使用 Jetpack 的 Android 导航组件销毁或重新创建的碎片

我试图在我现有的应用程序中实现 Navigation with Jetpack's architecture components。

我有一个单独的活动应用程序，其中的主要片段(ListFragment)是一个项目列表。目前，当用户点击一个列表项时，第二个片段通过 fragmentTransaction.add(R.id.main, detailFragment)添加到堆栈中。因此，当背部被按下的 DetailFragment是分离和 ListFragment再次显示。

使用导航体系结构，这是自动处理的。没有添加新的片段，而是添加了 被取代了，因此片段视图被销毁，当按回键重新创建视图时，将调用 onDestroyView()和 onCreateView()。

我知道这是一个很好的模式，用于 LiveData和 ViewModel，以避免使用过多的内存，但在我的情况下，这是恼人的，因为列表有一个复杂的布局和膨胀的时间和 CPU 消耗，也因为我需要保存列表的滚动位置，并再次滚动到相同的位置用户离开片段。这是可能的，但似乎它应该存在一个更好的方式。

我已经尝试在片段的私有字段中“保存”视图，并在 onCreateView()上重用它(如果已经存在) ，但它似乎是反模式的。

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？

42241 次浏览
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最佳答案

伊恩 · 雷克从谷歌回复我说，我们可以 将视图存储在变量中和 取而代之的充气一个新的布局，回来吧的实例 预先存储的视图上的 onCreateView()

资料来源: https://twitter.com/ianhlake/status/1103522856535638016

Leakcanary 可能显示为泄漏，但它的 假阳性. 。

小开

我试过了，对我很有效。

  • 通过 navGraphViewModels初始化 ViewModel(在导航范围上实时运行)
  • 在 ViewModel中存储任何要还原的状态
// 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
小开

您可以通过下面的实现获得片段的持久视图

基础碎片

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()
}
}
}

来源

小开

虽然我认为 NavigationAdvancedSample是一个更好的解决方案，但我也用@shahab-rauf 的代码解决了这个问题。因为我没有足够的时间把它应用到我的项目中。

Base Fragment

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")
}
}

工具

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()
}
}
小开

如果你跟随从谷歌高级样品，他们使用扩展。下面是它的修改版本。在我的情况下，我必须显示和隐藏碎片，而他们正在附加和分离:

/**
* 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"
小开

这个答案和@Shahab Rauf 建议的是一样的。唯一额外的事情就是加入 Databinding，并且只在 baseFragment 中实现 oncreateView，而不是在 Child Fragments 中实现 onCreateView。并且还在 BaseFragment 的 onViewCreated ()中初始化导航控制器。

BaseFragment

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 (任何正在扩展 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()
--------


}
小开

嗨，这个问题在最新版本2.4.0-alpha01中得到了修复，现在官方支持多重回溯导航

你可浏览以下连结: https://developer.android.com/jetpack/androidx/releases/navigation#version_240_2

小开

这将有助于加速片段的创建，当您使用数据绑定和 viewModel 时，数据仍将保存在视图中，以防后按。

按我说的做:

    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
}
小开

For Java Developers, As described and combined from above answers,

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;
}
}

在片段中实现,

@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


}


}




}
小开

如果您只想检查您的 碎片被重建了，我们可以简单地覆盖 onCreate () ，它在生命周期中只调用一次片段。

    override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
//Your onetime operation or function call here.
}

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