Navigation是什么

官方的话：

Navigation 是一个框架，用于在 Android 应用中的“目标”之间导航，该框架提供一致的 API，无论目标是作为 Fragment、Activity 还是其他组件实现。

自己的话：

Navigation是管理Fragment之间导航的组件库，特别在实现单个Activity多个Fragment的管理模式更加灵活

Navigation应用篇

在这里插入图片描述
1.Navigation Graph。这是一种新型的XML资源文件，其中包含应用程序所有的页面，以及页面间的关系。
2.NavHostFragment。这是一个特殊的Fragment，你可以认为它是其他Fragment的“容器”，Navigation Graph中的Fragment正是通过NavHostFragment进行展示的。3.NavController。这是一个Java/Kotlin对象，用于在代码中完成Navigation Graph中具体的页面切换工作。
当你想切换Fragment时，使用NavController对象，告诉它你想要去Navigation Graph中的哪个Fragment，NavController会将你想去的Fragment展示在NavHostFragment中。
具体的使用可以参考：https://blog.csdn.net/Lbsssss/article/details/121660838
这里就直接略过了。

Navigation源码原理篇

1.NavHostFragment.create

@NonNull
public static NavHostFragment create(@NavigationRes int graphResId) {
    return create(graphResId, null);
}

@NonNull
public static NavHostFragment create(@NavigationRes int graphResId,
        @Nullable Bundle startDestinationArgs) {
    Bundle b = null;
    if (graphResId != 0) {
        b = new Bundle();
        b.putInt(KEY_GRAPH_ID, graphResId); // 把graphID存入到Bundle
    }
    if (startDestinationArgs != null) {
        if (b == null) {
            b = new Bundle();
        }
        b.putBundle(KEY_START_DESTINATION_ARGS, startDestinationArgs);
    }

    final NavHostFragment result = new NavHostFragment();  初始化此对象
    if (b != null) {
        result.setArguments(b);
    }
    return result;
}

2.NavHostFragment.onInflate

@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; // 就把此属性解析出来，activity_main.xml 的 app:navGraph="@navigation/nav_graph_main"

    }
    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; // 就把此属性解析出来，activity_main.xml 的 app:defaultNavHost="true"
    }
    a.recycle();
}

3.NavHostFragment.onCreateNavController

@CallSuper
protected void onCreateNavController(@NonNull NavController navController) {
    // 这里是通过导航暴露者，获取到 NavigationProvider 对象
    navController.getNavigatorProvider().addNavigator(
            new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
    navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}

public class NavigatorProvider {
    // 把导航页面，例如 三个Fragment保存  与  key保存 记录
    private static final HashMap<Class<?>, String> sAnnotationNames = new HashMap<>();
}

protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
    return new FragmentNavigator(requireContext(), getChildFragmentManager(),
            getContainerId()); // 此ID，如果不写xml文件，单纯用代码实现的时候，需要得到一个父容器ID
}

4.NavHostFragment.onCreateNav

@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    final Context context = requireContext();

    mNavController = new NavHostController(context); // 此处 实例化出 NavHostController
    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);
   // 下面代码是容错处理
    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) { // 如果GraphID不等于空
        // Set from onInflate()
        mNavController.setGraph(mGraphId); // 设置GraphID，此处意义重大，会获取nav_graph_main里面的action等导航信息
    } 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);
        }
    }

    // We purposefully run this last as this will trigger the onCreate() of
    // child fragments, which may be relying on having the NavController already
    // created and having its state restored by that point.
    super.onCreate(savedInstanceState);
}

5.NavController.setGraph

@CallSuper
public void setGraph(@NavigationRes int graphResId) {
    setGraph(graphResId, null);
}

@CallSuper
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
   // getNavInflater().inflate(graphResId) 开始真正的解析 nav_graph_main里面的action等导航信息，还要确定startDestination的Fragment
   // startDestinationArgs 代表要启动 app:startDestination="@id/page1Fragment" 的参数信息
    setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}

@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();
        NavDestination destination = inflate(res, parser, attrs, graphResId); // 解析时，主要是把目的地获取到了
        if (!(destination instanceof NavGraph)) {
            throw new IllegalArgumentException("Root element <" + rootElement + ">"
                    + " did not inflate into a NavGraph");
        }
        return (NavGraph) destination; // 返回要导航的 目的地
    } catch (Exception e) {
        throw new RuntimeException("Exception inflating "
                + res.getResourceName(graphResId) + " line "
                + parser.getLineNumber(), e);
    } finally {
        parser.close();
    }
}

// 退会后，看到 setGraph 细节
@CallSuper // 可以看到 把 GraphID xml 里面的内容  转变成 NavGraph对象了
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); // 对象信息有了后，先从栈里面去弹，看能不能弹出来，第一次肯定是弹不出来的
    }
    mGraph = graph; // 先把对象 保存到成员
    onGraphCreated(startDestinationArgs); // 此函数就能看到，如何把我们的默认Fragment UI给显示出来了
}

6.NavController.onGraphCreated

private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
    // 这个 这种 if的 ，对我们的主线流程意义不大，先跳过
    if (mNavigatorStateToRestore != null) {
        ArrayList<String> navigatorNames = mNavigatorStateToRestore.getStringArrayList(
                KEY_NAVIGATOR_STATE_NAMES);
        if (navigatorNames != null) {
            for (String name : navigatorNames) {
                Navigator<?> navigator = mNavigatorProvider.getNavigator(name);
                Bundle bundle = mNavigatorStateToRestore.getBundle(name);
                if (bundle != null) {
                    navigator.onRestoreState(bundle);
                }
            }
        }
    }
    // 这里都是 栈状态的更新 保存 等处理，代码我们先不研究，不是说他不重要是，对主线流程研究 意义不大
    if (mBackStackToRestore != null) {
        for (Parcelable parcelable : mBackStackToRestore) {
            NavBackStackEntryState state = (NavBackStackEntryState) parcelable;
            NavDestination node = findDestination(state.getDestinationId());
            if (node == null) {
                final String dest = NavDestination.getDisplayName(mContext,
                        state.getDestinationId());
                throw new IllegalStateException("Restoring the Navigation back stack failed:"
                        + " destination " + dest
                        + " cannot be found from the current destination "
                        + getCurrentDestination());
            }
            Bundle args = state.getArgs();
            if (args != null) {
                args.setClassLoader(mContext.getClassLoader());
            }
            NavBackStackEntry entry = new NavBackStackEntry(mContext, node, args,
                    mLifecycleOwner, mViewModel,
                    state.getUUID(), state.getSavedState());
            mBackStack.add(entry);
        }
        updateOnBackPressedCallbackEnabled();
        mBackStackToRestore = null;
    }
    
    // 此代码就比较关键了，如果 前面辛辛苦苦保存的mGraph对象不为空，并且，栈里面是空的 取不出来的情况下， 
    if (mGraph != null && mBackStack.isEmpty()) {
        boolean deepLinked = !mDeepLinkHandled && mActivity != null
                && handleDeepLink(mActivity.getIntent());
        if (!deepLinked) {
             // 这里说的很清楚了，导航到图表中的第一个目的地
            // Navigate to the first destination in the graph
            // if we haven't deep linked to a destination
            navigate(mGraph, startDestinationArgs, null, null); // 看此处是如何导航的
        }
    } else {
        dispatchOnDestinationChanged();
    }
}

7. NavController.navigation

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());
        }
    }
    // 同学们注意：这里是关键，真正的导航过程了
    Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(node.getNavigatorName());
    // 导航需要的参数集
    Bundle finalArgs = node.addInDefaultArgs(args); 
    // 构建新的目的地对象，此对象，就是后续要完成的目标了
    NavDestination newDest = navigator.navigate(node, finalArgs,
            navOptions, navigatorExtras);
    if (newDest != null) {
        if (!(newDest instanceof FloatingWindow)) {
            // We've successfully navigating to the new destination, which means
            // we should pop any FloatingWindow destination off the back stack
            // before updating the back stack with our new destination
            //noinspection StatementWithEmptyBody
            while (!mBackStack.isEmpty()
                    && mBackStack.peekLast().getDestination() instanceof FloatingWindow
                    && popBackStackInternal(
                            mBackStack.peekLast().getDestination().getId(), true)) {
                // Keep popping
            }
        }

        // When you navigate() to a NavGraph, we need to ensure that a new instance
        // is always created vs reusing an existing copy of that destination
        ArrayDeque<NavBackStackEntry> hierarchy = new ArrayDeque<>();
        NavDestination destination = newDest;
        if (node instanceof NavGraph) {
            do {
                NavGraph parent = destination.getParent();
                if (parent != null) {
                    NavBackStackEntry entry = new NavBackStackEntry(mContext, parent,
                            finalArgs, mLifecycleOwner, mViewModel);
                    hierarchy.addFirst(entry);
                    // Pop any orphaned copy of that navigation graph off the back stack
                    if (!mBackStack.isEmpty()
                            && mBackStack.getLast().getDestination() == parent) {
                        popBackStackInternal(parent.getId(), true);
                    }
                }
                destination = parent;
            } while (destination != null && destination != node);
        }

        // Now collect the set of all intermediate NavGraphs that need to be put onto
        // the back stack
        destination = hierarchy.isEmpty()
                ? newDest
                : hierarchy.getFirst().getDestination();
        while (destination != null && findDestination(destination.getId()) == null) {
            NavGraph parent = destination.getParent();
            if (parent != null) {
                NavBackStackEntry entry = new NavBackStackEntry(mContext, parent, finalArgs,
                        mLifecycleOwner, mViewModel);
                hierarchy.addFirst(entry);
            }
            destination = parent;
        }
        NavDestination overlappingDestination = hierarchy.isEmpty()
                ? newDest
                : hierarchy.getLast().getDestination();
        // Pop any orphaned navigation graphs that don't connect to the new destinations
        //noinspection StatementWithEmptyBody
        while (!mBackStack.isEmpty()
                && mBackStack.getLast().getDestination() instanceof NavGraph
                && ((NavGraph) mBackStack.getLast().getDestination()).findNode(
                        overlappingDestination.getId(), false) == null
                && popBackStackInternal(mBackStack.getLast().getDestination().getId(), true)) {
            // Keep popping
        }
        mBackStack.addAll(hierarchy);
        // The mGraph should always be on the back stack after you navigate()
        if (mBackStack.isEmpty() || mBackStack.getFirst().getDestination() != mGraph) {
            NavBackStackEntry entry = new NavBackStackEntry(mContext, mGraph, finalArgs,
                    mLifecycleOwner, mViewModel);
            mBackStack.addFirst(entry);
        }
        // And finally, add the new destination with its default args
        NavBackStackEntry newBackStackEntry = new NavBackStackEntry(mContext, newDest,
                newDest.addInDefaultArgs(finalArgs), mLifecycleOwner, mViewModel);
        mBackStack.add(newBackStackEntry);
    } else if (navOptions != null && navOptions.shouldLaunchSingleTop()) {
        launchSingleTop = true;
        NavBackStackEntry singleTopBackStackEntry = mBackStack.peekLast();
        if (singleTopBackStackEntry != null) {
            singleTopBackStackEntry.replaceArguments(finalArgs);
        }
    }
    updateOnBackPressedCallbackEnabled();
    if (popped || newDest != null || launchSingleTop) {
        dispatchOnDestinationChanged();
    }
}

8.Navigator.navigate

@Nullable
public abstract NavDestination navigate(@NonNull D destination, @Nullable Bundle args,
        @Nullable NavOptions navOptions, @Nullable Extras navigatorExtras);

在这里插入图片描述

9.FragmentNavigator.navigate

@Nullable
@Override
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;
    }
   
    // 根据Destination目的地，获取到第一个Fragmet  也就是 MainPage1Fragment实例
    // 下面代码是反射 去 实例化 我们的 第一个Fragment
    String className = destination.getClassName();
    if (className.charAt(0) == '.') {
        className = mContext.getPackageName() + className;
    }
    // instanceFragment反射的细节，可以看看
    final Fragment frag = instantiateFragment(mContext, mFragmentManager,
            className, args);
    frag.setArguments(args);
    final FragmentTransaction ft = mFragmentManager.beginTransaction();

    // Fragment的进出 动画
    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);
    }

    // 然后把mContainerId，替换到ft，相当于把 我们的 第一个Fragment加入到了 UI上面了
    // 就是把 MainPage1Fragment 加入到 activity_main.xml 的 FragmentContainerView
    // mContainerId（就是 FragmentContainerView）相当于容器
    // frag（就是 我们的第一个Fragment实例 MainPage1Fragment）
    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;
    }
}

@Deprecated
@NonNull
public Fragment instantiateFragment(@NonNull Context context,
        @NonNull FragmentManager fragmentManager,
        @NonNull String className, @SuppressWarnings("unused") @Nullable Bundle args) {
    // instantiate 反射的细节看看
    return fragmentManager.getFragmentFactory().instantiate(
            context.getClassLoader(), className);
}

@NonNull
public Fragment instantiate(@NonNull ClassLoader classLoader, @NonNull String className) {
    try {
        Class<? extends Fragment> cls = loadFragmentClass(classLoader, className);
        return cls.getConstructor().newInstance(); // 把最终方式的Fragment结果 给返回
    } catch (java.lang.InstantiationException e) {
        throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (IllegalAccessException e) {
        throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
                + ": make sure class name exists, is public, and has an"
                + " empty constructor that is public", e);
    } catch (NoSuchMethodException e) {
        throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
                + ": could not find Fragment constructor", e);
    } catch (InvocationTargetException e) {
        throw new Fragment.InstantiationException("Unable to instantiate fragment " + className
                + ": calling Fragment constructor caused an exception", e);
    }
}

注意：但onCreate执行完成后，界面就展示了第一个Fragment的画面了

10.小结

第一步：解析所有的目的地
第二步：放到一个HashMap里面全部保存好
第三步：获取第一个目的地Fragment
第四步：初始化目的地的所有信息
第五步：显示第一个目的地Fragment 画面

11.NavHostFragment.onViwCreated

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    if (!(view instanceof ViewGroup)) {
        throw new IllegalStateException("created host view " + view + " is not a ViewGroup");
    }
    Navigation.setViewNavController(view, mNavController); // 会把我们的 mNavController 加入到 Navigation里面去

    // 说白了，如果是代码的方式去写的话，并非是xml的写法的话，就做下面的代码，但是 大部分都是 xml的形式
    // 当以编程方式添加时，我们需要在父级上设置 NavController - 即具有与此 NavHostFragment 匹配的 ID 的视图
    // When added programmatically, we need to set the NavController on the parent - i.e.,
    // the View that has the ID matching this NavHostFragment.
    if (view.getParent() != null) {
        mViewParent = (View) view.getParent();
        if (mViewParent.getId() == getId()) {
            Navigation.setViewNavController(mViewParent, mNavController);
        }
    }
}

navigation角度上分析源码

1. MainPage1Fragment

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    val btn = view.findViewById<Button>(R.id.btn)
    btn.setOnClickListener { view ->
        Navigation.findNavController(view).navigate(R.id.action_page2) // 在这里开始导航
    }
}

2.NavController.navigation

public void navigate(@IdRes int resId) {
    navigate(resId, null);
}

public void navigate(@IdRes int resId, @Nullable Bundle args) {
    navigate(resId, args, null);
}

public void navigate(@IdRes int resId, @Nullable Bundle args,
        @Nullable NavOptions navOptions) {
    navigate(resId, args, navOptions, null);
}

public void navigate(@IdRes int resId, @Nullable Bundle args, @Nullable NavOptions navOptions,
        @Nullable Navigator.Extras navigatorExtras) {
    // 目的 是在 Graph里面去寻找 当前节点的Fragment 为 目的地
    NavDestination currentNode = mBackStack.isEmpty() // 先去看看栈有没有
            ? mGraph
            : mBackStack.getLast().getDestination(); // 如果栈没有，就取最后一个
    if (currentNode == null) {
        throw new IllegalStateException("no current navigation node");
    }
    @IdRes int destId = resId;
    // 获取节点后，就要去获取 action 导航动作，只有action才能通过resId 获取下一个要导航的目标id
    final NavAction navAction = currentNode.getAction(resId); 
    Bundle combinedArgs = null;
    if (navAction != null) {
        if (navOptions == null) {
            navOptions = navAction.getNavOptions();
        }
        destId = navAction.getDestinationId(); // 获取下一个目的地的 id号信息等
        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");
    }
     
    // 下面代码只是一个检查健壮性而已，先不管
    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);
        }
    }
    // 这行代码就是我们要关心的主线流程
    navigate(node, combinedArgs, navOptions, navigatorExtras);
}

3.NavController.navigation

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());
        }
    }
    // 同学们，这里又回到了，最初开始的代码哦
    Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
            node.getNavigatorName());
    Bundle finalArgs = node.addInDefaultArgs(args);
    // 同学们 还记得 新的目的地么？ OK 分析完毕了，无效重复分析了
    NavDestination newDest = navigator.navigate(node, finalArgs,
            navOptions, navigatorExtras);

    if (newDest != null) {
        if (!(newDest instanceof FloatingWindow)) {
            // We've successfully navigating to the new destination, which means
            // we should pop any FloatingWindow destination off the back stack
            // before updating the back stack with our new destination
            //noinspection StatementWithEmptyBody
            while (!mBackStack.isEmpty()
                    && mBackStack.peekLast().getDestination() instanceof FloatingWindow
                    && popBackStackInternal(
                            mBackStack.peekLast().getDestination().getId(), true)) {
                // Keep popping
            }
        }

        // When you navigate() to a NavGraph, we need to ensure that a new instance
        // is always created vs reusing an existing copy of that destination
        ArrayDeque<NavBackStackEntry> hierarchy = new ArrayDeque<>();
        NavDestination destination = newDest;
        if (node instanceof NavGraph) {
            do {
                NavGraph parent = destination.getParent();
                if (parent != null) {
                    NavBackStackEntry entry = new NavBackStackEntry(mContext, parent,
                            finalArgs, mLifecycleOwner, mViewModel);
                    hierarchy.addFirst(entry);
                    // Pop any orphaned copy of that navigation graph off the back stack
                    if (!mBackStack.isEmpty()
                            && mBackStack.getLast().getDestination() == parent) {
                        popBackStackInternal(parent.getId(), true);
                    }
                }
                destination = parent;
            } while (destination != null && destination != node);
        }

        // Now collect the set of all intermediate NavGraphs that need to be put onto
        // the back stack
        destination = hierarchy.isEmpty()
                ? newDest
                : hierarchy.getFirst().getDestination();
        while (destination != null && findDestination(destination.getId()) == null) {
            NavGraph parent = destination.getParent();
            if (parent != null) {
                NavBackStackEntry entry = new NavBackStackEntry(mContext, parent, finalArgs,
                        mLifecycleOwner, mViewModel);
                hierarchy.addFirst(entry);
            }
            destination = parent;
        }
        NavDestination overlappingDestination = hierarchy.isEmpty()
                ? newDest
                : hierarchy.getLast().getDestination();
        // Pop any orphaned navigation graphs that don't connect to the new destinations
        //noinspection StatementWithEmptyBody
        while (!mBackStack.isEmpty()
                && mBackStack.getLast().getDestination() instanceof NavGraph
                && ((NavGraph) mBackStack.getLast().getDestination()).findNode(
                        overlappingDestination.getId(), false) == null
                && popBackStackInternal(mBackStack.getLast().getDestination().getId(), true)) {
            // Keep popping
        }
        mBackStack.addAll(hierarchy);
        // The mGraph should always be on the back stack after you navigate()
        if (mBackStack.isEmpty() || mBackStack.getFirst().getDestination() != mGraph) {
            NavBackStackEntry entry = new NavBackStackEntry(mContext, mGraph, finalArgs,
                    mLifecycleOwner, mViewModel);
            mBackStack.addFirst(entry); // 留意一下，每次都会管理 进栈
        }
        // And finally, add the new destination with its default args
        NavBackStackEntry newBackStackEntry = new NavBackStackEntry(mContext, newDest,
                newDest.addInDefaultArgs(finalArgs), mLifecycleOwner, mViewModel);
        mBackStack.add(newBackStackEntry);
    } else if (navOptions != null && navOptions.shouldLaunchSingleTop()) {
        launchSingleTop = true;
        NavBackStackEntry singleTopBackStackEntry = mBackStack.peekLast();
        if (singleTopBackStackEntry != null) {
            singleTopBackStackEntry.replaceArguments(finalArgs);
        }
    }
    updateOnBackPressedCallbackEnabled();
    if (popped || newDest != null || launchSingleTop) {
        dispatchOnDestinationChanged();
    }
}

在这里插入图片描述

1.官网上：MainActivity去绑定Navigation （实际上：源码理解 Fragment帖到MainActivity）

2.NavHostFragment create 实例化NavHostFragment 并且赋予丰富的信息而已

3.NavHostFragment.onInflate属于Fragment的生命周期 { app:defaultNavHost=“true”
app:navGraph=“@navigation/nav_graph_main” }

4.protected void onCreateNavController(@NonNull NavController navController) {
NavigatorProvider HashMap会去保存所有的Fragment信息 3个
getContainerId() 不写xml的时候，写Java代码的是才有用
}

5.onCreate
mContainerId
<androidx.fragment.app.FragmentContainerView
android:name=“androidx.navigation.fragment.NavHostFragment”>