每当我重新单击或导航到下一个片段时，我的片段都会不断重新创建
Kyl*_*tta 10 java android android-fragments
我已经在我的 android 应用程序上实现了新的架构组件，但不幸的是，处理这些片段的状态对我来说已经成为一场噩梦。每当我按下片段的图标时，每次导航时都会重新创建片段。我该如何处理这个或者更确切地说保存这些片段状态？
这是我处理五个片段的主要活动：
public class MainActivityCenterofInformation extends AppCompatActivity {
BottomNavigationView bottomNavigationView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate (savedInstanceState);
setContentView (R.layout.activity_maincict);
setUpNavigation ();
}
public void setUpNavigation(){
bottomNavigationView = findViewById (R.id.bottom_nav_cict);
NavHostFragment navHostFragment = (NavHostFragment)getSupportFragmentManager ()
.findFragmentById (R.id.nav_host_fragment_cict);
NavigationUI.setupWithNavController (bottomNavigationView, navHostFragment.getNavController ());
}
//adding animations to the fragment
}
我看不懂 Kotlin，所以请指导我使用 Java，谢谢。
Sye*_*mil 28
TL;DR：跳到只给我展示步骤！！！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，并且您假设使用onSaveInstanceState.
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，您可以使用View Model，它是以下推荐的 android 架构的一部分。它们是保留和恢复 UI 数据的好方法。
您可以按照此分步代码实验室学习如何实现此架构
编辑：解决方案
花了一段时间，但就在这里。解决方案目前不使用ViewModels。
请仔细阅读，因为每一步都很重要。本解决方案包括以下两部分
- 在返回键按下时实现正确的导航
- 在导航期间保持片段活着
背景 ：
Android Navigation 组件提供了一个NavController用于在不同目的地之间导航的类。在内部NavController使用Navigator实际进行导航的 。Navigator是一个抽象类，任何人都可以扩展/继承这个类来创建自己的自定义导航器，以根据目的地的类型提供自定义导航。当使用片段作为目的地时，NavHostFragment使用FragmentNavigator它的默认实现会在我们导航时替换片段，使用FragmentTransaction.replace()它完全破坏以前的片段并添加新片段。因此，我们必须创建自己的导航仪，而不是使用FragmentTransaction.replace()我们将使用的组合FragmentTransaction.hide()，并FragmentTransaction.show()避免被破坏片段。
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
您的后台堆栈将只有
[A, E]
正如你所看到的，片段 B、C、D 没有被添加到 backstack 中，所以按回按总是会让你得到片段 A，这是主片段
我们现在想要的行为：
我们想要一个简单而有效的行为。我们不想将所有片段都添加到 backstack，但是如果片段已经在 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，只应弹出最后一个片段，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 应该是这样的
[A, B]
我希望这种行为是有道理的。正如您将在下面看到的那样，这种行为很容易使用全局操作来实现，并且比默认行为更好。
好帅哥！怎么办 ？：
现在我们有两个选择
- 延长
FragmentNavigator - 复制粘贴
FragmentNavigator
好吧，我个人只想扩展FragmentNavigator和覆盖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，然后将整个代码中的名称从“FragmentNavigator”更改为我想要的任何名称。
只是告诉我步骤已经！：
- 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. FragmentNavigator除了navigate()方法之外，所有代码都相同。如您所见，它使用hide(),show()和add()方法而不是replace(). 逻辑很简单。隐藏上一个片段并显示目标片段。如果这是我们第一次访问特定目标片段，则添加该片段而不是显示它。
@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文件并使用您之前提供的任何名称重命名与底部导航相关的片段标签@Navigator.Name()。
<?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程序中的任何位置导航到目的地的方法。您可以使用可视化编辑器或直接使用 xml 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 : 自己
- singleTop : 真/检查
这是您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_v
-
@KyleMutta 很抱歉让你等了一个多星期。实际上，我一直在寻找一种需要最少代码并从导航组件库转移的解决方案。但最终我还是找到了一个。阅读更新的解决方案。它有点长但值得一读。希望能帮助到你。 (2认同)
-
应该没那么难 (2认同)
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