前言

Navigation庫并不像Lifecycle、LiveData、ViewModel能夠優(yōu)雅地解決我們在開發(fā)中常遇到的問題种呐。它只是對我們以前在Activity和Fragment的界面跳轉(zhuǎn)方式做了統(tǒng)一的封裝汁讼，可以讓開發(fā)者減少一些模版代碼的編寫，提升開發(fā)效率耸彪。

簡單使用

1. 添加依賴

implementation 'androidx.navigation:navigation-fragment:2.3.5'
implementation 'androidx.navigation:navigation-ui:2.3.5'

2. 創(chuàng)建兩個Fragment，分別叫做FirstFragment和SecondFragment

3. 在res目錄下新建一個名為navigation文件夾蜀肘，在文件夾內(nèi)新建一個名為demo_navigation.xml文件，文件內(nèi)容如下:

<?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/first_fragment">

    <!--  startDestination表示要加載首頁Fragment  -->

    <fragment
        android:id="@+id/first_fragment"
        android:name="com.pbl.navigation.navigationdemo.ui.fragment.FirstFragment"
        android:label="第一個fragment"
        tools:layout="@layout/fragment_first" />

    <fragment
        android:id="@+id/second_fragment"
        android:name="com.pbl.navigation.navigationdemo.ui.fragment.SecondFragment"
        android:label="第二個fragment"
        tools:layout="@layout/fragment_second" />
</navigation>

4. 在MainActivity的布局文件中添加承載Fragment的容器

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!--
        defaultNavHost表示是否要攔截系統(tǒng)返回鍵薄腻，在Fragment界面罢艾，
        攔截它是用來處理Fragment回退棧問題
    -->

    <!--
        navGraph給示導航圖童漩，也就是我們所有Fragment都會被囊括在這個圖內(nèi)期奔，
        這樣Navigation框架就可以根據(jù)id導航到目標Fragment
    -->


    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/nav_host_fragment_activity_main"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/demo_navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

從上面可以看出弥搞，承載Fragment的容器是FragmentContainerView，它是一個FrameLayout，而它承載的第一個Fragment是androidx.navigation.fragment.NavHostFragment。很顯然，這是一個占位Fragment赌躺。我們的FirstFragment在顯示的時候肯定會替換它掠归，因此可以推測出會調(diào)用Fragment的replace方法码泞。

5. 在MainActivity的onCreate使用它

// 使用1
NavHostFragment fragment = (NavHostFragment) getSupportFragmentManager()
        .findFragmentById(R.id.nav_host_fragment_activity_main);
// 使用2
NavController controller = fragment.getNavController();
// 使用3
NavigationUI.setupActionBarWithNavController(this, controller);

6. 在FirstFragment中有一個Button跳轉(zhuǎn)到SecondFragment, 它的onClickListener的實現(xiàn)為:

// 跳轉(zhuǎn)到某個Fragment
Navigation.findNavController(v).navigate(R.id.second_fragment);

// 返回上個Fragment
// Navigation.findNavController(v).navigateUp();

源碼分析

使用1處的代碼宋舷，以前在開發(fā)中使用Fragment時也會遇到绎狭，所以對于我們來說是不陌生的。通過xml文件中標明的id，找到Navigation框架給我們提供的占位Fragment: NavHostFragment摆屯。接下來我們就來看看這個Fragment的生命周期做了一些什么事富弦。

@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    final Context context = requireContext();
    // 注釋1
    mNavController = new NavHostController(context);
    // Lifecycle相關(guān)
    mNavController.setLifecycleOwner(this);
    // 返回鍵相關(guān)
    mNavController.setOnBackPressedDispatcher(requireActivity().getOnBackPressedDispatcher());
    // Set the default state - this will be updated whenever
    // onPrimaryNavigationFragmentChanged() is called
    mNavController.enableOnBackPressed(
            mIsPrimaryBeforeOnCreate != null && mIsPrimaryBeforeOnCreate);
    mIsPrimaryBeforeOnCreate = null;
    // ViewModel相關(guān)
    mNavController.setViewModelStore(getViewModelStore());
    // 注釋2
    onCreateNavController(mNavController);

    ...
    // 注釋3
    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);
        }
    }
    super.onCreate(savedInstanceState);
}

先看注釋1部分

public NavController(@NonNull Context context) {
    mContext = context;
    while (context instanceof ContextWrapper) {
        if (context instanceof Activity) {
            mActivity = (Activity) context;
            break;
        }
        context = ((ContextWrapper) context).getBaseContext();
    }
    mNavigatorProvider.addNavigator(new NavGraphNavigator(mNavigatorProvider));
    mNavigatorProvider.addNavigator(new ActivityNavigator(mContext));
}

NavHostController繼承于NavController蓖扑，創(chuàng)建其對象時就做了一件事柜去，添加了兩個Navigator浑厚，Navigator是用來控制界面導航的基類物蝙。在這里添加了一個ActivityNavigator，那肯定還會有一個FragmentNavigator

看看注釋2部分

@CallSuper
protected void onCreateNavController(@NonNull NavController navController) {
    navController.getNavigatorProvider().addNavigator(
            new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
    navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}


@Deprecated
@NonNull
protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
    return new FragmentNavigator(requireContext(), getChildFragmentManager(),
            getContainerId());
}

上面我們猜對了，又添加了兩個Navigator。我們繼續(xù)看注釋3部分湃望。到底是執(zhí)行if里面代碼還是else里面的代碼呢？我們先來看看這個mGraphId是在哪里賦值的。

@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;
    }
    navHost.recycle();

    final TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.NavHostFragment);
    final boolean defaultHost = a.getBoolean(R.styleable.NavHostFragment_defaultNavHost, false);
    if (defaultHost) {
        mDefaultNavHost = true;
    }
    a.recycle();
}

onInflate方法先于onCreate執(zhí)行官硝，它主要做了一件事傻咖，獲取xml文件中自定義屬性『τ伲回看上文簡單用法的第4步添加的xml內(nèi)容，有兩個自定義的屬性溪王，一個是navGraph，通過這個graphId就可以獲取我們寫在res/navigation/demo_navigation.xml里面的內(nèi)容了道伟。另一個是defaultNavHost，是否需要攔截系統(tǒng)返回鍵。

我們再回到注釋3盆色，很顯然這里會執(zhí)行if語句塊里面的代碼，我們繼續(xù)跟進：

@CallSuper
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
    setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}

inflate方法應該就是去解析我們那個demo_navigation.xml文件宣旱，來繼續(xù)跟蹤源碼

@SuppressLint("ResourceType")
@NonNull
public NavGraph inflate(@NavigationRes int graphResId) {
    Resources res = mContext.getResources();
    XmlResourceParser parser = res.getXml(graphResId);
    final AttributeSet attrs = Xml.asAttributeSet(parser);
    try {
        int type;
        while ((type = parser.next()) != XmlPullParser.START_TAG
                && type != XmlPullParser.END_DOCUMENT) {
            // Empty loop
        }
        if (type != XmlPullParser.START_TAG) {
            throw new XmlPullParserException("No start tag found");
        }

        String rootElement = parser.getName();
        // 真正解析xml文件在這里
        NavDestination destination = inflate(res, parser, attrs, graphResId);
        if (!(destination instanceof NavGraph)) {
            throw new IllegalArgumentException("Root element <" + rootElement + ">"
                    + " did not inflate into a NavGraph");
        }
        return (NavGraph) destination;
    } catch (Exception e) {
        throw new RuntimeException("Exception inflating "
                + res.getResourceName(graphResId) + " line "
                + parser.getLineNumber(), e);
    } finally {
        parser.close();
    }
}

@NonNull
private NavDestination inflate(@NonNull Resources res, @NonNull XmlResourceParser parser,
        @NonNull AttributeSet attrs, int graphResId)
        throws XmlPullParserException, IOException {
    // 注釋4
    Navigator<?> navigator = mNavigatorProvider.getNavigator(parser.getName());
    final NavDestination dest = navigator.createDestination();

    dest.onInflate(mContext, attrs);

    final int innerDepth = parser.getDepth() + 1;
    int type;
    int depth;
    while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
            && ((depth = parser.getDepth()) >= innerDepth
            || type != XmlPullParser.END_TAG)) {
        if (type != XmlPullParser.START_TAG) {
            continue;
        }

        if (depth > innerDepth) {
            continue;
        }

        final String name = parser.getName();
        if (TAG_ARGUMENT.equals(name)) {
            inflateArgumentForDestination(res, dest, attrs, graphResId);
        } else if (TAG_DEEP_LINK.equals(name)) {
            inflateDeepLink(res, dest, attrs);
        } else if (TAG_ACTION.equals(name)) {
            inflateAction(res, dest, attrs, parser, graphResId);
        } else if (TAG_INCLUDE.equals(name) && dest instanceof NavGraph) {
            final TypedArray a = res.obtainAttributes(
                    attrs, androidx.navigation.R.styleable.NavInclude);
            final int id = a.getResourceId(
                    androidx.navigation.R.styleable.NavInclude_graph, 0);
            ((NavGraph) dest).addDestination(inflate(id));
            a.recycle();
        } else if (dest instanceof NavGraph) {
            // 注釋5
            ((NavGraph) dest).addDestination(inflate(res, parser, attrs, graphResId));
        }
    }

    return dest;
}

注釋4處的navigator對象到底是指哪一個呢粪糙？再回看上文的注釋1部分說明，添加的第一個navigator對象就是NavGraphNavigator寞酿。它在這里做了一件事伐弹，通過createDestination方法創(chuàng)建了一個NavDestination對象，它是用來干嘛的呢日川？

根據(jù)注釋5得知，原來每一個fragment標簽信息都會被封裝成一個個NavDestination對象，這些destination對象的管者理就是NavGraph對象欧漱。

@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;
    onGraphCreated(startDestinationArgs);
}

當demo_navigation.xml被解析成一個NavGraph對象后，通過這個對象跳轉(zhuǎn)到

private void onGraphCreated(@Nullable Bundle startDestinationArgs) {

    ...
    // 首次進來硫椰，后退棧mBackStack數(shù)據(jù)是為空的
    if (mGraph != null && mBackStack.isEmpty()) {
        // mDeepLinkHandled第一次進來時為false
        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
            // 導航到導航圖中的第一個界面
            navigate(mGraph, startDestinationArgs, null, null);
        }
    } else {
        dispatchOnDestinationChanged();
    }
}

private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
        @Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
    boolean popped = false;
    boolean launchSingleTop = false;
    if (navOptions != null) {
        if (navOptions.getPopUpTo() != -1) {
            popped = popBackStackInternal(navOptions.getPopUpTo(),
                    navOptions.isPopUpToInclusive());
        }
    }
    // 這里由于是導航到第一個Fragment界面了靶草，所以這個navigator對象是FragmentNavigator
    Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
            node.getNavigatorName());
    Bundle finalArgs = node.addInDefaultArgs(args);
    NavDestination newDest = navigator.navigate(node, finalArgs,
            navOptions, navigatorExtras);
    ...
}

我們繼續(xù)跟進到FragmentNavigator#navigate方法中

@Nullable
@Override
public NavDestination navigate(@NonNull Destination destination, @Nullable Bundle args,
        @Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
    ...
    
    String className = destination.getClassName();
    if (className.charAt(0) == '.') {
        className = mContext.getPackageName() + className;
    }
    // 通過反射創(chuàng)建一個Fragment對象
    final Fragment frag = instantiateFragment(mContext, mFragmentManager,
            className, args);
    frag.setArguments(args);
    final FragmentTransaction ft = mFragmentManager.beginTransaction();
    
    // Fragment跳轉(zhuǎn)動畫相關(guān)
    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);
    }

    // 使用replace方法岳遥，我們的首頁Fragment就在這里替換掉了占位NavHostFragment
    ft.replace(mContainerId, frag);
    ft.setPrimaryNavigationFragment(frag);

    ...
    
    // FragmentTransaction事務提交
    ft.commit();
    // The commit succeeded, update our view of the world
    if (isAdded) {
        // 添加到后退棧中
        mBackStack.add(destId);
        return destination;
    } else {
        return null;
    }
}

分析到這里，我們已經(jīng)知道了Navigation框架是如何通過我們配置的demo_navigation.xml文件加載我們的首頁Fragment了浩蓉。再來看看從第一個Fragment跳轉(zhuǎn)到第二個Fragment都做了一些什么宾袜？

Navigation.findNavController(root).navigate(R.id.second_fragment);

先找到NavController對象庆猫，從這里可以看出绅络，導航的總控制器是這個對象。繼續(xù)跟進這個navigate方法恩急，最終會執(zhí)行下面這個方法

public void navigate(@IdRes int resId, @Nullable Bundle args, @Nullable NavOptions navOptions,
        @Nullable Navigator.Extras navigatorExtras) {
    // 因為我們已經(jīng)加載了第一個FirstFragment了，所以此時mBackStack不為空此叠，
    // 那么currentNode就是FirstFragment對應的NavDestination
    NavDestination currentNode = mBackStack.isEmpty()
            ? mGraph
            : mBackStack.getLast().getDestination();
    if (currentNode == null) {
        throw new IllegalStateException("no current navigation node");
    }
    @IdRes int destId = resId;
    final NavAction navAction = currentNode.getAction(resId);
    Bundle combinedArgs = null;
    if (navAction != null) {
        if (navOptions == null) {
            navOptions = navAction.getNavOptions();
        }
        destId = navAction.getDestinationId();
        Bundle navActionArgs = navAction.getDefaultArguments();
        if (navActionArgs != null) {
            combinedArgs = new Bundle();
            combinedArgs.putAll(navActionArgs);
        }
    }

    if (args != null) {
        if (combinedArgs == null) {
            combinedArgs = new Bundle();
        }
        combinedArgs.putAll(args);
    }

    if (destId == 0 && navOptions != null && navOptions.getPopUpTo() != -1) {
        popBackStack(navOptions.getPopUpTo(), navOptions.isPopUpToInclusive());
        return;
    }

    if (destId == 0) {
        throw new IllegalArgumentException("Destination id == 0 can only be used"
                + " in conjunction with a valid navOptions.popUpTo");
    }
    // 通過我們傳入的R.id.second_fragment這個ID值去查找對應的NavDestination
    // 去哪里查找随珠？上文有提到過所有界面的NavDestination都是由NavGraph對象管理的
    NavDestination node = findDestination(destId);
    if (node == null) {
        final String dest = NavDestination.getDisplayName(mContext, destId);
        if (navAction != null) {
            throw new IllegalArgumentException("Navigation destination " + dest
                    + " referenced from action "
                    + NavDestination.getDisplayName(mContext, resId)
                    + " cannot be found from the current destination " + currentNode);
        } else {
            throw new IllegalArgumentException("Navigation action/destination " + dest
                    + " cannot be found from the current destination " + currentNode);
        }
    }
    // 這里就會執(zhí)行到我們上文分析過的navigate方法了
    navigate(node, combinedArgs, navOptions, navigatorExtras);
}

源碼差不多分析完了，最后用一張類UML圖來總結(jié)一相涉及到的相關(guān)類牙丽。

navigation.png

從類的UML圖可以看出烤芦，Navigation框架使用了設計模式中的策略模式举娩，Navigator定義了一個導航規(guī)則构罗，具體怎么導航由子類來實現(xiàn)。這里的子類就是圖中的ActivityNavigator遂唧、FragmentNavigator等。

遇到的問題

• 通過上面源碼分析纹烹，Fragment的加載都是通過replace方法，也就意味著每次回退時界面都會重新渲染铺呵，很顯然這不符合我們的開發(fā)需求
• Fragment只能在xml文件配置隧熙，若是分模塊開發(fā)片挂，無法滿足業(yè)務需求

下篇文章將會介紹如何優(yōu)雅地解決上面的兩個問題。