Jetpack Navigation Drawer 始终重新创建第一个片段，即使在 onBackPress 中也是如此
Jetpack Navigation Drawer always recreates the first fragment even in onBackPress
标题本身就是我的问题，每当我打开 MainActivity 然后导航到 hamburger/drawer 菜单中可用的另一个片段，然后 press/swipe 回到主屏幕中的 return（第一个片段）它重新创建。 Nav Component 是否有办法让它不重新创建第一个片段？我正在使用 Android Studio 生成的 Jetpack Navigation 模板，这似乎是默认行为。
这是 MainActivity
class MainActivity : AppCompatActivity() {
private lateinit var appBarConfiguration: AppBarConfiguration
private var _binding: ActivityMainBinding? = null
// This property is only valid between onCreate and
// onDestroyView.
private val binding get() = _binding!!
private lateinit var drawerLayout: DrawerLayout
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
_binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
setSupportActionBar(binding.appBarMain.toolbar)
drawerLayout = binding.drawerLayout
val navView: NavigationView = binding.navView
val navController = findNavController(R.id.nav_host_fragment_content_main)
// Passing each menu ID as a set of Ids because each
// menu should be considered as top level destinations.
appBarConfiguration = AppBarConfiguration(setOf(
R.id.nav_home, R.id.nav_marketcap, R.id.nav_about), drawerLayout)
setupActionBarWithNavController(navController, appBarConfiguration)
navView.setupWithNavController(navController)
}
override fun onCreateOptionsMenu(menu: Menu): Boolean {
// Inflate the menu; this adds items to the action bar if it is present.
menuInflater.inflate(R.menu.main, menu)
menu.findItem(R.id.action_settings).isChecked = AppCompatDelegate.getDefaultNightMode() == AppCompatDelegate.MODE_NIGHT_YES
return true
}
override fun onSupportNavigateUp(): Boolean {
val navController = findNavController(R.id.nav_host_fragment_content_main)
return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
}
override fun onDestroy() {
super.onDestroy()
_binding = null
}
override fun onBackPressed() {
if (drawerLayout.isDrawerOpen(GravityCompat.START))
drawerLayout.closeDrawer(GravityCompat.START)
else
super.onBackPressed()
}
}
这是包含 child 片段 AssetFragment
的主页片段（MainActivity 中的第一个片段）
class HomeFragment : Fragment() {
private val homeViewModel: HomeViewModel by activityViewModels()
private var _binding: FragmentHomeBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!
private lateinit var viewPager : ViewPager2
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View {
_binding = FragmentHomeBinding.inflate(inflater, container, false)
val root: View = binding.root
viewPager = binding.viewPagerContainer
val bottomNav = binding.bottomNav
// val tabLayout = binding.tabLayout
val fragmentList : MutableList<Pair<String, Fragment>> = mutableListOf()
fragmentList.add(Pair(getString(R.string.assets), AssetFragment.newInstance()))
fragmentList.add(Pair(getString(R.string.news), NewsFragment.newInstance()))
fragmentList.add(Pair(getString(R.string.videos), VideosFragment.newInstance()))
val adapter = AppFragmentAdapter(fragmentList, this)
viewPager.adapter = adapter
viewPager.offscreenPageLimit = 2
viewPager.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
override fun onPageSelected(position: Int) {
super.onPageSelected(position)
bottomNav.menu.getItem(position).isChecked = true
homeViewModel.setTitle(adapter.getFragmentTabName(position))
}
})
val bottomNavListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
when(item.itemId) {
R.id.page_1 -> {
// Respond to navigation item 1 click
viewPager.setCurrentItem(0, true)
true
}
R.id.page_2 -> {
// Respond to navigation item 2 click
viewPager.setCurrentItem(1, true)
true
}
R.id.page_3 -> {
// Respond to navigation item 3 click
viewPager.setCurrentItem(2, true)
true
}
else -> false
}
}
bottomNav.setOnNavigationItemSelectedListener(bottomNavListener)
// val layoutInflater : LayoutInflater = LayoutInflater.from(context)
//Connect TabLayout with ViewPager2
// TabLayoutMediator(tabLayout, viewPager){ tab, position ->
// tab.customView = prepareTabView(layoutInflater, tabLayout, adapter.getFragmentTabName(position), tabIcons[position])
// }.attach()
return root
}
// private fun prepareTabView(
// layoutInflater: LayoutInflater,
// tabLayout: TabLayout,
// fragmentName: String,
// drawableId: Int
// ): View {
//
// val rootView : View = layoutInflater.inflate(R.layout.main_custom_tab_text, tabLayout, false)
//
// val tabName : AppCompatTextView = rootView.findViewById(R.id.tabName)
//
// tabName.text = fragmentName
// tabName.setCompoundDrawablesWithIntrinsicBounds(null, AppCompatResources.getDrawable(requireContext(), drawableId), null, null)
//
// return tabName
//
// }
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
override fun onResume() {
super.onResume()
requireView().isFocusableInTouchMode = true
requireView().requestFocus()
requireView().setOnKeyListener(object : View.OnKeyListener {
override fun onKey(v: View?, keyCode: Int, event: KeyEvent?): Boolean {
if (event!!.action == KeyEvent.ACTION_UP && keyCode == KeyEvent.KEYCODE_BACK) {
onBackPress()
return true
}
return false
}
})
}
fun onBackPress() {
if (viewPager.currentItem != 0)
viewPager.setCurrentItem(0, true)
else
requireActivity().onBackPressed()
}
}
这是显示在由 parent 片段 HomeFragment
托管的 ViewPager 中的 child 片段之一
class AssetFragment : Fragment() {
companion object {
fun newInstance() = AssetFragment()
}
private lateinit var viewModel: AssetViewModel
private var _binding: FragmentAssetsBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!
private lateinit var logTxt: AppCompatTextView
private lateinit var recyclerView: RecyclerView
private lateinit var swipeRefreshLayout: SwipeRefreshLayout
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?,
): View {
_binding = FragmentAssetsBinding.inflate(inflater, container, false)
val root: View = binding.root
recyclerView = binding.recyclerView
swipeRefreshLayout = binding.refreshLayout
logTxt = binding.errorLog
recyclerView.layoutManager = LinearLayoutManager(context)
adapter = AssetAdapter(requireContext(), this)
recyclerView.adapter = adapter
swipeRefreshLayout.isRefreshing = true
fetchAssets("30")
swipeRefreshLayout.setOnRefreshListener {
swipeRefreshLayout.isRefreshing = true
fetchAssets("30")
}
return root
}
private fun fetchAssets(limit: String) {
//Network stuff
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
viewModel = ViewModelProvider(this).get(AssetViewModel::class.java)
// TODO: Use the ViewModel
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
}
导航xml
这是将显示在抽屉菜单中的片段
<?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/mobile_navigation"
app:startDestination="@+id/nav_home">
<fragment
android:id="@+id/nav_home"
android:name="com.myapp.ui.home.HomeFragment"
android:label="@string/home"
tools:layout="@layout/fragment_home" />
<fragment
android:id="@+id/nav_marketcap"
android:name="com.myapp.ui.marketcap.MarketCapFragment"
android:label="@string/marketCap"
tools:layout="@layout/fragment_marketcap" />
<fragment
android:id="@+id/nav_about"
android:name="com.myapp.ui.about.AboutFragment"
android:label="@string/about"
tools:layout="@layout/fragment_about" />
</navigation>
menu.xml
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
tools:showIn="navigation_view">
<group android:checkableBehavior="single">
<item android:title="@string/menu">
<menu>
<item
android:id="@+id/nav_home"
android:icon="@drawable/ic_assets"
android:title="@string/home" />
<item
android:id="@+id/nav_marketcap"
android:icon="@drawable/ic_marketcap"
android:title="@string/marketCap" />
<item
android:id="@+id/nav_about"
android:icon="@drawable/ic_about"
android:title="@string/about" />
</menu>
</item>
</group>
<item android:title="@string/connect">
<menu>
<item
android:id="@+id/email_connect"
android:icon="@drawable/ic_email"
android:title="@string/fui_email_hint" />
</menu>
</item>
</menu>
流量：
打开应用程序
启动 MainActivity
显示 HomeFragment (AssetFragment)
打开抽屉菜单
Select 项，例如关于 (AboutFragment)
Press/Swipe 返回
这里有问题 HomeFragment onCreateView 再次被触发
预期行为 HomeFragment 将不再需要膨胀视图，因为我们只是让用户回到最初的目的地。除非用户自己按下抽屉菜单中的 Home 项，否则 HomeFragment 将在此时重新创建。
根据 Saving state with fragments guide，您的 Fragment 视图（而不是 Fragment 本身）应该在返回堆栈上时被销毁并重新创建。
根据该指南，一种状态类型是非配置状态：
NonConfig: data pulled from an external source, such as a server or local repository, or user-created data that is sent to a server once committed.
NonConfig data should be placed outside of your fragment, such as in a ViewModel. The ViewModel class inherently allows data to survive configuration changes, such as screen rotations, and remains in memory when the fragment is placed on the back stack.
因此您的片段应该永远不会在onCreateView()中调用fetchAssets("30")。相反，此逻辑应该发生在 ViewModel 内部，以便当片段 returns 来自返回堆栈时它 立即 可用。根据 ViewModel guide，您的 fetchAssets 应该在 ViewModel 中完成，您的 Fragment 会观察到该数据。
标题本身就是我的问题，每当我打开 MainActivity 然后导航到 hamburger/drawer 菜单中可用的另一个片段，然后 press/swipe 回到主屏幕中的 return（第一个片段）它重新创建。 Nav Component 是否有办法让它不重新创建第一个片段？我正在使用 Android Studio 生成的 Jetpack Navigation 模板，这似乎是默认行为。
这是 MainActivity
class MainActivity : AppCompatActivity() {
private lateinit var appBarConfiguration: AppBarConfiguration
private var _binding: ActivityMainBinding? = null
// This property is only valid between onCreate and
// onDestroyView.
private val binding get() = _binding!!
private lateinit var drawerLayout: DrawerLayout
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
_binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
setSupportActionBar(binding.appBarMain.toolbar)
drawerLayout = binding.drawerLayout
val navView: NavigationView = binding.navView
val navController = findNavController(R.id.nav_host_fragment_content_main)
// Passing each menu ID as a set of Ids because each
// menu should be considered as top level destinations.
appBarConfiguration = AppBarConfiguration(setOf(
R.id.nav_home, R.id.nav_marketcap, R.id.nav_about), drawerLayout)
setupActionBarWithNavController(navController, appBarConfiguration)
navView.setupWithNavController(navController)
}
override fun onCreateOptionsMenu(menu: Menu): Boolean {
// Inflate the menu; this adds items to the action bar if it is present.
menuInflater.inflate(R.menu.main, menu)
menu.findItem(R.id.action_settings).isChecked = AppCompatDelegate.getDefaultNightMode() == AppCompatDelegate.MODE_NIGHT_YES
return true
}
override fun onSupportNavigateUp(): Boolean {
val navController = findNavController(R.id.nav_host_fragment_content_main)
return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
}
override fun onDestroy() {
super.onDestroy()
_binding = null
}
override fun onBackPressed() {
if (drawerLayout.isDrawerOpen(GravityCompat.START))
drawerLayout.closeDrawer(GravityCompat.START)
else
super.onBackPressed()
}
}
这是包含 child 片段 AssetFragment
的主页片段（MainActivity 中的第一个片段）class HomeFragment : Fragment() {
private val homeViewModel: HomeViewModel by activityViewModels()
private var _binding: FragmentHomeBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!
private lateinit var viewPager : ViewPager2
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View {
_binding = FragmentHomeBinding.inflate(inflater, container, false)
val root: View = binding.root
viewPager = binding.viewPagerContainer
val bottomNav = binding.bottomNav
// val tabLayout = binding.tabLayout
val fragmentList : MutableList<Pair<String, Fragment>> = mutableListOf()
fragmentList.add(Pair(getString(R.string.assets), AssetFragment.newInstance()))
fragmentList.add(Pair(getString(R.string.news), NewsFragment.newInstance()))
fragmentList.add(Pair(getString(R.string.videos), VideosFragment.newInstance()))
val adapter = AppFragmentAdapter(fragmentList, this)
viewPager.adapter = adapter
viewPager.offscreenPageLimit = 2
viewPager.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
override fun onPageSelected(position: Int) {
super.onPageSelected(position)
bottomNav.menu.getItem(position).isChecked = true
homeViewModel.setTitle(adapter.getFragmentTabName(position))
}
})
val bottomNavListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
when(item.itemId) {
R.id.page_1 -> {
// Respond to navigation item 1 click
viewPager.setCurrentItem(0, true)
true
}
R.id.page_2 -> {
// Respond to navigation item 2 click
viewPager.setCurrentItem(1, true)
true
}
R.id.page_3 -> {
// Respond to navigation item 3 click
viewPager.setCurrentItem(2, true)
true
}
else -> false
}
}
bottomNav.setOnNavigationItemSelectedListener(bottomNavListener)
// val layoutInflater : LayoutInflater = LayoutInflater.from(context)
//Connect TabLayout with ViewPager2
// TabLayoutMediator(tabLayout, viewPager){ tab, position ->
// tab.customView = prepareTabView(layoutInflater, tabLayout, adapter.getFragmentTabName(position), tabIcons[position])
// }.attach()
return root
}
// private fun prepareTabView(
// layoutInflater: LayoutInflater,
// tabLayout: TabLayout,
// fragmentName: String,
// drawableId: Int
// ): View {
//
// val rootView : View = layoutInflater.inflate(R.layout.main_custom_tab_text, tabLayout, false)
//
// val tabName : AppCompatTextView = rootView.findViewById(R.id.tabName)
//
// tabName.text = fragmentName
// tabName.setCompoundDrawablesWithIntrinsicBounds(null, AppCompatResources.getDrawable(requireContext(), drawableId), null, null)
//
// return tabName
//
// }
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
override fun onResume() {
super.onResume()
requireView().isFocusableInTouchMode = true
requireView().requestFocus()
requireView().setOnKeyListener(object : View.OnKeyListener {
override fun onKey(v: View?, keyCode: Int, event: KeyEvent?): Boolean {
if (event!!.action == KeyEvent.ACTION_UP && keyCode == KeyEvent.KEYCODE_BACK) {
onBackPress()
return true
}
return false
}
})
}
fun onBackPress() {
if (viewPager.currentItem != 0)
viewPager.setCurrentItem(0, true)
else
requireActivity().onBackPressed()
}
}
这是显示在由 parent 片段 HomeFragment
托管的 ViewPager 中的 child 片段之一class AssetFragment : Fragment() {
companion object {
fun newInstance() = AssetFragment()
}
private lateinit var viewModel: AssetViewModel
private var _binding: FragmentAssetsBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!
private lateinit var logTxt: AppCompatTextView
private lateinit var recyclerView: RecyclerView
private lateinit var swipeRefreshLayout: SwipeRefreshLayout
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?,
): View {
_binding = FragmentAssetsBinding.inflate(inflater, container, false)
val root: View = binding.root
recyclerView = binding.recyclerView
swipeRefreshLayout = binding.refreshLayout
logTxt = binding.errorLog
recyclerView.layoutManager = LinearLayoutManager(context)
adapter = AssetAdapter(requireContext(), this)
recyclerView.adapter = adapter
swipeRefreshLayout.isRefreshing = true
fetchAssets("30")
swipeRefreshLayout.setOnRefreshListener {
swipeRefreshLayout.isRefreshing = true
fetchAssets("30")
}
return root
}
private fun fetchAssets(limit: String) {
//Network stuff
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
viewModel = ViewModelProvider(this).get(AssetViewModel::class.java)
// TODO: Use the ViewModel
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
}
导航xml
这是将显示在抽屉菜单中的片段
<?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/mobile_navigation"
app:startDestination="@+id/nav_home">
<fragment
android:id="@+id/nav_home"
android:name="com.myapp.ui.home.HomeFragment"
android:label="@string/home"
tools:layout="@layout/fragment_home" />
<fragment
android:id="@+id/nav_marketcap"
android:name="com.myapp.ui.marketcap.MarketCapFragment"
android:label="@string/marketCap"
tools:layout="@layout/fragment_marketcap" />
<fragment
android:id="@+id/nav_about"
android:name="com.myapp.ui.about.AboutFragment"
android:label="@string/about"
tools:layout="@layout/fragment_about" />
</navigation>
menu.xml
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
tools:showIn="navigation_view">
<group android:checkableBehavior="single">
<item android:title="@string/menu">
<menu>
<item
android:id="@+id/nav_home"
android:icon="@drawable/ic_assets"
android:title="@string/home" />
<item
android:id="@+id/nav_marketcap"
android:icon="@drawable/ic_marketcap"
android:title="@string/marketCap" />
<item
android:id="@+id/nav_about"
android:icon="@drawable/ic_about"
android:title="@string/about" />
</menu>
</item>
</group>
<item android:title="@string/connect">
<menu>
<item
android:id="@+id/email_connect"
android:icon="@drawable/ic_email"
android:title="@string/fui_email_hint" />
</menu>
</item>
</menu>
流量：
打开应用程序
启动 MainActivity
显示 HomeFragment (AssetFragment)
打开抽屉菜单
Select 项，例如关于 (AboutFragment)
Press/Swipe 返回
这里有问题 HomeFragment onCreateView 再次被触发
预期行为 HomeFragment 将不再需要膨胀视图，因为我们只是让用户回到最初的目的地。除非用户自己按下抽屉菜单中的 Home 项，否则 HomeFragment 将在此时重新创建。
根据 Saving state with fragments guide，您的 Fragment 视图（而不是 Fragment 本身）应该在返回堆栈上时被销毁并重新创建。
根据该指南，一种状态类型是非配置状态：
NonConfig: data pulled from an external source, such as a server or local repository, or user-created data that is sent to a server once committed.
NonConfig data should be placed outside of your fragment, such as in a ViewModel. The ViewModel class inherently allows data to survive configuration changes, such as screen rotations, and remains in memory when the fragment is placed on the back stack.
因此您的片段应该永远不会在onCreateView()中调用fetchAssets("30")。相反，此逻辑应该发生在 ViewModel 内部，以便当片段 returns 来自返回堆栈时它 立即 可用。根据 ViewModel guide，您的 fetchAssets 应该在 ViewModel 中完成，您的 Fragment 会观察到该数据。