Android中导航组件Navigation的实现原理

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);
            }
        }
    }

    /**
     * 创建导航控制器，在导航图中，导航到的目的地可以是fragment、activity、dialog、子导航图，
     * 导航到不同的目的地使用不同的控制器，此处提供的是dialog和fragment
     */
    @SuppressWarnings({"WeakerAccess", "deprecation"})
    @CallSuper
    protected void onCreateNavController(@NonNull NavController navController) {
        navController.getNavigatorProvider().addNavigator(
                new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
        navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
    }

    /**
     * 创建fragment的控制器
     */
    @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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