对于导航组件的使用方式不是本文的重点，具体使用可以参考官方文档，导航组件框架是通过fragment来实现的，其核心类主要可以分为三个NavGraph、NavHostController、NavHostFragment，这三个类的作用分别是：
NavGraph：
解析导航图xml获取到的对象，其内部主要维护了一个集合用来存储目的地，当导航到目的地时，会传递进来一个id，这个id可能导航图xml中fragment的id，也有可能是fragment节点下action节点的id，如果是action节点的id，内部会转换成fragment的id（这也就是说，action节点不加也是可以的），这样就可以寻找到对应的fragment。
NavHostController：
导航控制的核心类，内部持有解析导航图xml的对象，还维护了导航回退栈，管理着导航中的逻辑处理。
NavHostFragment：
导航组件的入口，主要是初始化一些相关类，最主要的是持有NavHostController，可以控制整个导航图。
这里先看下在布局文件xml中的简单使用：
<fragment
android:id="@+id/nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:defaultNavHost="true"
app:layout_constraintBottom_toTopOf="@id/nav_view"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/mobile_navigation" />这里的name属性指定了androidx.navigation.fragment.NavHostFragment，熟悉fragment的应该知道，这里会去加载NavHostFragment，
public class NavHostFragment extends Fragment implements NavHost {
private static final String KEY_GRAPH_ID = "android-support-nav:fragment:graphId";
private static final String KEY_START_DESTINATION_ARGS =
"android-support-nav:fragment:startDestinationArgs";
private static final String KEY_NAV_CONTROLLER_STATE =
"android-support-nav:fragment:navControllerState";
private static final String KEY_DEFAULT_NAV_HOST = "android-support-nav:fragment:defaultHost";
private NavHostController mNavController;
private Boolean mIsPrimaryBeforeOnCreate = null;
private View mViewParent;
// State that will be saved and restored
private int mGraphId;
private boolean mDefaultNavHost;
@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;
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) {
// 会去解析xml导航图，mGraphId是从onInflate()设置进来的
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);
}
}
}
@SuppressWarnings({"WeakerAccess", "deprecation"})
@CallSuper
protected void onCreateNavController(@NonNull NavController navController) {
navController.getNavigatorProvider().addNavigator(
new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}
@SuppressWarnings("DeprecatedIsStillUsed")
@Deprecated
@NonNull
protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
return new FragmentNavigator(requireContext(), getChildFragmentManager(),
getContainerId());
}
@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;
}
... ...
}
}NavHostFragment这个类代码行数不多，这里在精简了下，保留了几个在初始化流程上的方法，布局中遇到fragment标签，会先进行创建view，执行到NavHostFragment就会先执行这里的onInflate()，可以看到这里获取到了导航图的id，并赋值给了变量mGraphId。接着就会调用到fragment的生命周期方法，也就是这里的onCreate()方法，在这里会先初始化NavHostController对象，然后调用了onCreateNavController()方法，这个方法和NavHostController的构造函数都创建了导航控制器并添加NavigatorProvider对象中，导航到指定页面时用到的就是这里的控制器，之后调用mNavController.setGraph(mGraphId)：
public void setGraph(@NavigationRes int graphResId) {
setGraph(graphResId, null);
}
@CallSuper
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}
@CallSuper
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);
}
mGraph = graph;
// 在导航图中配置的startDestination默认显示页面就是在这个方法中处理的
onGraphCreated(startDestinationArgs);
}可以看到，这里对导航图xml进行了解析，最终结果存储在NavGraph中，这里对xml的解析类似于布局xml的解析，这里就不进去看了，感兴趣的可以自己看看，在导航图的根标签下通常会配置startDestination属性指定启动的默认fragment，对这个属性的处理就在onGraphCreate()方法中：
private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
... ...
if (mGraph != null && mBackStack.isEmpty()) {
boolean deepLinked = !mDeepLinkHandled && mActivity != null
&& handleDeepLink(mActivity.getIntent());
if (!deepLinked) {
// Navigate to the first destination in the graph
// if we haven't deep linked to a destination
navigate(mGraph, startDestinationArgs, null, null);
}
} else {
dispatchOnDestinationChanged();
}
}这里会调用到navigate()这个方法，传递的是导航图中的根对象：
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
... ...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
... ...
}这里先获取到导航控制器，然后导航到对应的界面，关于导航控制器的添加，前面有说到，这里再来看下具体的添加：
public NavController(@NonNull Context context) {
... ...
mNavigatorProvider.addNavigator(new NavGraphNavigator(mNavigatorProvider));
mNavigatorProvider.addNavigator(new ActivityNavigator(mContext));
}调用的是NavigatorProvider的addNavigator()方法：
private final HashMap<String, Navigator<? extends NavDestination>> mNavigators = new HashMap<>();
public final Navigator<? extends NavDestination> addNavigator(
@NonNull Navigator<? extends NavDestination> navigator) {
String name = getNameForNavigator(navigator.getClass());
return addNavigator(name, navigator);
}
@CallSuper
@Nullable
public Navigator<? extends NavDestination> addNavigator(@NonNull String name,
@NonNull Navigator<? extends NavDestination> navigator) {
if (!validateName(name)) {
throw new IllegalArgumentException("navigator name cannot be an empty string");
}
return mNavigators.put(name, navigator);
}这里拿到的name是导航控制器类上的注解，比如：
@Navigator.Name("navigation")
public class NavGraphNavigator extends Navigator<NavGraph> {
... ...
}这里获取到的name就是这个navigation，并以这个name为key保存对应的导航控制器，这里回到上面的navigate()方法：
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
... ...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
... ...
}传入的node是导航图的根对象，node.getNavigatorName()获取到的值是navigation，故这里获取到的导航控制器是NavGraphNavigator，接着调用它的navigate()方法：
public NavDestination navigate(@NonNull NavGraph destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Extras navigatorExtras) {
int startId = destination.getStartDestination();
... ...
NavDestination startDestination = destination.findNode(startId, false);
... ...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
startDestination.getNavigatorName());
return navigator.navigate(startDestination, startDestination.addInDefaultArgs(args),
navOptions, navigatorExtras);
}先获取到导航图中配置的默认显示视图id，然后根据id找到对应的导航目的地，根据导航目的地获取对应导航控制器，以如下导航图xml为例：
<?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">
<fragment
android:id="@+id/navigation_home"
android:name="com.tangedegushi.jetpack_navigation.ui.home.HomeFragment"
android:label="@string/title_home"
tools:layout="@layout/fragment_home" />
</navigation>startDestination.getNavigatorName()获取到就是fragment，那对应的导航控制器是FragmentNavigator，接着调用它的navigate()方法：
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;
}
String className = destination.getClassName();
if (className.charAt(0) == '.') {
className = mContext.getPackageName() + className;
}
final Fragment frag = instantiateFragment(mContext, mFragmentManager,
className, args);
frag.setArguments(args);
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);
}
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;
}
}这里就是对fragment的操作了，执行完成后对应的视图也就显示出来了，关于点击导航的也类似，这里就不在赘述了。
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