Navigation——Fragment建立新的例項問題詳解

摘要： 背景 上一篇文章 Navigation——Fragment建立新的例項問題 ，我們簡述了我們在使用Navigation遇到的Fragment建立新的例項的問題。接下來，我們在這篇文章就來解決一下我們遇到的這個問題 原始碼追蹤 開啟 MainActivity 的佈局檔案，我們可以...

背景

上一篇文章 Navigation——Fragment建立新的例項問題 ，我們簡述了我們在使用Navigation遇到的Fragment建立新的例項的問題。接下來，我們在這篇文章就來解決一下我們遇到的這個問題

原始碼追蹤

開啟 MainActivity 的佈局檔案，我們可以看到在佈局檔案當中， Frangmet 這裡，有一個來自於 androidx的NavHostFragment。

<fragment
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/my_nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
app:navGraph="@navigation/nav_graph"
app:defaultNavHost="true"
/>

進入 NavHostFragment 的原始碼，我們一探究竟。

在 NavHostFragment 原始碼的 onCreate 方法當中，我們找到了答案。

完整的 onCreat 方法

@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final Context context = requireContext();

mNavController = new NavController(context);
mNavController.getNavigatorProvider().addNavigator(createFragmentNavigator());

Bundle navState = null;
if (savedInstanceState != null) {
navState = savedInstanceState.getBundle(KEY_NAV_CONTROLLER_STATE);
if (savedInstanceState.getBoolean(KEY_DEFAULT_NAV_HOST, false)) {
mDefaultNavHost = true;
requireFragmentManager().beginTransaction()
.setPrimaryNavigationFragment(this)
.commit();
}
}

if (navState != null) {
// Navigation controller state overrides arguments
mNavController.restoreState(navState);
}
if (mGraphId != 0) {
// Set from onInflate()
mNavController.setGraph(mGraphId);
} else {
// See if it was set by NavHostFragment.create()
final Bundle args = getArguments();
final int graphId = args != null ? args.getInt(KEY_GRAPH_ID) : 0;
final Bundle startDestinationArgs = args != null
? args.getBundle(KEY_START_DESTINATION_ARGS)
: null;
if (graphId != 0) {
mNavController.setGraph(graphId, startDestinationArgs);
}
}
}

在這其中，有一行

mNavController = new NavController(context);
mNavController.getNavigatorProvider().addNavigator(createFragmentNavigator());

也就是說，只要新增一個 Fragment 就會在 NavController 當中 Add 一個 FragmentNavigator ，而在 createFragmentNavigator 方法當中，Navigator 方法裡對 Fragment 進行了處理

@NonNull
protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
return new FragmentNavigator(requireContext(), getChildFragmentManager(), getId());
}

解決問題

既然，我們都已經找到導致沒次都建立新的例項的根結所在，那麼我們現在來解決一下問題。1那麼我們只需要重新寫一個NavHostFragment的createFragmentNavigator的方法，來滿足我們的要求。

/**
 * 複用的NavHostFragment （預設不是複用 引起一個問題就是 不儲存fragment狀態）
 */
class TabNavHostFragment : NavHostFragment() {
override fun createFragmentNavigator(): Navigator<out FragmentNavigator.Destination> {
return MyNavigator(requireContext(), childFragmentManager, id)
}

//參考相關連結
// https://stackoverflow.com/questions/50485988/is-there-a-way-to-keep-fragment-alive-when-using-bottomnavigationview-with-new-n/51684125
@Navigator.Name("tab_fragment")// 這個名稱在 navigation.xml 當中使用。
open class MyNavigator(var mContext: Context, var mFragmentManager: FragmentManager, var mContainerId: Int) :
FragmentNavigator(mContext, mFragmentManager, mContainerId) {
override fun navigate(
destination: Destination,
args: Bundle?,
navOptions: NavOptions?,
navigatorExtras: Navigator.Extras?
): NavDestination? {
try {
//反射獲取mBackStack mIsPendingBackStackOperation
val mBackStackField = FragmentNavigator::class.java.getDeclaredField("mBackStack")
mBackStackField.isAccessible = true
var mBackStack: ArrayDeque<Int> = mBackStackField.get(this) as ArrayDeque<Int>

val mIsPendingBackStackOperationField =
FragmentNavigator::class.java.getDeclaredField("mIsPendingBackStackOperation")
mIsPendingBackStackOperationField.isAccessible = true
var mIsPendingBackStackOperation: Boolean = mIsPendingBackStackOperationField.get(this) as Boolean

if (mFragmentManager.isStateSaved) {
//Log.i("TAG", "Ignoring navigate() call: FragmentManager has already" + " saved its state")
return null
}
var className = destination.className
if (className[0] == '.') {
className = mContext.packageName + className
}

val ft = mFragmentManager.beginTransaction()

var enterAnim = navOptions?.enterAnim ?: -1
var exitAnim = navOptions?.exitAnim ?: -1
var popEnterAnim = navOptions?.popEnterAnim ?: -1
var popExitAnim = navOptions?.popExitAnim ?: -1
if (enterAnim != -1 || exitAnim != -1 || popEnterAnim != -1 || popExitAnim != -1) {
enterAnim = if (enterAnim != -1) enterAnim else 0
exitAnim = if (exitAnim != -1) exitAnim else 0
popEnterAnim = if (popEnterAnim != -1) popEnterAnim else 0
popExitAnim = if (popExitAnim != -1) popExitAnim else 0
ft.setCustomAnimations(enterAnim, exitAnim, popEnterAnim, popExitAnim)
}

val tag = destination.id.toString()
//ft.replace(mContainerId, frag)

val currentFragment = mFragmentManager.primaryNavigationFragment
if (currentFragment != null) {
ft.hide(currentFragment)
}

var frag = mFragmentManager.findFragmentByTag(tag)
if (frag == null) {
frag = instantiateFragment(
mContext, mFragmentManager,
className, args
)
frag.arguments = args
ft.add(mContainerId, frag, tag)
} else {
ft.show(frag)
}

ft.setPrimaryNavigationFragment(frag)

@IdRes val destId = destination.id
val initialNavigation = mBackStack.isEmpty()
// TODO Build first class singleTop behavior for fragments
val isSingleTopReplacement = (navOptions != null && !initialNavigation
&& navOptions.shouldLaunchSingleTop()
&& mBackStack.peekLast().toInt() == destId)

val isAdded: Boolean
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.popBackStack(
generateMyBackStackName(mBackStack.size, mBackStack.peekLast()),
FragmentManager.POP_BACK_STACK_INCLUSIVE
)
ft.addToBackStack(generateMyBackStackName(mBackStack.size, destId))
mIsPendingBackStackOperation = true
mIsPendingBackStackOperationField.set(this, true)
}
isAdded = false
} else {
ft.addToBackStack(generateMyBackStackName(mBackStack.size + 1, destId))
mIsPendingBackStackOperation = true
mIsPendingBackStackOperationField.set(this, true)
isAdded = true
}
if (navigatorExtras is Extras) {
val extras = navigatorExtras as Extras?
for ((key, value) in extras!!.sharedElements) {
ft.addSharedElement(key, value)
}
}
ft.setReorderingAllowed(true)
ft.commit()
// The commit succeeded, update our view of the world
return if (isAdded) {
mBackStack.add(destId)
destination
} else {
null
}
} catch (e: Throwable) {
return super.navigate(destination, args, navOptions, navigatorExtras)
}
}

private fun generateMyBackStackName(backStackIndex: Int, destId: Int): String {
return "$backStackIndex-$destId"
}
}
}

然後，在我們的程式碼當中，引入我們自定修改之後的這個 TabNavHostFragment

在 MianActivity 的佈局檔案當中修改為

<fragment
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/my_nav_host_fragment"
android:name="com.demo.navigationcomponent.TabNavHostFragment"
app:navGraph="@navigation/nav_graph"
app:defaultNavHost="true"
/>

然後，在 nav_graph.xml檔案當中，修改為：

<tab_fragment android:id="@+id/oneFragment" android:name="com.demo.navigationcomponent.OneFragment"
android:label="fragment_one" tools:layout="@layout/fragment_one">
<action android:id="@+id/action_oneFragment_to_twoFragment"
app:destination="@id/twoFragment"
app:popUpTo="@id/oneFragment"
app:popUpToInclusive="true"/>
</tab_fragment>

至此，我們大功告成了，當我們在新增新的 Fragment 的時候，當已經建立過 Fragment 的例項的時候，就不會建立新的例項了。

最後

通過以上的方法，可以實現我們想要的效果，但是我認為這只是一個臨時的解決方案，修改原始碼這種方式，並不是一個特別好的解決方案。如果有其他更好的方法，方案，歡迎給我公共號「朝陽楊大爺」給我留言，討論。

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