这一节主要是了解一下Navigation的源码，主要涉及如下类: 1 NavHostFragment：它相当于是Fragment和NavController的纽带，并提供导航的容器布局；2 NavController 导航组件的核心，用它来加载xml中Fragment节点并转化成NavDestination,再通过navigate(）方法切换NavDestionation,以实现Fragment的切换。NavigationUI,它为各个View设置接口监听，将View的UI状态和NavController中的Fragment做了绑定。
从xml布局入手:
<fragment
android:id="@+id/my_nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
app:defaultNavHost="true"
app:navGraph="@navigation/mobile_navigation" />
说明:引用androidx库中NavHostFragment,既承载导航界面，又管理控制导航行为，看看它在创建时做了什么:
@CallSuper
@Override
public void onAttach(@NonNull Context context) {
super.onAttach(context);
// TODO This feature should probably be a first-class feature of the Fragment system,
// but it can stay here until we can add the necessary attr resources to
// the fragment lib.
if (mDefaultNavHost) {
requireFragmentManager().beginTransaction()
.setPrimaryNavigationFragment(this)
.commit();
}
}在onAttach生命周期 通过defaultNavHost 条件判读 开启事务将它自己设置成了PrimaryFragment，
@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final Context context = requireContext();
mNavController = new NavHostController(context);
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);
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) {
// 如果存在之前Fragment,恢复状态
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);
}
}
}
protected void onCreateNavController(@NonNull NavController navController) {
navController.getNavigatorProvider().addNavigator(
new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}
protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
return new FragmentNavigator(requireContext(), getChildFragmentManager(), getId());
}创建了一个NavController，并且通过navController 创建一个FragmentNavigator, 这个类继承Navigator, 它为每个Navigation设置策略，Fragment之间的导航切换是通过它来实现的; 然后看一下NavController设置setGraph(),在 xml里面定义的navGraph，导航布局里面的Fragment及action跳转等信息；它是如何将Fragment添加进去的 NavController.setGraph()
NavController.java
public void setGraph(@NavigationRes int graphResId) {
setGraph(graphResId, null);
}
//恢复状态
private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
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);
}
}
}
}
...
if (mGraph != null && mBackStack.isEmpty()) {
boolean deepLinked = !mDeepLinkHandled && mActivity != null
&& handleDeepLink(mActivity.getIntent());
if (!deepLinked) {
// 如果没有深层链接 导航到目标Fragment
navigate(mGraph, startDestinationArgs, null, null);
}
}
}
如果设置的graph不为null，它执行了popBackStackInternal
boolean popBackStackInternal(@IdRes int destinationId, boolean inclusive) {
if (mBackStack.isEmpty()) {
// Nothing to pop if the back stack is empty
return false;
}
ArrayList<Navigator> popOperations = new ArrayList<>();
Iterator<NavBackStackEntry> iterator = mBackStack.descendingIterator();
boolean foundDestination = false;
while (iterator.hasNext()) {
NavDestination destination = iterator.next().getDestination();
Navigator navigator = mNavigatorProvider.getNavigator(
destination.getNavigatorName());
if (inclusive || destination.getId() != destinationId) {
popOperations.add(navigator);
}
if (destination.getId() == destinationId) {
foundDestination = true;
break;
}
}
...
boolean popped = false;
for (Navigator navigator : popOperations) {
if (navigator.popBackStack()) {
NavBackStackEntry entry = mBackStack.removeLast();
if (mViewModel != null) {
mViewModel.clear(entry.mId);
}
popped = true;
} else {
// The pop did not complete successfully, so stop immediately
break;
}
}
updateOnBackPressedCallbackEnabled();
return popped;
}说明:remove之前所有的navigator,mBackStack是在哪里初始化的 如下:
private final Deque<NavBackStackEntry> mBackStack = new ArrayDeque<>();
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
boolean popped = false;
if (navOptions != null) {
if (navOptions.getPopUpTo() != -1) {
popped = popBackStackInternal(navOptions.getPopUpTo(),
navOptions.isPopUpToInclusive());
}
}
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
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
}
}
// The mGraph should always be on the back stack after you navigate()
if (mBackStack.isEmpty()) {
mBackStack.add(new NavBackStackEntry(mGraph, finalArgs, mViewModel));
}
// Now ensure all intermediate NavGraphs are put on the back stack
// to ensure that global actions work.
ArrayDeque<NavBackStackEntry> hierarchy = new ArrayDeque<>();
NavDestination destination = newDest;
while (destination != null && findDestination(destination.getId()) == null) {
NavGraph parent = destination.getParent();
if (parent != null) {
hierarchy.addFirst(new NavBackStackEntry(parent, finalArgs, mViewModel));
}
destination = parent;
}
mBackStack.addAll(hierarchy);
// 添加新的Destination
NavBackStackEntry newBackStackEntry = new NavBackStackEntry(newDest,
newDest.addInDefaultArgs(finalArgs), mViewModel);
mBackStack.add(newBackStackEntry);
}
updateOnBackPressedCallbackEnabled();
if (popped || newDest != null) {
dispatchOnDestinationChanged();
}
}这个方法的调用是在 手动切换fragment时 调用:
findNavConroller().navigate(R.id.bottomNavFragment);//Dispatch changes to all OnDestinationChangedListeners.
private boolean dispatchOnDestinationChanged() {
// We never want to leave NavGraphs on the top of the stack
//noinspection StatementWithEmptyBody
while (!mBackStack.isEmpty()
&& mBackStack.peekLast().getDestination() instanceof NavGraph
&& popBackStackInternal(mBackStack.peekLast().getDestination().getId(), true)) {
// Keep popping
}
if (!mBackStack.isEmpty()) {
NavBackStackEntry backStackEntry = mBackStack.peekLast();
for (OnDestinationChangedListener listener :
mOnDestinationChangedListeners) {
listener.onDestinationChanged(this, backStackEntry.getDestination(),
backStackEntry.getArguments());
}
return true;
}
return false;
}说明: 分发目标界面切换。看一下这个接口的实现类，AbstractAppBarOnDestinationChangedListener
@Override
public void onDestinationChanged(@NonNull NavController controller,
@NonNull NavDestination destination, @Nullable Bundle arguments) {
if (destination instanceof FloatingWindow) {
return;
}
DrawerLayout drawerLayout = mDrawerLayoutWeakReference != null
? mDrawerLayoutWeakReference.get()
: null;
if (mDrawerLayoutWeakReference != null && drawerLayout == null) {
controller.removeOnDestinationChangedListener(this);
return;
}
CharSequence label = destination.getLabel();
if (!TextUtils.isEmpty(label)) {
// Fill in the data pattern with the args to build a valid URI
StringBuffer title = new StringBuffer();
Pattern fillInPattern = Pattern.compile("\\{(.+?)\\}");
Matcher matcher = fillInPattern.matcher(label);
while (matcher.find()) {
String argName = matcher.group(1);
if (arguments != null && arguments.containsKey(argName)) {
matcher.appendReplacement(title, "");
//noinspection ConstantConditions
title.append(arguments.get(argName).toString());
} else {
throw new IllegalArgumentException("Could not find " + argName + " in "
+ arguments + " to fill label " + label);
}
}
matcher.appendTail(title);
setTitle(title);
}
boolean isTopLevelDestination = NavigationUI.matchDestinations(destination,
mTopLevelDestinations);
if (drawerLayout == null && isTopLevelDestination) {
setNavigationIcon(null, 0);//设置icon
} else {
//设置返回箭头
setActionBarUpIndicator(drawerLayout != null && isTopLevelDestination);
}
}到此 大体能知道切换Fragment的流程: 1 将目标Fragment放到栈顶 2 分发目标Fragment状态 3 设置 ActionBar 4 设置setNavigationIcon
看一下Navigation.findNavController(this,R.id.fragment_home)
@NonNull
public static NavController findNavController(@NonNull Activity activity, @IdRes int viewId) {
View view = ActivityCompat.requireViewById(activity, viewId);
//通过递归View层级 来获取Controller
NavController navController = findViewNavController(view);
if (navController == null) {
throw new IllegalStateException("Activity " + activity
+ " does not have a NavController set on " + viewId);
}
return navController;
}
@Nullable
private static NavController findViewNavController(@NonNull View view) {
while (view != null) {
NavController controller = getViewNavController(view);
if (controller != null) {
return controller;
}
ViewParent parent = view.getParent();
view = parent instanceof View ? (View) parent : null;
}
return null;
}什么时候setViewNavController的？NavHostFragment 初始化的时候 ：
NavHostFragment.java
@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);
// 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) {
View rootView = (View) view.getParent();
if (rootView.getId() == getId()) {
Navigation.setViewNavController(rootView, mNavController);
}
}
}在视图创建的时候设置的，简单的一个navigation源码分析到此完毕。