Jetpack之Navigation技术解密
Navigation是什么
官方的话：
Navigation 是一个框架，用于在 Android 应用中的“目标”之间导航，该框架提供一致的 API，无论目标是作为 Fragment、Activity 还是其他组件实现。
自己的话：
Navigation是管理Fragment之间导航的组件库，特别在实现单个Activity多个Fragment的管理模式更加灵活
Navigation应用篇
1.Navigation Graph。这是一种新型的XML资源文件，其中包含应用程序所有的页面，以及页面间的关系。
2.NavHostFragment。这是一个特殊的Fragment，你可以认为它是其他Fragment的“容器”，Navigation Graph中的Fragment正是通过NavHostFragment进行展示的。3.NavController。这是一个Java/Kotlin对象，用于在代码中完成Navigation Graph中具体的页面切换工作。
当你想切换Fragment时，使用NavController对象，告诉它你想要去Navigation Graph中的哪个Fragment，NavController会将你想去的Fragment展示在NavHostFragment中。
具体的使用可以参考：https://blog.csdn.net/Lbsssss/article/details/121660838
这里就直接略过了。
Navigation源码原理篇
1.NavHostFragment.create
@NonNull
public static NavHostFragment create(@NavigationRes int graphResId) {
return create(graphResId, null);
}
@NonNull
public static NavHostFragment create(@NavigationRes int graphResId,
@Nullable Bundle startDestinationArgs) {
Bundle b = null;
if (graphResId != 0) {
b = new Bundle();
b.putInt(KEY_GRAPH_ID, graphResId); // 把graphID存入到Bundle
}
if (startDestinationArgs != null) {
if (b == null) {
b = new Bundle();
}
b.putBundle(KEY_START_DESTINATION_ARGS, startDestinationArgs);
}
final NavHostFragment result = new NavHostFragment(); 初始化此对象
if (b != null) {
result.setArguments(b);
}
return result;
}
2.NavHostFragment.onInflate
@CallSuper
@Override
public void onInflate(@NonNull Context context, @NonNull AttributeSet attrs,
@Nullable Bundle savedInstanceState) {
super.onInflate(context, attrs, savedInstanceState);
final TypedArray navHost = context.obtainStyledAttributes(attrs,
androidx.navigation.R.styleable.NavHost);
final int graphId = navHost.getResourceId(
androidx.navigation.R.styleable.NavHost_navGraph, 0);
if (graphId != 0) {
mGraphId = graphId; // 就把此属性解析出来，activity_main.xml 的 app:navGraph="@navigation/nav_graph_main"
}
navHost.recycle();
final TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.NavHostFragment);
final boolean defaultHost = a.getBoolean(R.styleable.NavHostFragment_defaultNavHost, false);
if (defaultHost) {
mDefaultNavHost = true; // 就把此属性解析出来，activity_main.xml 的 app:defaultNavHost="true"
}
a.recycle();
}
3.NavHostFragment.onCreateNavController
@CallSuper
protected void onCreateNavController(@NonNull NavController navController) {
// 这里是通过导航暴露者，获取到 NavigationProvider 对象
navController.getNavigatorProvider().addNavigator(
new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}
public class NavigatorProvider {
// 把导航页面，例如 三个Fragment保存 与 key保存 记录
private static final HashMap<Class<?>, String> sAnnotationNames = new HashMap<>();
}
protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
return new FragmentNavigator(requireContext(), getChildFragmentManager(),
getContainerId()); // 此ID，如果不写xml文件，单纯用代码实现的时候，需要得到一个父容器ID
}
4.NavHostFragment.onCreateNav
@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
final Context context = requireContext();
mNavController = new NavHostController(context); // 此处 实例化出 NavHostController
mNavController.setLifecycleOwner(this);
mNavController.setOnBackPressedDispatcher(requireActivity().getOnBackPressedDispatcher());
// Set the default state - this will be updated whenever
// onPrimaryNavigationFragmentChanged() is called
mNavController.enableOnBackPressed(
mIsPrimaryBeforeOnCreate != null && mIsPrimaryBeforeOnCreate);
mIsPrimaryBeforeOnCreate = null;
mNavController.setViewModelStore(getViewModelStore());
onCreateNavController(mNavController);
// 下面代码是容错处理
Bundle navState = null;
if (savedInstanceState != null) {
navState = savedInstanceState.getBundle(KEY_NAV_CONTROLLER_STATE);
if (savedInstanceState.getBoolean(KEY_DEFAULT_NAV_HOST, false)) {
mDefaultNavHost = true;
getParentFragmentManager().beginTransaction()
.setPrimaryNavigationFragment(this)
.commit();
}
mGraphId = savedInstanceState.getInt(KEY_GRAPH_ID);
}
if (navState != null) {
// Navigation controller state overrides arguments
mNavController.restoreState(navState);
}
if (mGraphId != 0) { // 如果GraphID不等于空
// Set from onInflate()
mNavController.setGraph(mGraphId); // 设置GraphID，此处意义重大，会获取nav_graph_main里面的action等导航信息
} 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);
}
}
// We purposefully run this last as this will trigger the onCreate() of
// child fragments, which may be relying on having the NavController already
// created and having its state restored by that point.
super.onCreate(savedInstanceState);
}
5.NavController.setGraph
@CallSuper
public void setGraph(@NavigationRes int graphResId) {
setGraph(graphResId, null);
}
@CallSuper
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
// getNavInflater().inflate(graphResId) 开始真正的解析 nav_graph_main里面的action等导航信息，还要确定startDestination的Fragment
// startDestinationArgs 代表要启动 app:startDestination="@id/page1Fragment" 的参数信息
setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}
@SuppressLint("ResourceType")
@NonNull
public NavGraph inflate(@NavigationRes int graphResId) {
Resources res = mContext.getResources();
XmlResourceParser parser = res.getXml(graphResId);
final AttributeSet attrs = Xml.asAttributeSet(parser);
try {
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG
&& type != XmlPullParser.END_DOCUMENT) {
// Empty loop
}
if (type != XmlPullParser.START_TAG) {
throw new XmlPullParserException("No start tag found");
}
String rootElement = parser.getName();
NavDestination destination = inflate(res, parser, attrs, graphResId); // 解析时，主要是把目的地获取到了
if (!(destination instanceof NavGraph)) {
throw new IllegalArgumentException("Root element <" + rootElement + ">"
+ " did not inflate into a NavGraph");
}
return (NavGraph) destination; // 返回要导航的 目的地
} catch (Exception e) {
throw new RuntimeException("Exception inflating "
+ res.getResourceName(graphResId) + " line "
+ parser.getLineNumber(), e);
} finally {
parser.close();
}
}
// 退会后，看到 setGraph 细节
@CallSuper // 可以看到 把 GraphID xml 里面的内容 转变成 NavGraph对象了
public void setGraph(@NonNull NavGraph graph, @Nullable Bundle startDestinationArgs) {
if (mGraph != null) {
// Pop everything from the old graph off the back stack
popBackStackInternal(mGraph.getId(), true); // 对象信息有了后，先从栈里面去弹，看能不能弹出来，第一次肯定是弹不出来的
}
mGraph = graph; // 先把对象 保存到成员
onGraphCreated(startDestinationArgs); // 此函数就能看到，如何把我们的默认Fragment UI给显示出来了
}
6.NavController.onGraphCreated
private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
// 这个 这种 if的 ，对我们的主线流程意义不大，先跳过
if (mNavigatorStateToRestore != null) {
ArrayList<String> navigatorNames = mNavigatorStateToRestore.getStringArrayList(
KEY_NAVIGATOR_STATE_NAMES);
if (navigatorNames != null) {
for (String name : navigatorNames) {
Navigator<?> navigator = mNavigatorProvider.getNavigator(name);
Bundle bundle = mNavigatorStateToRestore.getBundle(name);
if (bundle != null) {
navigator.onRestoreState(bundle);
}
}
}
}
// 这里都是 栈状态的更新 保存 等处理，代码我们先不研究，不是说他不重要是，对主线流程研究 意义不大
if (mBackStackToRestore != null) {
for (Parcelable parcelable : mBackStackToRestore) {
NavBackStackEntryState state = (NavBackStackEntryState) parcelable;
NavDestination node = findDestination(state.getDestinationId());
if (node == null) {
final String dest = NavDestination.getDisplayName(mContext,
state.getDestinationId());
throw new IllegalStateException("Restoring the Navigation back stack failed:"
+ " destination " + dest
+ " cannot be found from the current destination "
+ getCurrentDestination());
}
Bundle args = state.getArgs();
if (args != null) {
args.setClassLoader(mContext.getClassLoader());
}
NavBackStackEntry entry = new NavBackStackEntry(mContext, node, args,
mLifecycleOwner, mViewModel,
state.getUUID(), state.getSavedState());
mBackStack.add(entry);
}
updateOnBackPressedCallbackEnabled();
mBackStackToRestore = null;
}
// 此代码就比较关键了，如果 前面辛辛苦苦保存的mGraph对象不为空，并且，栈里面是空的 取不出来的情况下，
if (mGraph != null && mBackStack.isEmpty()) {
boolean deepLinked = !mDeepLinkHandled && mActivity != null
&& handleDeepLink(mActivity.getIntent());
if (!deepLinked) {
// 这里说的很清楚了，导航到图表中的第一个目的地
// Navigate to the first destination in the graph
// if we haven't deep linked to a destination
navigate(mGraph, startDestinationArgs, null, null); // 看此处是如何导航的
}
} else {
dispatchOnDestinationChanged();
}
}
7. NavController.navigation
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
boolean popped = false;
boolean launchSingleTop = false;
if (navOptions != null) {
if (navOptions.getPopUpTo() != -1) {
popped = popBackStackInternal(navOptions.getPopUpTo(),
navOptions.isPopUpToInclusive());
}
}
// 同学们注意：这里是关键，真正的导航过程了
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(node.getNavigatorName());
// 导航需要的参数集
Bundle finalArgs = node.addInDefaultArgs(args);
// 构建新的目的地对象，此对象，就是后续要完成的目标了
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
if (newDest != null) {
if (!(newDest instanceof FloatingWindow)) {
// We've successfully navigating to the new destination, which means
// we should pop any FloatingWindow destination off the back stack
// before updating the back stack with our new destination
//noinspection StatementWithEmptyBody
while (!mBackStack.isEmpty()
&& mBackStack.peekLast().getDestination() instanceof FloatingWindow
&& popBackStackInternal(
mBackStack.peekLast().getDestination().getId(), true)) {
// Keep popping
}
}
// When you navigate() to a NavGraph, we need to ensure that a new instance
// is always created vs reusing an existing copy of that destination
ArrayDeque<NavBackStackEntry> hierarchy = new ArrayDeque<>();
NavDestination destination = newDest;
if (node instanceof NavGraph) {
do {
NavGraph parent = destination.getParent();
if (parent != null) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, parent,
finalArgs, mLifecycleOwner, mViewModel);
hierarchy.addFirst(entry);
// Pop any orphaned copy of that navigation graph off the back stack
if (!mBackStack.isEmpty()
&& mBackStack.getLast().getDestination() == parent) {
popBackStackInternal(parent.getId(), true);
}
}
destination = parent;
} while (destination != null && destination != node);
}
// Now collect the set of all intermediate NavGraphs that need to be put onto
// the back stack
destination = hierarchy.isEmpty()
? newDest
: hierarchy.getFirst().getDestination();
while (destination != null && findDestination(destination.getId()) == null) {
NavGraph parent = destination.getParent();
if (parent != null) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, parent, finalArgs,
mLifecycleOwner, mViewModel);
hierarchy.addFirst(entry);
}
destination = parent;
}
NavDestination overlappingDestination = hierarchy.isEmpty()
? newDest
: hierarchy.getLast().getDestination();
// Pop any orphaned navigation graphs that don't connect to the new destinations
//noinspection StatementWithEmptyBody
while (!mBackStack.isEmpty()
&& mBackStack.getLast().getDestination() instanceof NavGraph
&& ((NavGraph) mBackStack.getLast().getDestination()).findNode(
overlappingDestination.getId(), false) == null
&& popBackStackInternal(mBackStack.getLast().getDestination().getId(), true)) {
// Keep popping
}
mBackStack.addAll(hierarchy);
// The mGraph should always be on the back stack after you navigate()
if (mBackStack.isEmpty() || mBackStack.getFirst().getDestination() != mGraph) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, mGraph, finalArgs,
mLifecycleOwner, mViewModel);
mBackStack.addFirst(entry);
}
// And finally, add the new destination with its default args
NavBackStackEntry newBackStackEntry = new NavBackStackEntry(mContext, newDest,
newDest.addInDefaultArgs(finalArgs), mLifecycleOwner, mViewModel);
mBackStack.add(newBackStackEntry);
} else if (navOptions != null && navOptions.shouldLaunchSingleTop()) {
launchSingleTop = true;
NavBackStackEntry singleTopBackStackEntry = mBackStack.peekLast();
if (singleTopBackStackEntry != null) {
singleTopBackStackEntry.replaceArguments(finalArgs);
}
}
updateOnBackPressedCallbackEnabled();
if (popped || newDest != null || launchSingleTop) {
dispatchOnDestinationChanged();
}
}
8.Navigator.navigate
@Nullable
public abstract NavDestination navigate(@NonNull D destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Extras navigatorExtras);
9.FragmentNavigator.navigate
@Nullable
@Override
public NavDestination navigate(@NonNull 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;
}
// 根据Destination目的地，获取到第一个Fragmet 也就是 MainPage1Fragment实例
// 下面代码是反射 去 实例化 我们的 第一个Fragment
String className = destination.getClassName();
if (className.charAt(0) == '.') {
className = mContext.getPackageName() + className;
}
// instanceFragment反射的细节，可以看看
final Fragment frag = instantiateFragment(mContext, mFragmentManager,
className, args);
frag.setArguments(args);
final FragmentTransaction ft = mFragmentManager.beginTransaction();
// Fragment的进出 动画
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);
}
// 然后把mContainerId，替换到ft，相当于把 我们的 第一个Fragment加入到了 UI上面了
// 就是把 MainPage1Fragment 加入到 activity_main.xml 的 FragmentContainerView
// mContainerId（就是 FragmentContainerView）相当于容器
// frag（就是 我们的第一个Fragment实例 MainPage1Fragment）
ft.replace(mContainerId, frag);
ft.setPrimaryNavigationFragment(frag);
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.popBackStack(
generateBackStackName(mBackStack.size(), mBackStack.peekLast()),
FragmentManager.POP_BACK_STACK_INCLUSIVE);
ft.addToBackStack(generateBackStackName(mBackStack.size(), destId));
}
isAdded = false;
} else {
ft.addToBackStack(generateBackStackName(mBackStack.size() + 1, destId));
isAdded = true;
}
if (navigatorExtras instanceof Extras) {
Extras extras = (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;
}
}
@Deprecated
@NonNull
public Fragment instantiateFragment(@NonNull Context context,
@NonNull FragmentManager fragmentManager,
@NonNull String className, @SuppressWarnings("unused") @Nullable Bundle args) {
// instantiate 反射的细节看看
return fragmentManager.getFragmentFactory().instantiate(
context.getClassLoader(), className);
}
@NonNull
public Fragment instantiate(@NonNull ClassLoader classLoader, @NonNull String className) {
try {
Class<? extends Fragment> cls = loadFragmentClass(classLoader, className);
return cls.getConstructor().newInstance(); // 把最终方式的Fragment结果 给返回
} catch (java.lang.InstantiationException e) {
throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
+ ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (IllegalAccessException e) {
throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
+ ": make sure class name exists, is public, and has an"
+ " empty constructor that is public", e);
} catch (NoSuchMethodException e) {
throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
+ ": could not find Fragment constructor", e);
} catch (InvocationTargetException e) {
throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
+ ": calling Fragment constructor caused an exception", e);
}
}
注意：但onCreate执行完成后，界面就展示了第一个Fragment的画面了
10.小结
第一步：解析所有的目的地
第二步：放到一个HashMap里面全部保存好
第三步：获取第一个目的地Fragment
第四步：初始化目的地的所有信息
第五步：显示第一个目的地Fragment 画面
11.NavHostFragment.onViwCreated
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if (!(view instanceof ViewGroup)) {
throw new IllegalStateException("created host view " + view + " is not a ViewGroup");
}
Navigation.setViewNavController(view, mNavController); // 会把我们的 mNavController 加入到 Navigation里面去
// 说白了，如果是代码的方式去写的话，并非是xml的写法的话，就做下面的代码，但是 大部分都是 xml的形式
// 当以编程方式添加时，我们需要在父级上设置 NavController - 即具有与此 NavHostFragment 匹配的 ID 的视图
// When added programmatically, we need to set the NavController on the parent - i.e.,
// the View that has the ID matching this NavHostFragment.
if (view.getParent() != null) {
mViewParent = (View) view.getParent();
if (mViewParent.getId() == getId()) {
Navigation.setViewNavController(mViewParent, mNavController);
}
}
}
navigation角度上分析源码
1. MainPage1Fragment
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
val btn = view.findViewById<Button>(R.id.btn)
btn.setOnClickListener { view ->
Navigation.findNavController(view).navigate(R.id.action_page2) // 在这里开始导航
}
}
2.NavController.navigation
public void navigate(@IdRes int resId) {
navigate(resId, null);
}
public void navigate(@IdRes int resId, @Nullable Bundle args) {
navigate(resId, args, null);
}
public void navigate(@IdRes int resId, @Nullable Bundle args,
@Nullable NavOptions navOptions) {
navigate(resId, args, navOptions, null);
}
public void navigate(@IdRes int resId, @Nullable Bundle args, @Nullable NavOptions navOptions,
@Nullable Navigator.Extras navigatorExtras) {
// 目的 是在 Graph里面去寻找 当前节点的Fragment 为 目的地
NavDestination currentNode = mBackStack.isEmpty() // 先去看看栈有没有
? mGraph
: mBackStack.getLast().getDestination(); // 如果栈没有，就取最后一个
if (currentNode == null) {
throw new IllegalStateException("no current navigation node");
}
@IdRes int destId = resId;
// 获取节点后，就要去获取 action 导航动作，只有action才能通过resId 获取下一个要导航的目标id
final NavAction navAction = currentNode.getAction(resId);
Bundle combinedArgs = null;
if (navAction != null) {
if (navOptions == null) {
navOptions = navAction.getNavOptions();
}
destId = navAction.getDestinationId(); // 获取下一个目的地的 id号信息等
Bundle navActionArgs = navAction.getDefaultArguments();
if (navActionArgs != null) {
combinedArgs = new Bundle();
combinedArgs.putAll(navActionArgs);
}
}
if (args != null) {
if (combinedArgs == null) {
combinedArgs = new Bundle();
}
combinedArgs.putAll(args);
}
if (destId == 0 && navOptions != null && navOptions.getPopUpTo() != -1) {
popBackStack(navOptions.getPopUpTo(), navOptions.isPopUpToInclusive());
return;
}
if (destId == 0) {
throw new IllegalArgumentException("Destination id == 0 can only be used"
+ " in conjunction with a valid navOptions.popUpTo");
}
// 下面代码只是一个检查健壮性而已，先不管
NavDestination node = findDestination(destId);
if (node == null) {
final String dest = NavDestination.getDisplayName(mContext, destId);
if (navAction != null) {
throw new IllegalArgumentException("Navigation destination " + dest
+ " referenced from action "
+ NavDestination.getDisplayName(mContext, resId)
+ " cannot be found from the current destination " + currentNode);
} else {
throw new IllegalArgumentException("Navigation action/destination " + dest
+ " cannot be found from the current destination " + currentNode);
}
}
// 这行代码就是我们要关心的主线流程
navigate(node, combinedArgs, navOptions, navigatorExtras);
}
3.NavController.navigation
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
boolean popped = false;
boolean launchSingleTop = false;
if (navOptions != null) {
if (navOptions.getPopUpTo() != -1) {
popped = popBackStackInternal(navOptions.getPopUpTo(),
navOptions.isPopUpToInclusive());
}
}
// 同学们，这里又回到了，最初开始的代码哦
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
// 同学们 还记得 新的目的地么？ OK 分析完毕了，无效重复分析了
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
if (newDest != null) {
if (!(newDest instanceof FloatingWindow)) {
// We've successfully navigating to the new destination, which means
// we should pop any FloatingWindow destination off the back stack
// before updating the back stack with our new destination
//noinspection StatementWithEmptyBody
while (!mBackStack.isEmpty()
&& mBackStack.peekLast().getDestination() instanceof FloatingWindow
&& popBackStackInternal(
mBackStack.peekLast().getDestination().getId(), true)) {
// Keep popping
}
}
// When you navigate() to a NavGraph, we need to ensure that a new instance
// is always created vs reusing an existing copy of that destination
ArrayDeque<NavBackStackEntry> hierarchy = new ArrayDeque<>();
NavDestination destination = newDest;
if (node instanceof NavGraph) {
do {
NavGraph parent = destination.getParent();
if (parent != null) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, parent,
finalArgs, mLifecycleOwner, mViewModel);
hierarchy.addFirst(entry);
// Pop any orphaned copy of that navigation graph off the back stack
if (!mBackStack.isEmpty()
&& mBackStack.getLast().getDestination() == parent) {
popBackStackInternal(parent.getId(), true);
}
}
destination = parent;
} while (destination != null && destination != node);
}
// Now collect the set of all intermediate NavGraphs that need to be put onto
// the back stack
destination = hierarchy.isEmpty()
? newDest
: hierarchy.getFirst().getDestination();
while (destination != null && findDestination(destination.getId()) == null) {
NavGraph parent = destination.getParent();
if (parent != null) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, parent, finalArgs,
mLifecycleOwner, mViewModel);
hierarchy.addFirst(entry);
}
destination = parent;
}
NavDestination overlappingDestination = hierarchy.isEmpty()
? newDest
: hierarchy.getLast().getDestination();
// Pop any orphaned navigation graphs that don't connect to the new destinations
//noinspection StatementWithEmptyBody
while (!mBackStack.isEmpty()
&& mBackStack.getLast().getDestination() instanceof NavGraph
&& ((NavGraph) mBackStack.getLast().getDestination()).findNode(
overlappingDestination.getId(), false) == null
&& popBackStackInternal(mBackStack.getLast().getDestination().getId(), true)) {
// Keep popping
}
mBackStack.addAll(hierarchy);
// The mGraph should always be on the back stack after you navigate()
if (mBackStack.isEmpty() || mBackStack.getFirst().getDestination() != mGraph) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, mGraph, finalArgs,
mLifecycleOwner, mViewModel);
mBackStack.addFirst(entry); // 留意一下，每次都会管理 进栈
}
// And finally, add the new destination with its default args
NavBackStackEntry newBackStackEntry = new NavBackStackEntry(mContext, newDest,
newDest.addInDefaultArgs(finalArgs), mLifecycleOwner, mViewModel);
mBackStack.add(newBackStackEntry);
} else if (navOptions != null && navOptions.shouldLaunchSingleTop()) {
launchSingleTop = true;
NavBackStackEntry singleTopBackStackEntry = mBackStack.peekLast();
if (singleTopBackStackEntry != null) {
singleTopBackStackEntry.replaceArguments(finalArgs);
}
}
updateOnBackPressedCallbackEnabled();
if (popped || newDest != null || launchSingleTop) {
dispatchOnDestinationChanged();
}
}
1.官网上：MainActivity去绑定Navigation （实际上：源码理解 Fragment帖到MainActivity）
2.NavHostFragment create 实例化NavHostFragment 并且 赋予丰富的信息而已
3.NavHostFragment.onInflate属于Fragment的生命周期 { app:defaultNavHost=“true”
app:navGraph=“@navigation/nav_graph_main” }
4.protected void onCreateNavController(@NonNull NavController navController) {
NavigatorProvider HashMap会去保存所有的Fragment信息 3个
getContainerId() 不写xml的时候，写Java代码的是才有用
}
5.onCreate
mContainerId