
        android:icon="@drawable/ic_organization"
        android:title="@string/title_organization" />

    <item
        android:id="@+id/navigation_business"
        android:icon="@drawable/ic_business"
        android:title="@string/title_business" />

    <item
        android:id="@+id/navigation_tasks"
        android:icon="@drawable/ic_dashboard"
        android:title="@string/title_tasks" />

</menu>

还补充道

bottomNavigation.setupWithNavController(Navigation.findNavController(this, R.id.my_nav_host_fragment))

我得到了一个答案Levi Moreira,如下

navigation.setOnNavigationItemSelectedListener {item ->

            onNavDestinationSelected(item, Navigation.findNavController(this, R.id.my_nav_host_fragment))

        }

但通过这样做只发生了最后打开的片段的实例再次创建.

为BottomNavigationView提供适当的后向导航

Answer 1

你并不真的需要ViewPager一起工作BottomNavigation和新的导航架构组件.我一直在使用一个完全使用这两个的示例应用程序,请参阅此处.

基本的概念就是这个,你有主要的活动来托管BottomNavigationView导航图的导航主机,这就是它的xml的样子:

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".main.MainActivity">

    <fragment
        android:id="@+id/my_nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@+id/navigation"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation" />

</android.support.constraint.ConstraintLayout>

导航菜单(标签菜单)BottomNavigationView如下所示:

navigation.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <item
        android:id="@+id/navigation_home"
        android:icon="@drawable/ic_home"
        android:title="@string/title_home" />

    <item
        android:id="@+id/navigation_people"
        android:icon="@drawable/ic_group"
        android:title="@string/title_people" />

    <item
        android:id="@+id/navigation_organization"
        android:icon="@drawable/ic_organization"
        android:title="@string/title_organization" />

    <item
        android:id="@+id/navigation_business"
        android:icon="@drawable/ic_business"
        android:title="@string/title_business" />

    <item
        android:id="@+id/navigation_tasks"
        android:icon="@drawable/ic_dashboard"
        android:title="@string/title_tasks" />

</menu>

所有这些只是BottomNavigationView设置.现在要使它与导航拱组件一起工作,你需要进入导航图编辑器,添加所有的片段目的地(在我的情况下,我有5个,每个选项卡一个),并设置目的地的ID相同将name命名为navigation.xml文件中的名称:

这将告诉android在选项卡和片段之间建立链接,现在每次用户单击"主页"选项卡时,android将负责加载正确的片段.还有一段kotlin代码需要添加到你的NavHost(主要活动)来连接以下内容BottomNavigationView:

您需要添加onCreate:

bottomNavigation.setupWithNavController(Navigation.findNavController(this, R.id.my_nav_host_fragment))

这告诉android在导航架构组件和BottomNavigationView之间进行连接.在文档中查看更多内容.

要获得使用youtube时的相同行为,只需添加:

navigation.setOnNavigationItemSelectedListener {item ->

            onNavDestinationSelected(item, Navigation.findNavController(this, R.id.my_nav_host_fragment))

        }

这将使目的地进入后台,因此当您点击后退按钮时,将弹出最后访问的目的地.

Answer 2

你必须像xml下面设置主机导航:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@color/colorPrimary" />

    <fragment
        android:id="@+id/navigation_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        app:defaultNavHost="true"
        app:navGraph="@navigation/nav_graph" />

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/bottom_navigation_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:itemIconTint="@drawable/color_state_list"
        app:itemTextColor="@drawable/color_state_list"
        app:menu="@menu/menu_bottom_navigation" />
</LinearLayout>

使用导航控制器设置:

NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.navigation_host_fragment);
NavigationUI.setupWithNavController(bottomNavigationView, navHostFragment.getNavController());

menu_bottom_navigation.xml:

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@id/tab1"  // Id of navigation graph 
        android:icon="@mipmap/ic_launcher"
        android:title="@string/tab1" />
    <item
        android:id="@id/tab2" // Id of navigation graph
        android:icon="@mipmap/ic_launcher"
        android:title="@string/tab2" />

    <item
        android:id="@id/tab3" // Id of navigation graph
        android:icon="@mipmap/ic_launcher"
        android:title="@string/tab3" />
</menu>

nav_graph.xml:

<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"
    app:startDestination="@id/tab1">
    <fragment
        android:id="@+id/tab1"
        android:name="com.navigationsample.Tab1Fragment"
        android:label="@string/tab1"
        tools:layout="@layout/fragment_tab_1" />

    <fragment
        android:id="@+id/tab2"
        android:name="com.navigationsample.Tab2Fragment"
        android:label="@string/tab2"
        tools:layout="@layout/fragment_tab_2"/>

    <fragment
        android:id="@+id/tab3"
        android:name="com.simform.navigationsample.Tab3Fragment"
        android:label="@string/tab3"
        tools:layout="@layout/fragment_tab_3"/>
</navigation>

通过将"nav_graph"的相同ID设置为"menu_bottom_navigation"将处理底部导航的单击.

您可以使用标记中的popUpTo属性处理后退操作action.

Answer 3

首先，让我澄清一下Youtube和Instagram如何处理片段导航。

• 当用户使用详细片段时，向后或向上弹出堆栈一次，并正确恢复状态。再次单击已选择的底部栏项目，将所有堆栈弹出到根目录，刷新它
• 当用户使用根片段时，返回到底部栏上选择的最后一个菜单，显示最后一个细节片段，并正确恢复状态（JetPack不会）
• 当用户位于起始目标片段上时，后退完成活动

上面的其他答案都无法使用jetpack导航解决所有这些问题。

JetPack导航没有执行此操作的标准方法，我发现更简单的方法是将每个底部导航项的导航xml图分为一个，使用活动FragmentManager和JetPack NavController自己处理导航项之间的反向堆栈处理根片段和细节片段之间的内部导航（其实现使用childFragmentManager堆栈）。

假设您的navigation文件夹中有以下3个xml：

res/navigation/
    navigation_feed.xml
    navigation_explore.xml
    navigation_profile.xml

将导航xml中的destinationId与bottomNavigationBar菜单ID相同。另外，对每个xml都将设置为app:startDestination您想要作为导航项目根目录的片段。

创建一个类BottomNavController.kt：

class BottomNavController(
        val context: Context,
        @IdRes val containerId: Int,
        @IdRes val appStartDestinationId: Int
) {
    private val navigationBackStack = BackStack.of(appStartDestinationId)
    lateinit var activity: Activity
    lateinit var fragmentManager: FragmentManager
    private var listener: OnNavigationItemChanged? = null
    private var navGraphProvider: NavGraphProvider? = null

    interface OnNavigationItemChanged {
        fun onItemChanged(itemId: Int)
    }

    interface NavGraphProvider {
        @NavigationRes
        fun getNavGraphId(itemId: Int): Int
    }

    init {
        var ctx = context
        while (ctx is ContextWrapper) {
            if (ctx is Activity) {
                activity = ctx
                fragmentManager = (activity as FragmentActivity).supportFragmentManager
                break
            }
            ctx = ctx.baseContext
        }
    }

    fun setOnItemNavigationChanged(listener: (itemId: Int) -> Unit) {
        this.listener = object : OnNavigationItemChanged {
            override fun onItemChanged(itemId: Int) {
                listener.invoke(itemId)
            }
        }
    }

    fun setNavGraphProvider(provider: NavGraphProvider) {
        navGraphProvider = provider
    }

    fun onNavigationItemReselected(item: MenuItem) {
        // If the user press a second time the navigation button, we pop the back stack to the root
        activity.findNavController(containerId).popBackStack(item.itemId, false)
    }

    fun onNavigationItemSelected(itemId: Int = navigationBackStack.last()): Boolean {

        // Replace fragment representing a navigation item
        val fragment = fragmentManager.findFragmentByTag(itemId.toString())
                ?: NavHostFragment.create(navGraphProvider?.getNavGraphId(itemId)
                        ?: throw RuntimeException("You need to set up a NavGraphProvider with " +
                                "BottomNavController#setNavGraphProvider")
                )
        fragmentManager.beginTransaction()
                .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
                )
                .replace(containerId, fragment, itemId.toString())
                .addToBackStack(null)
                .commit()

        // Add to back stack
        navigationBackStack.moveLast(itemId)

        listener?.onItemChanged(itemId)

        return true
    }

    fun onBackPressed() {
        val childFragmentManager = fragmentManager.findFragmentById(containerId)!!
                .childFragmentManager
        when {
            // We should always try to go back on the child fragment manager stack before going to
            // the navigation stack. It's important to use the child fragment manager instead of the
            // NavController because if the user change tabs super fast commit of the
            // supportFragmentManager may mess up with the NavController child fragment manager back
            // stack
            childFragmentManager.popBackStackImmediate() -> {
            }
            // Fragment back stack is empty so try to go back on the navigation stack
            navigationBackStack.size > 1 -> {
                // Remove last item from back stack
                navigationBackStack.removeLast()

                // Update the container with new fragment
                onNavigationItemSelected()
            }
            // If the stack has only one and it's not the navigation home we should
            // ensure that the application always leave from startDestination
            navigationBackStack.last() != appStartDestinationId -> {
                navigationBackStack.removeLast()
                navigationBackStack.add(0, appStartDestinationId)
                onNavigationItemSelected()
            }
            // Navigation stack is empty, so finish the activity
            else -> activity.finish()
        }
    }

    private class BackStack : ArrayList<Int>() {
        companion object {
            fun of(vararg elements: Int): BackStack {
                val b = BackStack()
                b.addAll(elements.toTypedArray())
                return b
            }
        }

        fun removeLast() = removeAt(size - 1)
        fun moveLast(item: Int) {
            remove(item)
            add(item)
        }
    }
}

// Convenience extension to set up the navigation
fun BottomNavigationView.setUpNavigation(bottomNavController: BottomNavController, onReselect: ((menuItem: MenuItem) -> Unit)? = null) {
    setOnNavigationItemSelectedListener {
        bottomNavController.onNavigationItemSelected(it.itemId)
    }
    setOnNavigationItemReselectedListener {
        bottomNavController.onNavigationItemReselected(it)
        onReselect?.invoke(it)
    }
    bottomNavController.setOnItemNavigationChanged { itemId ->
        menu.findItem(itemId).isChecked = true
    }
}

做你这样的布局main.xml：

<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">

    <FrameLayout
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@id/bottomNavigationView"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomNavigationView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:menu="@menu/navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

像这样在您的活动中使用：

class MainActivity : AppCompatActivity(),
        BottomNavController.NavGraphProvider  {

    private val navController by lazy(LazyThreadSafetyMode.NONE) {
        Navigation.findNavController(this, R.id.container)
    }

    private val bottomNavController by lazy(LazyThreadSafetyMode.NONE) {
        BottomNavController(this, R.id.container, R.id.navigation_feed)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.main)

        bottomNavController.setNavGraphProvider(this)
        bottomNavigationView.setUpNavigation(bottomNavController)
        if (savedInstanceState == null) bottomNavController
                .onNavigationItemSelected()

        // do your things...
    }

    override fun getNavGraphId(itemId: Int) = when (itemId) {
        R.id.navigation_feed -> R.navigation.navigation_feed
        R.id.navigation_explore -> R.navigation.navigation_explore
        R.id.navigation_profile -> R.navigation.navigation_profile
        else -> R.navigation.navigation_feed
    }

    override fun onSupportNavigateUp(): Boolean = navController
            .navigateUp()

    override fun onBackPressed() = bottomNavController.onBackPressed()
}

Answer 4

您可以使用底部导航视图设置viewpager.viewpager中的每个片段都是一个容器片段,它将具有带有自己的backstack的子片段.您可以通过这种方式为viewpager中的每个选项卡维护backstack

Answer 5

有了导航包2.4.0版本，终于正式支持了！

https://developer.android.com/jetpack/androidx/releases/navigation#version_240_2

不仅如此：将导航库上传到此版本后，此功能是默认行为。作为旁注，现在这个默认行为包括在它们之间导航时不会重新创建片段，这似乎是非常需要的。

Answer 6

拥有保持状态的适当后退堆栈的关键点是拥有NavHostFragment具有 childFragmentManager 和他们自己的后退堆栈的'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 不存在于上述函数的后堆栈中，则获取它并将其添加到后堆栈中。commitNow是同步的，不像commit

private fun gainNavHostFragment( fragmentManager: FragmentManager, fragmentTag: String, navGraphId: Int, containerId: Int ): NavHostFragment { // 如果导航主机片段存在，则返回它 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，如下所示

带有嵌套导航。

导航图很相似，所以我只加了一个

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>

包含FragmentContainerView和的MainActivity 的布局BottomNavigationView

activiy_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>

主活动.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。你可以查看我构建的完整示例，或者谷歌的存储库来检查高级导航或其他示例的扩展。

Answer 7

从 Navigation 版本 2.4.0 开始，带有 NavHostFragment 的 BottomNavigationView 支持每个选项卡都有一个单独的返回堆栈，因此Elyeante 的答案是 50% 正确的。但它不支持主选项卡的返回堆栈。例如，如果我们有4 个主要片段（选项卡）A、B、C 和 D，startDestination则为A。D具有子片段D1、D2 和 D3。如果用户像A -> B -> C ->D -> D1 -> D2-> D3导航，如果用户单击官方库的后退按钮，导航将是D3 -> D2-> D1-> D由A . 这意味着主选项卡B 和 C将不会位于返回堆栈中。

为了支持主选项卡返回堆栈，我创建了一个带有主选项卡导航引用的堆栈。在用户后退单击时，我根据创建的堆栈更新了 BottomNavigationView 的所选项目。

我创建了这个Github 存储库来展示我所做的事情。我通过以下媒体文章得出了这个答案。

• https://medium.com/r?url=https%3A%2F%2Fvedraj360.medium.com%2Fyoutube-like-backstack-in-jetpack-navigation-component-android-2537b446668d
• https://medium.com/androiddevelopers/navigation-multiple-back-stacks-6c67ba41952f

实施步骤

将最新的导航库添加到 Gradle 并遵循官方存储库以支持子片段的返回堆栈。

我们必须为每个底部栏项目创建单独的导航图，而不是创建单个 nav_graph，并且这三个图应添加到一个主图，如下所示

<navigation
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/nav_graph"
    app:startDestination="@+id/home">

    <include app:graph="@navigation/home"/>
    <include app:graph="@navigation/list"/>
    <include app:graph="@navigation/form"/>

</navigation>

并将底部导航视图和导航主机片段链接到setupWithNavController

现在应用程序将支持子片段的返回堆栈。为了支持主要的后退导航，我们需要添加更多行。

private var addToBackStack: Boolean = true
private lateinit var fragmentBackStack: Stack<Int>

fragmentBackStack将帮助我们将所有访问过的目的地保存在堆栈中，addToBackStack是一个检查器，它将帮助我们确定是否要将当前目的地添加到堆栈中。

navHostFragment.findNavController().addOnDestinationChangedListener { _, destination, _ ->
    val bottomBarId = findBottomBarIdFromFragment(destination.id)
    if (!::fragmentBackStack.isInitialized){
        fragmentBackStack = Stack()
    }
    if (needToAddToBackStack && bottomBarId!=null) {
        if (!fragmentBackStack.contains(bottomBarId)) {
            fragmentBackStack.add(bottomBarId)
        } else if (fragmentBackStack.contains(bottomBarId)) {
            if (bottomBarId == R.id.home) {
                val homeCount =
                    Collections.frequency(fragmentBackStack, R.id.home)
                if (homeCount < 2) {
                    fragmentBackStack.push(bottomBarId)
                } else {
                    fragmentBackStack.asReversed().remove(bottomBarId)
                    fragmentBackStack.push(bottomBarId)
                }
            } else {
                fragmentBackStack.remove(bottomBarId)
                fragmentBackStack.push(bottomBarId)
            }
        }

    }
    needToAddToBackStack = true
}

当navHostFragment更改片段时，我们会收到回调addOnDestinationChangedListener，并检查该片段是否已存在于堆栈中。如果不是，我们将添加到堆栈顶部，如果是，我们将交换位置到堆栈顶部。由于我们现在为每个选项卡使用单独的图表，因此 BottomNavigationView 中的 idaddOnDestinationChangedListener会有所不同，因此我们使用findBottomBarIdFromFragment从目标片段中查找 BottomNavigationView 项目 id。

private fun findBottomBarIdFromFragment(fragmentId:Int?):Int?{
    if (fragmentId!=null){
        val bottomBarId = when(fragmentId){
            R.id.register ->{
                R.id.form
            }
            R.id.leaderboard -> {
                R.id.list
            }
            R.id.titleScreen ->{
                R.id.home
            }
            else -> {
                null
            }
        }
        return bottomBarId
    } else {
        return null
    }
}

当用户点击返回时，我们覆盖活动的onBackPressed方法（注意：已弃用，一旦找到内部onBackPressed的替代方法，我将更新答案）super.onBackPressed()override fun onBackPressed()

override fun onBackPressed() {
    val bottomBarId = if (::navController.isInitialized){
        findBottomBarIdFromFragment(navController.currentDestination?.id)
    } else {
        null
    }
    if (bottomBarId!=null) {
        if (::fragmentBackStack.isInitialized && fragmentBackStack.size > 1) {
            if (fragmentBackStack.size == 2 && fragmentBackStack.lastElement() == fragmentBackStack.firstElement()){
                finish()
            } else {
                fragmentBackStack.pop()
                val fragmentId = fragmentBackStack.lastElement()
                needToAddToBackStack = false
                bottomNavigationView.selectedItemId = fragmentId
            }
        } else {
            if (::fragmentBackStack.isInitialized && fragmentBackStack.size == 1) {
                finish()
            } else {
                super.onBackPressed()
            }
        }
    } else super.onBackPressed()
}

当用户单击返回时，我们将从 Stack 中弹出最后一个片段，并在底部导航视图中设置所选项目 id。

中等链接