Android中导航组件Navigation的实现原理

Kiona ·

更新时间:2024-09-20

· 481 次阅读

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。

到此这篇关于Android中导航组件Navigation的实现原理的文章就介绍到这了,更多相关Android导航组件Navigation内容请搜索软件开发网以前的文章或继续浏览下面的相关文章希望大家以后多多支持软件开发网！

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