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Android导航组件Navigation

tangedegushi 人气:0

        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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