前言

最近项目中使用到了BottomNavigationView结合Navigation实现底部导航栏切换页面业务。

NavigationUI.setupWithNavController(bottomNavigationView, navController);

结果发现每次点击底部导航栏切换的时候都会重建Fragment，于是分析了源码，并研究了解决方案。

源码分析：

首先看布局文件：
<?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:layout_width="match_parent"
    android:layout_height="match_parent">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@+id/bottom_nav_view"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottom_nav_view"
        android:layout_width="0dp"
        android:layout_height="50dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/bottom_menu" />
</androidx.constraintlayout.widget.ConstraintLayout>

当调用NavigationUI.setupWithNavController(BottomNavigationView, NavController)以后，setupWithNavController方法内部其实通过调用BottomNavigationView#setOnNavigationItemSelectedListener方法监听导航栏选中事件。
在BottomNavigationView.OnNavigationItemSelectedListener监听中，最终会调用到NavController#navigate方法，进入Navigation源码中。

Navigation源码分析

首先看NavHostFragment的执行流程。

1. NavHostFragment#onInflate

因为在xml中声明fragment因此，首先调用Fragment的onInflate方法。

@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方法中主要是从XML属性中解析navGraph属性和defaultNavHost属性值。

2. NavHostFragment#onAttach

根据Fragment生命周期，然后执行的是onAttach方法。

@CallSuper
@Override
public void onAttach(@NonNull Context context) {
    super.onAttach(context);
    if (mDefaultNavHost) {
        getParentFragmentManager().beginTransaction()
                .setPrimaryNavigationFragment(this).commit();
    }
}

onAttach方法中主要是设置NavHostFragment为导航器的主导航容器。

3. NavHostFragment#onCreate

@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    final Context context = requireContext();
// 1. 实例化NavHostController
    mNavController = new NavHostController(context);
    mNavController.setLifecycleOwner(this);
 	mNavController.setOnBackPressedDispatcher(requireActivity().getOnBackPressedDispatcher());
    mNavController.enableOnBackPressed(mIsPrimaryBeforeOnCreate != null && mIsPrimaryBeforeOnCreate);
    mIsPrimaryBeforeOnCreate = null;
    mNavController.setViewModelStore(getViewModelStore());
    // 2. 创建DialogFragmentNavigator和FragmentNavigator并添加示例到NavigatorProvider中
    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) {
        mNavController.restoreState(navState);
    }
    if (mGraphId != 0) {
    	// 3. 设置导航配置文件
        mNavController.setGraph(mGraphId);
    } else {
        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);
}
onCreate方法中主要做三件事：
  • 实例化NavHostController对象
  • 创建DialogFragmentNavigator和FragmentNavigator并添加到NavHostController的父类NavController的NavigatorProvider类型的成员变量mNavigatorProvider中
  • 调用NavHostController#setGraph方法设置导航配置文件nav_graph
public class NavHostController extends NavController {

    public NavHostController(@NonNull Context context) {
        super(context);
    }
    ...
}
主要看父类初始化方法：
public class NavController {
	private NavigatorProvider mNavigatorProvider = new NavigatorProvider();
	public NavController(@NonNull Context context) {
       	...
        mNavigatorProvider.addNavigator(new NavGraphNavigator(mNavigatorProvider));
        mNavigatorProvider.addNavigator(new ActivityNavigator(mContext));
    }
}

主要是创建NavGraphNavigator和ActivityNavigator实例并添加到NavController的成员变量mNavigatorProvider中。

4. NavHostFragment#onCreateNavController

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

onCreate方法中调用了onCreateNavController方法添加DialogFragmentNavigator和FragmentNavigator示例。

5. NavigatorProvider

public class NavigatorProvider {
    private static final HashMap<Class<?>, String> sAnnotationNames = new HashMap<>();
	@NonNull
    static String getNameForNavigator(@NonNull Class<? extends Navigator> navigatorClass) {
        String name = sAnnotationNames.get(navigatorClass);
        if (name == null) {
        	// 自定义Navigator类的注解Navigator.Name
            Navigator.Name annotation = navigatorClass.getAnnotation(Navigator.Name.class);
            name = annotation != null ? annotation.value() : null;
            ...
            sAnnotationNames.put(navigatorClass, name);
        }
        return name;
    }
	private final HashMap<String, Navigator<? extends NavDestination>> mNavigators = new HashMap<>()
	@Nullable
    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) {
        return mNavigators.put(name, navigator);
    }
}

NavigatorProvider类内部主要是存储了键值为自定义Navigator时注解Navigator.Name指定的名称，值为对应的Navigator示例。

因此onCreate方法执行以后，NavigatorProvider中的mNavigators的值为：
("navigation", NavGraphNavigator)
("activity", ActivityNavigator)
("dialog", DialogFragmentNavigator)
("fragment", FragmentNavigator)

6. NavController#setGraph

@CallSuper
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) {
        popBackStackInternal(mGraph.getId(), true);
    }
    mGraph = graph;
    onGraphCreated(startDestinationArgs);
}

@NonNull
public NavInflater getNavInflater() {
    if (mInflater == null) {
        mInflater = new NavInflater(mContext, mNavigatorProvider);
    }
    return mInflater;
}

这个方法中首先是实例化NavInflater并调用NavInflater#inflate解析导航配置文件，解析以后的结构存放在NavGraph类中。NavGraph是可以按ID获取的NavDestination节点的树形结构。

7. NavInflater#inflate

public final class NavInflater {
	private Context mContext;
	private NavigatorProvider mNavigatorProvider;
    public NavInflater(@NonNull Context context, @NonNull NavigatorProvider navigatorProvider) {
        mContext = context;
        mNavigatorProvider = navigatorProvider;
    }
	    @NonNull
    public NavGraph inflate(@NavigationRes int graphResId) {
        ...
        NavDestination destination = inflate(res, parser, attrs, graphResId);
        ...
    }
	 @NonNull
    private NavDestination inflate(@NonNull Resources res, @NonNull XmlResourceParser parser, @NonNull AttributeSet attrs, int graphResId) throws XmlPullParserException, IOException {
        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) {
                ((NavGraph) dest).addDestination(inflate(res, parser, attrs, graphResId));
            }
        }
        return dest;
    }
	...
}

NavInflater的主要工作就是解析导航配置文件。接下来再回头看setGraph方法中调用的onGraphCreated方法。

8. NavController#onGraphCreated

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();
    }
}

刚开始的时候会执行到navigate方法。

9. NavController#navigate

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());
        }
    }
    Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(node.getNavigatorName());
    Bundle finalArgs = node.addInDefaultArgs(args);
    NavDestination newDest = navigator.navigate(node, finalArgs, navOptions, navigatorExtras);
    ...
}

根据分析得出getNavigator获取到的Navigator是NavGraphNavigator实例。

10. NavGraphNavigator#navigate

@Nullable
@Override
public NavDestination navigate(@NonNull NavGraph destination, @Nullable Bundle args,
            @Nullable NavOptions navOptions, @Nullable Extras navigatorExtras) {
    int startId = destination.getStartDestination();
    if (startId == 0) {
        throw new IllegalStateException("no start destination defined via"
                + " app:startDestination for "
                + destination.getDisplayName());
    }
    NavDestination startDestination = destination.findNode(startId, false);
    if (startDestination == null) {
        final String dest = destination.getStartDestDisplayName();
        throw new IllegalArgumentException("navigation destination " + dest
                + " is not a direct child of this NavGraph");
    }
    Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
            startDestination.getNavigatorName());
    return navigator.navigate(startDestination, startDestination.addInDefaultArgs(args),
            navOptions, navigatorExtras);
}

navigate方法中通过startId找到NavDestination变量，再根据NavDestination#getNavigatorName方法获取到的名称得到对应的Navigator实例，此处获取到的是FragmentNavigator实例。

11. FragmentNavigator#navigate


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;
    }
}

navigate方法中使用的是FragmentFactory（反射）创建fragment实例。最后通过FragmentTransaction#replace方法添加fragment实例。

再回头看NavHostFragment的生命周期onCreateView方法。

# android # android jetpack
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