TL;DR: 跳到 只需向我展示已完成的步骤！！！ 部分
这是片段的正常行为。假设每次删除或替换它们时都会重新创建它们，并且假设您使用 onSaveInstanceState 恢复它们的状态。
这是一篇很好的文章，描述了如何做到这一点：Saving Fragment States
除此之外，您还可以使用View Model，这是以下推荐的 android 架构的一部分。它们是保留和恢复 UI 数据的好方法。
你可以按照这个步骤一步一步来学习如何实现这个架构code lab
编辑：解决方案
花了一段时间，但它就在这里。解决方案目前不使用ViewModels。
仔细阅读，因为每一步都很重要。该解决方案包括以下两部分
- 在后退键按下时实现正确导航
- 在导航期间保持片段活动
背景：
Android Navigation 组件提供了一个NavController 类，您可以使用它在不同的目的地之间导航。在内部 NavController 使用 Navigator 实际进行导航。 Navigator 是一个抽象类，任何人都可以扩展/继承此类以创建自己的自定义导航器，以根据目的地类型提供自定义导航。当使用片段作为目的地时，NavHostFragment 使用FragmentNavigator，其默认实现会在我们使用FragmentTransaction.replace() 导航时替换片段，这会完全破坏先前的片段并添加新片段。所以我们必须创建自己的导航器，而不是使用FragmentTransaction.replace()，我们将使用FragmentTransaction.hide() 和FragmentTransaction.show() 的组合来避免片段被破坏。
导航 UI 的默认行为：
默认情况下，每当您导航到除主/起始片段以外的任何其他片段时，它们都不会被添加到后台堆栈中，因此假设您按以下顺序选择片段
A -> B -> C -> D -> E
你的后台只有
[A, E]
你可以看到片段 B、C、D 没有被添加到 backstack，所以按下回压总是会让你回到片段 A，它是主片段
我们现在想要的行为：
我们想要一个简单而有效的行为。我们不希望将所有片段都添加到 backstack，但如果片段已经在 backstack 中，我们希望将所有片段弹出到所选片段。
假设我按以下顺序选择片段
A -> B -> C -> D -> E
backstack也应该是
[A, B, C, D, E]
按下后，只有最后一个片段应该被弹出，并且 backstack 应该是这样的
[A, B, C, D]
但是如果我们导航到片段 B，因为 B 已经在堆栈中，那么 B 上面的所有片段都应该被弹出，我们的 backstack 应该看起来像这样
[A, B]
我希望这种行为是有意义的。使用全局操作很容易实现此行为，如下所示，并且比默认行为更好。
好帅！怎么办 ？ ：
现在我们有两个选择
- 扩展
FragmentNavigator
- 复制/粘贴
FragmentNavigator
我个人只想扩展 FragmentNavigator 并覆盖 navigate() 方法，但由于它的所有成员变量都是私有的，我无法实现正确的导航。
所以我决定复制粘贴整个 FragmentNavigator 类，并将整个代码中的名称从 "FragmentNavigator" 更改为我想调用的任何名称。
只要告诉我步骤就行了！！！ ：
- 创建自定义导航器
- 使用自定义标签
- 添加全局操作
- 使用全局操作
- 将自定义导航器添加到 NavController
第 1 步：创建自定义导航器
这是我的自定义导航器，名为StickyCustomNavigator。除了navigate() 方法外，所有代码都与FragmentNavigator 相同。如您所见，它使用hide()、show() 和add() 方法而不是replace()。逻辑很简单。隐藏前一个片段并显示目标片段。如果这是我们第一次访问特定的目标片段，请添加片段而不是显示它。
@Navigator.Name("sticky_fragment")
public class StickyFragmentNavigator extends Navigator<StickyFragmentNavigator.Destination> {
private static final String TAG = "StickyFragmentNavigator";
private static final String KEY_BACK_STACK_IDS = "androidx-nav-fragment:navigator:backStackIds";
private final Context mContext;
@SuppressWarnings("WeakerAccess") /* synthetic access */
final FragmentManager mFragmentManager;
private final int mContainerId;
@SuppressWarnings("WeakerAccess") /* synthetic access */
ArrayDeque<Integer> mBackStack = new ArrayDeque<>();
@SuppressWarnings("WeakerAccess") /* synthetic access */
boolean mIsPendingBackStackOperation = false;
private final FragmentManager.OnBackStackChangedListener mOnBackStackChangedListener =
new FragmentManager.OnBackStackChangedListener() {
@SuppressLint("RestrictedApi")
@Override
public void onBackStackChanged() {
// If we have pending operations made by us then consume this change, otherwise
// detect a pop in the back stack to dispatch callback.
if (mIsPendingBackStackOperation) {
mIsPendingBackStackOperation = !isBackStackEqual();
return;
}
// The initial Fragment won't be on the back stack, so the
// real count of destinations is the back stack entry count + 1
int newCount = mFragmentManager.getBackStackEntryCount() + 1;
if (newCount < mBackStack.size()) {
// Handle cases where the user hit the system back button
while (mBackStack.size() > newCount) {
mBackStack.removeLast();
}
dispatchOnNavigatorBackPress();
}
}
};
public StickyFragmentNavigator(@NonNull Context context, @NonNull FragmentManager manager,
int containerId) {
mContext = context;
mFragmentManager = manager;
mContainerId = containerId;
}
@Override
protected void onBackPressAdded() {
mFragmentManager.addOnBackStackChangedListener(mOnBackStackChangedListener);
}
@Override
protected void onBackPressRemoved() {
mFragmentManager.removeOnBackStackChangedListener(mOnBackStackChangedListener);
}
@Override
public boolean popBackStack() {
if (mBackStack.isEmpty()) {
return false;
}
if (mFragmentManager.isStateSaved()) {
Log.i(TAG, "Ignoring popBackStack() call: FragmentManager has already"
+ " saved its state");
return false;
}
if (mFragmentManager.getBackStackEntryCount() > 0) {
mFragmentManager.popBackStack(
generateBackStackName(mBackStack.size(), mBackStack.peekLast()),
FragmentManager.POP_BACK_STACK_INCLUSIVE);
mIsPendingBackStackOperation = true;
} // else, we're on the first Fragment, so there's nothing to pop from FragmentManager
mBackStack.removeLast();
return true;
}
@NonNull
@Override
public StickyFragmentNavigator.Destination createDestination() {
return new StickyFragmentNavigator.Destination(this);
}
@NonNull
public Fragment instantiateFragment(@NonNull Context context,
@SuppressWarnings("unused") @NonNull FragmentManager fragmentManager,
@NonNull String className, @Nullable Bundle args) {
return Fragment.instantiate(context, className, args);
}
@Nullable
@Override
public NavDestination navigate(@NonNull StickyFragmentNavigator.Destination destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
if (mFragmentManager.isStateSaved()) {
Log.i(TAG, "Ignoring navigate() call: FragmentManager has already"
+ " saved its state");
return null;
}
String className = destination.getClassName();
if (className.charAt(0) == '.') {
className = mContext.getPackageName() + className;
}
final FragmentTransaction ft = mFragmentManager.beginTransaction();
int enterAnim = navOptions != null ? navOptions.getEnterAnim() : -1;
int exitAnim = navOptions != null ? navOptions.getExitAnim() : -1;
int popEnterAnim = navOptions != null ? navOptions.getPopEnterAnim() : -1;
int popExitAnim = navOptions != null ? navOptions.getPopExitAnim() : -1;
if (enterAnim != -1 || exitAnim != -1 || popEnterAnim != -1 || popExitAnim != -1) {
enterAnim = enterAnim != -1 ? enterAnim : 0;
exitAnim = exitAnim != -1 ? exitAnim : 0;
popEnterAnim = popEnterAnim != -1 ? popEnterAnim : 0;
popExitAnim = popExitAnim != -1 ? popExitAnim : 0;
ft.setCustomAnimations(enterAnim, exitAnim, popEnterAnim, popExitAnim);
}
String tag = Integer.toString(destination.getId());
Fragment primaryNavigationFragment = mFragmentManager.getPrimaryNavigationFragment();
if(primaryNavigationFragment != null)
ft.hide(primaryNavigationFragment);
Fragment destinationFragment = mFragmentManager.findFragmentByTag(tag);
if(destinationFragment == null) {
destinationFragment = instantiateFragment(mContext, mFragmentManager, className, args);
destinationFragment.setArguments(args);
ft.add(mContainerId, destinationFragment , tag);
}
else
ft.show(destinationFragment);
ft.setPrimaryNavigationFragment(destinationFragment);
final @IdRes int destId = destination.getId();
final boolean initialNavigation = mBackStack.isEmpty();
// TODO Build first class singleTop behavior for fragments
final boolean isSingleTopReplacement = navOptions != null && !initialNavigation
&& navOptions.shouldLaunchSingleTop()
&& mBackStack.peekLast() == destId;
boolean isAdded;
if (initialNavigation) {
isAdded = true;
} else if (isSingleTopReplacement) {
// Single Top means we only want one instance on the back stack
if (mBackStack.size() > 1) {
// If the Fragment to be replaced is on the FragmentManager's
// back stack, a simple replace() isn't enough so we
// remove it from the back stack and put our replacement
// on the back stack in its place
mFragmentManager.popBackStackImmediate(
generateBackStackName(mBackStack.size(), mBackStack.peekLast()), 0);
mIsPendingBackStackOperation = false;
}
isAdded = false;
} else {
ft.addToBackStack(generateBackStackName(mBackStack.size() + 1, destId));
mIsPendingBackStackOperation = true;
isAdded = true;
}
if (navigatorExtras instanceof FragmentNavigator.Extras) {
FragmentNavigator.Extras extras = (FragmentNavigator.Extras) navigatorExtras;
for (Map.Entry<View, String> sharedElement : extras.getSharedElements().entrySet()) {
ft.addSharedElement(sharedElement.getKey(), sharedElement.getValue());
}
}
ft.setReorderingAllowed(true);
ft.commit();
// The commit succeeded, update our view of the world
if (isAdded) {
mBackStack.add(destId);
return destination;
} else {
return null;
}
}
@Override
@Nullable
public Bundle onSaveState() {
Bundle b = new Bundle();
int[] backStack = new int[mBackStack.size()];
int index = 0;
for (Integer id : mBackStack) {
backStack[index++] = id;
}
b.putIntArray(KEY_BACK_STACK_IDS, backStack);
return b;
}
@Override
public void onRestoreState(@Nullable Bundle savedState) {
if (savedState != null) {
int[] backStack = savedState.getIntArray(KEY_BACK_STACK_IDS);
if (backStack != null) {
mBackStack.clear();
for (int destId : backStack) {
mBackStack.add(destId);
}
}
}
}
@NonNull
private String generateBackStackName(int backStackIndex, int destId) {
return backStackIndex + "-" + destId;
}
private int getDestId(@Nullable String backStackName) {
String[] split = backStackName != null ? backStackName.split("-") : new String[0];
if (split.length != 2) {
throw new IllegalStateException("Invalid back stack entry on the "
+ "NavHostFragment's back stack - use getChildFragmentManager() "
+ "if you need to do custom FragmentTransactions from within "
+ "Fragments created via your navigation graph.");
}
try {
// Just make sure the backStackIndex is correctly formatted
Integer.parseInt(split[0]);
return Integer.parseInt(split[1]);
} catch (NumberFormatException e) {
throw new IllegalStateException("Invalid back stack entry on the "
+ "NavHostFragment's back stack - use getChildFragmentManager() "
+ "if you need to do custom FragmentTransactions from within "
+ "Fragments created via your navigation graph.");
}
}
@SuppressWarnings("WeakerAccess") /* synthetic access */
boolean isBackStackEqual() {
int fragmentBackStackCount = mFragmentManager.getBackStackEntryCount();
// Initial fragment won't be on the FragmentManager's back stack so +1 its count.
if (mBackStack.size() != fragmentBackStackCount + 1) {
return false;
}
// From top to bottom verify destination ids match in both back stacks/
Iterator<Integer> backStackIterator = mBackStack.descendingIterator();
int fragmentBackStackIndex = fragmentBackStackCount - 1;
while (backStackIterator.hasNext() && fragmentBackStackIndex >= 0) {
int destId = backStackIterator.next();
try {
int fragmentDestId = getDestId(mFragmentManager
.getBackStackEntryAt(fragmentBackStackIndex--)
.getName());
if (destId != fragmentDestId) {
return false;
}
} catch (NumberFormatException e) {
throw new IllegalStateException("Invalid back stack entry on the "
+ "NavHostFragment's back stack - use getChildFragmentManager() "
+ "if you need to do custom FragmentTransactions from within "
+ "Fragments created via your navigation graph.");
}
}
return true;
}
@NavDestination.ClassType(Fragment.class)
public static class Destination extends NavDestination {
private String mClassName;
public Destination(@NonNull NavigatorProvider navigatorProvider) {
this(navigatorProvider.getNavigator(StickyFragmentNavigator.class));
}
public Destination(@NonNull Navigator<? extends StickyFragmentNavigator.Destination> fragmentNavigator) {
super(fragmentNavigator);
}
@CallSuper
@Override
public void onInflate(@NonNull Context context, @NonNull AttributeSet attrs) {
super.onInflate(context, attrs);
TypedArray a = context.getResources().obtainAttributes(attrs,
R.styleable.FragmentNavigator);
String className = a.getString(R.styleable.FragmentNavigator_android_name);
if (className != null) {
setClassName(className);
}
a.recycle();
}
@NonNull
public final StickyFragmentNavigator.Destination setClassName(@NonNull String className) {
mClassName = className;
return this;
}
@NonNull
public final String getClassName() {
if (mClassName == null) {
throw new IllegalStateException("Fragment class was not set");
}
return mClassName;
}
}
public static final class Extras implements Navigator.Extras {
private final LinkedHashMap<View, String> mSharedElements = new LinkedHashMap<>();
Extras(Map<View, String> sharedElements) {
mSharedElements.putAll(sharedElements);
}
@NonNull
public Map<View, String> getSharedElements() {
return Collections.unmodifiableMap(mSharedElements);
}
public static final class Builder {
private final LinkedHashMap<View, String> mSharedElements = new LinkedHashMap<>();
@NonNull
public StickyFragmentNavigator.Extras.Builder addSharedElements(@NonNull Map<View, String> sharedElements) {
for (Map.Entry<View, String> sharedElement : sharedElements.entrySet()) {
View view = sharedElement.getKey();
String name = sharedElement.getValue();
if (view != null && name != null) {
addSharedElement(view, name);
}
}
return this;
}
@NonNull
public StickyFragmentNavigator.Extras.Builder addSharedElement(@NonNull View sharedElement, @NonNull String name) {
mSharedElements.put(sharedElement, name);
return this;
}
@NonNull
public StickyFragmentNavigator.Extras build() {
return new StickyFragmentNavigator.Extras(mSharedElements);
}
}
}
}
第 2 步：使用自定义标签
现在打开您的 navigation.xml 文件，并使用您之前在 @Navigator.Name() 中提供的任何名称重命名与底部导航相关的 fragment 标签。
<?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/navigation_home">
<sticky_fragment
android:id="@+id/navigation_home"
android:name="com.example.bottomnavigationlogic.ui.home.HomeFragment"
android:label="@string/title_home"
tools:layout="@layout/fragment_home" />
</navigation>
第 3 步：添加全局操作
全局操作是一种从应用中的任何位置导航到目的地的方式。您可以使用可视化编辑器或直接使用 xml 来添加全局操作。使用以下设置对每个片段设置全局操作
- 目的地：自己
- popUpTo：自我
- singleTop : true/checked
这就是你的 navigation.xml 添加全局操作后的样子
<?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/navigation_home">
<sticky_fragment
android:id="@+id/navigation_home"
android:name="com.example.bottomnavigationlogic.ui.home.HomeFragment"
android:label="@string/title_home"
tools:layout="@layout/fragment_home" />
<sticky_fragment
android:id="@+id/navigation_images"
android:name="com.example.bottomnavigationlogic.ui.images.ImagesFragment"
android:label="@string/title_images"
tools:layout="@layout/fragment_images" />
<sticky_fragment
android:id="@+id/navigation_videos"
android:name="com.example.bottomnavigationlogic.ui.videos.VideosFragment"
android:label="@string/title_videos"
tools:layout="@layout/fragment_videos" />
<sticky_fragment
android:id="@+id/navigation_songs"
android:name="com.example.bottomnavigationlogic.ui.songs.SongsFragment"
android:label="@string/title_songs"
tools:layout="@layout/fragment_songs" />
<sticky_fragment
android:id="@+id/navigation_notifications"
android:name="com.example.bottomnavigationlogic.ui.notifications.NotificationsFragment"
android:label="@string/title_notifications"
tools:layout="@layout/fragment_notifications" />
<action
android:id="@+id/action_global_navigation_home"
app:destination="@id/navigation_home"
app:launchSingleTop="true"
app:popUpTo="@id/navigation_home" />
<action
android:id="@+id/action_global_navigation_notifications"
app:destination="@id/navigation_notifications"
app:launchSingleTop="true"
app:popUpTo="@id/navigation_notifications" />
<action
android:id="@+id/action_global_navigation_songs"
app:destination="@id/navigation_songs"
app:launchSingleTop="true"
app:popUpTo="@id/navigation_songs" />
<action
android:id="@+id/action_global_navigation_videos"
app:destination="@id/navigation_videos"
app:launchSingleTop="true"
app:popUpTo="@id/navigation_videos" />
</navigation>
第 4 步：使用全局操作
你写的时候
NavigationUI.setupWithNavController (bottomNavigationView, navHostFragment.getNavController ());
然后在setupWithNavController() 内部，NavigationUI 使用bottomNavigationView.setOnNavigationItemSelectedListener() 根据单击的菜单项的 ID 导航到正确的片段。它的默认行为就像我之前提到的那样。我们将向其中添加我们自己的实现，并使用全局操作来实现我们想要的背压行为。
这就是你在MainActivity中的简单做法
bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
int id = menuItem.getItemId();
if (menuItem.isChecked()) return false;
switch (id)
{
case R.id.navigation_home :
navController.navigate(R.id.action_global_navigation_home);
break;
case R.id.navigation_images :
navController.navigate(R.id.action_global_navigation_images);
break;
case R.id.navigation_videos :
navController.navigate(R.id.action_global_navigation_videos);
break;
case R.id.navigation_songs :
navController.navigate(R.id.action_global_navigation_songs);
break;
case R.id.navigation_notifications :
navController.navigate(R.id.action_global_navigation_notifications);
break;
}
return true;
}
});
最后一步：将您的自定义导航器添加到 NavController
在 MainActivity 中添加导航器，如下所示。确保您传递的是childFragmentManager 的NavHostFragment。
navController.getNavigatorProvider().addNavigator(new StickyFragmentNavigator(this, navHostFragment.getChildFragmentManager(),R.id.nav_host_fragment));
在此处也可以使用setGraph() 方法将导航图添加到NavController，如下所示。
这是我的MainActivity 在步骤 4 和 步骤 5
之后的样子
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
BottomNavigationView navView = findViewById(R.id.nav_view);
AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
R.id.navigation_home, R.id.navigation_images, R.id.navigation_videos,R.id.navigation_songs,R.id.navigation_notifications)
.build();
NavHostFragment navHostFragment = (NavHostFragment)getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);
final NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
navController.getNavigatorProvider().addNavigator(new StickyFragmentNavigator(this, navHostFragment.getChildFragmentManager(),R.id.nav_host_fragment));
navController.setGraph(R.navigation.mobile_navigation);
NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
NavigationUI.setupWithNavController(navView,navController);
navView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
int id = menuItem.getItemId();
if (menuItem.isChecked()) return false;
switch (id)
{
case R.id.navigation_home :
navController.navigate(R.id.action_global_navigation_home);
break;
case R.id.navigation_images :
navController.navigate(R.id.action_global_navigation_images);
break;
case R.id.navigation_videos :
navController.navigate(R.id.action_global_navigation_videos);
break;
case R.id.navigation_songs :
navController.navigate(R.id.action_global_navigation_songs);
break;
case R.id.navigation_notifications :
navController.navigate(R.id.action_global_navigation_notifications);
break;
}
return true;
}
});
}
}
希望这会有所帮助。