使用 Jetpack 的 Android 导航组件销毁/重新创建的片段
Fragments destroyed / recreated with Jetpack's Android Navigation components
-
android
-
android-fragments
-
android-jetpack
-
android-architecture-navigation
-
android-jetpack-navigation
我正在尝试在我现有的应用程序中实施 Navigation with Jetpack's architecture components。
我有一个 activity 应用程序，其中主要片段 (ListFragment) 是一个项目列表。当前，当用户点击列表项时，fragmentTransaction.add(R.id.main, detailFragment) 会将第二个片段添加到堆栈中。因此，当按下后退时，DetailFragment 会分离，ListFragment 会再次显示。
对于导航架构，这是自动处理的。它不是添加新片段而是 replaced，因此片段视图被销毁，调用 onDestroyView() 并在按下返回以重新创建视图时调用 onCreateView()。
我知道这是与 LiveData and ViewModel 一起使用的一个很好的模式，可以避免使用不必要的内存，但在我的情况下，这很烦人，因为列表的布局很复杂，并且需要时间和 CPU 消耗，还因为我需要保存列表的滚动位置并再次滚动到用户离开片段的相同位置。这是可能的，但似乎应该有更好的方法。
我已经尝试 "save" 片段私有字段中的视图，并在 onCreateView() 上重新使用它（如果已经存在），但这似乎是一种反模式。
private View view = null;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
if (view == null) {
view = inflater.inflate(R.layout.fragment_list, container, false);
//...
}
return view;
}
有没有其他更优雅的方法来避免重新膨胀布局？
来自 google 的 Ian Lake 回复我说我们可以 将视图存储在变量中 和 而不是 膨胀新布局，只是 return onCreateView() 上 预存储视图 的实例]
来源：https://twitter.com/ianhlake/status/1103522856535638016
Leakcanary 可能将此显示为泄漏，但其 误报 ..
我这样试过，对我有用。
- 通过
navGraphViewModels初始化ViewModel（在导航范围内）
- 在
ViewModel 中存储任何to-restore状态
// fragment.kt
private val vm by navGraphViewModels<VM>(R.id.nav_graph) { vmFactory }
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
// Restore state
vm.state?.let {
(recycler.layoutManager as GridLayoutManager).onRestoreInstanceState(it)
}
}
override fun onPause() {
super.onPause()
// Store state
vm.state = (recycler.layoutManager as GridLayoutManager).onSaveInstanceState()
}
// vm.kt
var state:Parcelable? = null
您可以通过以下实现为您的片段提供持久视图
基础片段
open class BaseFragment : Fragment(){
var hasInitializedRootView = false
private var rootView: View? = null
fun getPersistentView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?, layout: Int): View? {
if (rootView == null) {
// Inflate the layout for this fragment
rootView = inflater?.inflate(layout,container,false)
} else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove rootView from the existing parent view group
// (it will be added back).
(rootView?.getParent() as? ViewGroup)?.removeView(rootView)
}
return rootView
}
}
MainFragment
class MainFragment : BaseFragment() {
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
return getPersistentView(inflater, container, savedInstanceState, R.layout.content_main)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
if (!hasInitializedRootView) {
hasInitializedRootView = true
setListeners()
loadViews()
}
}
}
虽然我认为 NavigationAdvancedSample 是更好的解决方案，但我也使用 @shahab-rauf 的代码解决了这个问题。因为我没有足够的时间将其应用到我的项目中。
基础片段
abstract class AppFragment: Fragment() {
private var persistingView: View? = null
private fun persistingView(view: View): View {
val root = persistingView
if (root == null) {
persistingView = view
return view
} else {
(root.parent as? ViewGroup)?.removeView(root)
return root
}
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?): View? {
val p = if (persistingView == null) {
onCreatePersistentView(inflater, container, savedInstanceState)
} else {
persistingView // prevent inflating
}
if (p != null) {
return persistingView(p)
}
return super.onCreateView(inflater, container, savedInstanceState)
}
protected open fun onCreatePersistentView(inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?): View? {
return null
}
override fun onViewCreated(view: View, savedInstanceState:Bundle?) {
super.onViewCreated(view, savedInstanceState)
if (persistingView != null) {
onPersistentViewCreated(view, savedInstanceState)
}
}
protected open fun onPersistentViewCreated(view: View, savedInstanceState: Bundle?) {
logv("onPersistentViewCreated")
}
}
实施
class DetailFragment : AppFragment() {
override fun onCreatePersistentView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// I used data-binding
binding = DataBindingUtil.inflate(inflater, R.layout.fragment_program_detail, container, false)
binding.model = viewModel
binding.lifecycleOwner = this
return binding.root
}
override fun onPersistentViewCreated(view: View, savedInstanceState: Bundle?) {
super.onPersistentViewCreated(view, savedInstanceState)
// RecyclerView bind with adapter
binding.curriculumRecycler.adapter = adapter
binding.curriculumRecycler.apply {
layoutManager = LinearLayoutManager(context)
setHasFixedSize(true)
}
viewModel.curriculums.observe(viewLifecycleOwner, Observer {
adapter.applyItems(it ?: emptyList())
})
viewModel.refresh()
}
}
如果您正在关注 google 中的高级示例，他们会使用扩展名。这是它的修改版本。在我的例子中，我必须在附加和分离时显示和隐藏片段：
/**
* 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, fragmentTag)
} 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.
if (!selectedFragment.isAdded) {
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
)
.add(selectedFragment, newlySelectedItemTag)
.setPrimaryNavigationFragment(selectedFragment)
.apply {
// Detach all other Fragments
graphIdToTagMap.forEach { _, fragmentTagIter ->
if (fragmentTagIter != newlySelectedItemTag) {
hide(fragmentManager.findFragmentByTag(firstFragmentTag)!!)
}
}
}
.addToBackStack(firstFragmentTag)
.setReorderingAllowed(true)
.commit()
} else {
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
)
.show(selectedFragment)
.setPrimaryNavigationFragment(selectedFragment)
.apply {
// Detach all other Fragments
graphIdToTagMap.forEach { _, fragmentTagIter ->
if (fragmentTagIter != newlySelectedItemTag) {
hide(fragmentManager.findFragmentByTag(firstFragmentTag)!!)
}
}
}
.addToBackStack(firstFragmentTag)
.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.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 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 detachNavHostFragment(
fragmentManager: FragmentManager,
navHostFragment: NavHostFragment
) {
fragmentManager.beginTransaction()
.hide(navHostFragment)
.commitNow()
}
private fun attachNavHostFragment(
fragmentManager: FragmentManager,
navHostFragment: NavHostFragment,
isPrimaryNavFragment: Boolean,
fragmentTag: String
) {
if (navHostFragment.isAdded) return
fragmentManager.beginTransaction()
.add(navHostFragment, fragmentTag)
.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"
这与@Shahab Rauf 建议的答案相同，唯一的额外内容是仅在 BaseFragment 而不是 Child Fragment 中包含数据绑定和实现 onCreateView。并在 BaseFragment 的 onViewCreated() 中初始化 navController。
基础片段
abstract class BaseFragment<T : ViewDataBinding, VM : BaseViewModel<UiState>> : Fragment() {
protected lateinit var binding: T
var hasInitializedRootView = false
private var rootView: View? = null
protected abstract val mViewModel: ViewModel
protected lateinit var navController: NavController
fun getPersistentView(
inflater: LayoutInflater?,
container: ViewGroup?,
savedInstanceState: Bundle?,
layout: Int
): View? {
if (rootView == null) {
binding = DataBindingUtil.inflate(inflater!!, getFragmentView(), container, false)
//setting the viewmodel
binding.setVariable(BR.mViewModel, mViewModel)
// Inflate the layout for this fragment
rootView = binding.root
} else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove rootView from the existing parent view group
// (it will be added back).
(rootView?.getParent() as? ViewGroup)?.removeView(rootView)
}
return rootView
}
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? = getPersistentView(inflater, container, savedInstanceState, getFragmentView())
//this method is used to get the fragment layout file
abstract fun getFragmentView(): Int
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
navController = Navigation.findNavController(view)
}
}
HomeFragment（任何扩展 BaseFragment 的片段）
class HomeFragment : BaseFragment<HomeFragmentBinding, HomeViewModel>(),
RecycleViewClickListener {
override val mViewModel by viewModel<HomeViewModel>()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
if (!hasInitializedRootView) {hasInitializedRootView = true
setListeners()
loadViews()
--------
}
您好，问题已在最新版本 2.4.0-alpha01 中修复，现在官方支持多个后台导航
查看 link：
https://developer.android.com/jetpack/androidx/releases/navigation#version_240_2
这将有助于加快片段创建速度，并且当您使用数据绑定和 viewModel 时，数据仍将保存在视图中以防按下后退。
只需这样做：
lateinit var binding: FragmentConnectBinding
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View {
if (this::binding.isInitialized) {
binding
} else {
binding = FragmentConnectBinding.inflate(inflater, container, false)
binding.viewModel = viewModel
binding.model = connectModel
binding.lifecycleOwner = viewLifecycleOwner
viewModel.buildAllProfiles()
// do what ever you need to do in first creation
}
setupObservers()
return binding.root
}
对于 Java 开发人员，如上述答案所述和结合，
BaseFragment.java
public abstract class BaseFragment<T extends ViewDataBinding, V extends BaseViewModel> extends Fragment {
private View mRootView;
private T mViewDataBinding;
private V mViewModel;
public boolean hasInitializedRootView = false;
private View rootView = null;
public View getPersistentView(LayoutInflater layoutInflater, ViewGroup container, Bundle saveInstanceState, int layout) {
if (rootView == null) {
mViewDataBinding = DataBindingUtil.inflate(layoutInflater, layout, container, false);
mViewDataBinding.setVariable(getBindingVariable(),mViewModel);
rootView = mViewDataBinding.getRoot();
}else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove rootView from the existing parent view group
// (it will be added back).
ViewGroup viewGroup = (ViewGroup) rootView.getParent();
if (viewGroup != null){
viewGroup.removeView(rootView);
}
}
return rootView;
}
}
在您的片段中实施为，
@AndroidEntryPoint
public class YourFragment extends BaseFragment<YourFragmentBinding, YourViewModel> {
@Override
public View onCreateView(@NonNull @NotNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return getPersistentView(inflater, container, savedInstanceState, getLayoutId());
}
@Override
public void onViewCreated(@NonNull @NotNull View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if (!hasInitializedRootView){
hasInitializedRootView = true;
// do your work here
}
}
}
android
android-fragments
android-jetpack
android-architecture-navigation
android-jetpack-navigation
我正在尝试在我现有的应用程序中实施 Navigation with Jetpack's architecture components。
我有一个 activity 应用程序，其中主要片段 (ListFragment) 是一个项目列表。当前，当用户点击列表项时，fragmentTransaction.add(R.id.main, detailFragment) 会将第二个片段添加到堆栈中。因此，当按下后退时，DetailFragment 会分离，ListFragment 会再次显示。
对于导航架构，这是自动处理的。它不是添加新片段而是 replaced，因此片段视图被销毁，调用 onDestroyView() 并在按下返回以重新创建视图时调用 onCreateView()。
我知道这是与 LiveData and ViewModel 一起使用的一个很好的模式，可以避免使用不必要的内存，但在我的情况下，这很烦人，因为列表的布局很复杂，并且需要时间和 CPU 消耗，还因为我需要保存列表的滚动位置并再次滚动到用户离开片段的相同位置。这是可能的，但似乎应该有更好的方法。
我已经尝试 "save" 片段私有字段中的视图，并在 onCreateView() 上重新使用它（如果已经存在），但这似乎是一种反模式。
private View view = null;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
if (view == null) {
view = inflater.inflate(R.layout.fragment_list, container, false);
//...
}
return view;
}
有没有其他更优雅的方法来避免重新膨胀布局？
来自 google 的 Ian Lake 回复我说我们可以 将视图存储在变量中 和 而不是 膨胀新布局，只是 return onCreateView() 上 预存储视图 的实例]
来源：https://twitter.com/ianhlake/status/1103522856535638016
Leakcanary 可能将此显示为泄漏，但其 误报 ..
我这样试过，对我有用。
- 通过
navGraphViewModels初始化ViewModel（在导航范围内） - 在
ViewModel中存储任何to-restore状态
// fragment.kt
private val vm by navGraphViewModels<VM>(R.id.nav_graph) { vmFactory }
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
// Restore state
vm.state?.let {
(recycler.layoutManager as GridLayoutManager).onRestoreInstanceState(it)
}
}
override fun onPause() {
super.onPause()
// Store state
vm.state = (recycler.layoutManager as GridLayoutManager).onSaveInstanceState()
}
// vm.kt
var state:Parcelable? = null
您可以通过以下实现为您的片段提供持久视图
基础片段
open class BaseFragment : Fragment(){
var hasInitializedRootView = false
private var rootView: View? = null
fun getPersistentView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?, layout: Int): View? {
if (rootView == null) {
// Inflate the layout for this fragment
rootView = inflater?.inflate(layout,container,false)
} else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove rootView from the existing parent view group
// (it will be added back).
(rootView?.getParent() as? ViewGroup)?.removeView(rootView)
}
return rootView
}
}
MainFragment
class MainFragment : BaseFragment() {
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
return getPersistentView(inflater, container, savedInstanceState, R.layout.content_main)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
if (!hasInitializedRootView) {
hasInitializedRootView = true
setListeners()
loadViews()
}
}
}
虽然我认为 NavigationAdvancedSample 是更好的解决方案，但我也使用 @shahab-rauf 的代码解决了这个问题。因为我没有足够的时间将其应用到我的项目中。
基础片段
abstract class AppFragment: Fragment() {
private var persistingView: View? = null
private fun persistingView(view: View): View {
val root = persistingView
if (root == null) {
persistingView = view
return view
} else {
(root.parent as? ViewGroup)?.removeView(root)
return root
}
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?): View? {
val p = if (persistingView == null) {
onCreatePersistentView(inflater, container, savedInstanceState)
} else {
persistingView // prevent inflating
}
if (p != null) {
return persistingView(p)
}
return super.onCreateView(inflater, container, savedInstanceState)
}
protected open fun onCreatePersistentView(inflater: LayoutInflater, container: ViewGroup?,savedInstanceState: Bundle?): View? {
return null
}
override fun onViewCreated(view: View, savedInstanceState:Bundle?) {
super.onViewCreated(view, savedInstanceState)
if (persistingView != null) {
onPersistentViewCreated(view, savedInstanceState)
}
}
protected open fun onPersistentViewCreated(view: View, savedInstanceState: Bundle?) {
logv("onPersistentViewCreated")
}
}
实施
class DetailFragment : AppFragment() {
override fun onCreatePersistentView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// I used data-binding
binding = DataBindingUtil.inflate(inflater, R.layout.fragment_program_detail, container, false)
binding.model = viewModel
binding.lifecycleOwner = this
return binding.root
}
override fun onPersistentViewCreated(view: View, savedInstanceState: Bundle?) {
super.onPersistentViewCreated(view, savedInstanceState)
// RecyclerView bind with adapter
binding.curriculumRecycler.adapter = adapter
binding.curriculumRecycler.apply {
layoutManager = LinearLayoutManager(context)
setHasFixedSize(true)
}
viewModel.curriculums.observe(viewLifecycleOwner, Observer {
adapter.applyItems(it ?: emptyList())
})
viewModel.refresh()
}
}
如果您正在关注 google 中的高级示例，他们会使用扩展名。这是它的修改版本。在我的例子中，我必须在附加和分离时显示和隐藏片段：
/**
* 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, fragmentTag)
} 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.
if (!selectedFragment.isAdded) {
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
)
.add(selectedFragment, newlySelectedItemTag)
.setPrimaryNavigationFragment(selectedFragment)
.apply {
// Detach all other Fragments
graphIdToTagMap.forEach { _, fragmentTagIter ->
if (fragmentTagIter != newlySelectedItemTag) {
hide(fragmentManager.findFragmentByTag(firstFragmentTag)!!)
}
}
}
.addToBackStack(firstFragmentTag)
.setReorderingAllowed(true)
.commit()
} else {
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
)
.show(selectedFragment)
.setPrimaryNavigationFragment(selectedFragment)
.apply {
// Detach all other Fragments
graphIdToTagMap.forEach { _, fragmentTagIter ->
if (fragmentTagIter != newlySelectedItemTag) {
hide(fragmentManager.findFragmentByTag(firstFragmentTag)!!)
}
}
}
.addToBackStack(firstFragmentTag)
.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.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 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 detachNavHostFragment(
fragmentManager: FragmentManager,
navHostFragment: NavHostFragment
) {
fragmentManager.beginTransaction()
.hide(navHostFragment)
.commitNow()
}
private fun attachNavHostFragment(
fragmentManager: FragmentManager,
navHostFragment: NavHostFragment,
isPrimaryNavFragment: Boolean,
fragmentTag: String
) {
if (navHostFragment.isAdded) return
fragmentManager.beginTransaction()
.add(navHostFragment, fragmentTag)
.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"
这与@Shahab Rauf 建议的答案相同，唯一的额外内容是仅在 BaseFragment 而不是 Child Fragment 中包含数据绑定和实现 onCreateView。并在 BaseFragment 的 onViewCreated() 中初始化 navController。
基础片段
abstract class BaseFragment<T : ViewDataBinding, VM : BaseViewModel<UiState>> : Fragment() {
protected lateinit var binding: T
var hasInitializedRootView = false
private var rootView: View? = null
protected abstract val mViewModel: ViewModel
protected lateinit var navController: NavController
fun getPersistentView(
inflater: LayoutInflater?,
container: ViewGroup?,
savedInstanceState: Bundle?,
layout: Int
): View? {
if (rootView == null) {
binding = DataBindingUtil.inflate(inflater!!, getFragmentView(), container, false)
//setting the viewmodel
binding.setVariable(BR.mViewModel, mViewModel)
// Inflate the layout for this fragment
rootView = binding.root
} else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove rootView from the existing parent view group
// (it will be added back).
(rootView?.getParent() as? ViewGroup)?.removeView(rootView)
}
return rootView
}
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? = getPersistentView(inflater, container, savedInstanceState, getFragmentView())
//this method is used to get the fragment layout file
abstract fun getFragmentView(): Int
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
navController = Navigation.findNavController(view)
}
}
HomeFragment（任何扩展 BaseFragment 的片段）
class HomeFragment : BaseFragment<HomeFragmentBinding, HomeViewModel>(),
RecycleViewClickListener {
override val mViewModel by viewModel<HomeViewModel>()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
if (!hasInitializedRootView) {hasInitializedRootView = true
setListeners()
loadViews()
--------
}
您好，问题已在最新版本 2.4.0-alpha01 中修复，现在官方支持多个后台导航
查看 link： https://developer.android.com/jetpack/androidx/releases/navigation#version_240_2
这将有助于加快片段创建速度，并且当您使用数据绑定和 viewModel 时，数据仍将保存在视图中以防按下后退。
只需这样做：
lateinit var binding: FragmentConnectBinding
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View {
if (this::binding.isInitialized) {
binding
} else {
binding = FragmentConnectBinding.inflate(inflater, container, false)
binding.viewModel = viewModel
binding.model = connectModel
binding.lifecycleOwner = viewLifecycleOwner
viewModel.buildAllProfiles()
// do what ever you need to do in first creation
}
setupObservers()
return binding.root
}
对于 Java 开发人员，如上述答案所述和结合，
BaseFragment.java
public abstract class BaseFragment<T extends ViewDataBinding, V extends BaseViewModel> extends Fragment {
private View mRootView;
private T mViewDataBinding;
private V mViewModel;
public boolean hasInitializedRootView = false;
private View rootView = null;
public View getPersistentView(LayoutInflater layoutInflater, ViewGroup container, Bundle saveInstanceState, int layout) {
if (rootView == null) {
mViewDataBinding = DataBindingUtil.inflate(layoutInflater, layout, container, false);
mViewDataBinding.setVariable(getBindingVariable(),mViewModel);
rootView = mViewDataBinding.getRoot();
}else {
// Do not inflate the layout again.
// The returned View of onCreateView will be added into the fragment.
// However it is not allowed to be added twice even if the parent is same.
// So we must remove rootView from the existing parent view group
// (it will be added back).
ViewGroup viewGroup = (ViewGroup) rootView.getParent();
if (viewGroup != null){
viewGroup.removeView(rootView);
}
}
return rootView;
}
}
在您的片段中实施为，
@AndroidEntryPoint
public class YourFragment extends BaseFragment<YourFragmentBinding, YourViewModel> {
@Override
public View onCreateView(@NonNull @NotNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return getPersistentView(inflater, container, savedInstanceState, getLayoutId());
}
@Override
public void onViewCreated(@NonNull @NotNull View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if (!hasInitializedRootView){
hasInitializedRootView = true;
// do your work here
}
}
}