Jetpack学习-Navigation

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Jetpack学习-Navigation

Navigation是什么

Navigation翻译过来就是导航。

导航是指支持用户导航、进入和退出应用中不同内容片段的交互。Android Jetpack 的导航组件可帮助您实现导航，无论是简单的按钮点击，还是应用栏和抽屉式导航栏等更为复杂的模式，该组件均可应对。导航组件还通过遵循一套既定原则来确保一致且可预测的用户体验。

导航组件由以下三个关键部分组成：

• 导航图：在一个集中位置包含所有导航相关信息的 XML 资源。这包括应用内所有单个内容区域（称为目标）以及用户可以通过应用获取的可能路径。

• NavHost：显示导航图中目标的空白容器。导航组件包含一个默认 NavHost 实现 (NavHostFragment)，可显示 Fragment 目标。

• NavController：在 NavHost 中管理应用导航的对象。当用户在整个应用中移动时，NavController 会安排 NavHost 中目标内容的交换。

在应用中导航时，您告诉 NavController，您想沿导航图中的特定路径导航至特定目标，或直接导航至特定目标。NavController 便会在 NavHost 中显示相应目标。

导航组件提供各种其他优势，包括以下内容：

• 处理 Fragment 事务。

• 默认情况下，正确处理往返操作。

• 为动画和转换提供标准化资源。

• 实现和处理深层链接。

• 包括导航界面模式（例如抽屉式导航栏和底部导航），用户只需完成极少的额外工作。

• Safe Args - 可在目标之间导航和传递数据时提供类型安全的 Gradle 插件。

• ViewModel 支持 - 您可以将 ViewModel 的范围限定为导航图，以在图表的目标之间共享与界面相关的数据。

• 此外，您还可以使用 Android Studio 的 Navigation Editor 来查看和编辑导航图。

以上内容来自官方文档(我只是一个搬运工(^o^)/)

简单使用

引入Navigation

在需要使用Navigation的模块的build.gradle中引入

    def nav_version = "2.3.0-alpha01"
    implementation "androidx.navigation:navigation-fragment:$nav_version"
    implementation "androidx.navigation:navigation-ui:$nav_version"

建立导航图

在res目录右键-New-Android Resource File

在弹出的界面中，File name可随意输入，Resource type选择Navigation,点击确定

点击确定后，会在res目录下创建navigation目录，以及刚才定义的导航文件

双击打开刚才创建的导航文件，在Design界面可以看到目前还没有内容，可以点击上方的+号图标添加fragment，也可以自己手动在xml中添加

我们需要为这个文件指定startDestination，即起始的界面

startDestination指定为mainFragment，mainFragment对应的布局为fragment_main

Navigation首先会加载一个默认的Fragment，这个需要在Activity中指定

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/nav_graph" />

</LinearLayout>

配置defaultNavHost为true,即指定这个fragment为默认的NavHost，每个导航图只能指定一个默认的NavHost。这里的name配置为androidx.navigation.fragment.NavHostFragment,navGraph配置为nav_graph，即指定nav_graph为导航图。这样当Activity启动时，会首先通过activity布局里的fragment去加载导航图中的startDestination配置的fragment。

导航

通过一个Fragment导航到另一个Fragment，可以通过

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_main, container, false);
        loginBtn = view.findViewById(R.id.fragment_main_login);
        loginBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Bundle bundle = new Bundle();
                bundle.putString("name", "zs");
                Navigation.findNavController(v).navigate(R.id.action_mainFragment_to_loginFragment, bundle);
            }
        });
        return view;
    }

这里通过点击一个按钮进行跳转，通过Navigation.findNavController(v).navigate()方法导航。这里还可以通过Bundle进行传值。

在目的Fragment,可以通过getArguments()来获取到传递过来的数据

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        String name = getArguments().getString("name", "null");
        Toast.makeText(getContext(), name, Toast.LENGTH_SHORT).show();
        View view = inflater.inflate(R.layout.fragment_login, container, false);
        backBtn = view.findViewById(R.id.fragment_login_back);
        backBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Navigation.findNavController(v).popBackStack();
            }
        });
        return view;
    }

在目的Fragment，还可以通过一个按钮返回上一个Fragment：Navigation.findNavController(v).popBackStack()

原理

Navigation的简单使用流程就介绍到这，可以在官方文档上看更多相关的使用方法。下面来分析下Navigation的流程

显示起始Fragment

在Activity启动时，会先实例化NavHostFragment,这个是我们前面在布局中指定的。

首先会执行onInflate方法

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();
    }

这个方法，其实就是解析出来我们要使用哪个导航图，获取到了graphId。还获取了是否为默认的Host：defaultHost

然后会执行onAttach方法

public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        // TODO This feature should probably be a first-class feature of the Fragment system,
        // but it can stay here until we can add the necessary attr resources to
        // the fragment lib.
        if (mDefaultNavHost) {
            getParentFragmentManager().beginTransaction()
                    .setPrimaryNavigationFragment(this)
                    .commit();
        }
    }

由于在onInflate已经获取到mDefaultNavHost为true,因此这里会将当前Fragment通过commit加入到FragmentManager()中

然后执行onCreate方法

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

        //设置NavHostController,NavHostController里初始化了NavigatorProvider
        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) {
            // Set from onInflate()
            //前面执行onInflate后，已经获取到mGraphId，因此会执行下面的setGraph代码
            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);
            }
        }
    }

在这个方法里，设置了NavHostController及NavigatorProvider，然后执行NavController.setGraph方法

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

继续调用setGraph

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

继续调用setGraph

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);
    }

调用onGraphCreated

private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
        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);
                    }
                }
            }
        }
        if (mBackStackToRestore != null) {
            for (Parcelable parcelable : mBackStackToRestore) {
                NavBackStackEntryState state = (NavBackStackEntryState) parcelable;
                NavDestination node = findDestination(state.getDestinationId());
                if (node == null) {
                    throw new IllegalStateException("unknown destination during restore: "
                            + mContext.getResources().getResourceName(state.getDestinationId()));
                }
                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;
        }
        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

                //首次进入时，会执行这个代码
                navigate(mGraph, startDestinationArgs, null, null);
            }
        }
    }

首次进入Activity，会最终执行navigate(mGraph, startDestinationArgs, null, null)方法来导航到起始的目的Fragment

private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
            @Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
        boolean popped = false;
       //...
        Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
                node.getNavigatorName());
        Bundle finalArgs = node.addInDefaultArgs(args);
        NavDestination newDest = navigator.navigate(node, finalArgs,
                navOptions, navigatorExtras);
        //...
    }

这个方法，会调用navigator.navigate(node, finalArgs,navOptions, navigatorExtras)方法，这个方法的实现在NavGraphNavigator

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);
    }

在这个方法里调用navigator.navigate方法，这个方法实现在FragmentNavigator

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;
        }
        //通过反射实例化Fragment
        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);
        }

        //替换fragment
        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);
        //提交
        ft.commit();
        // The commit succeeded, update our view of the world
        if (isAdded) {
            mBackStack.add(destId);
            return destination;
        } else {
            return null;
        }
    }

在这个方法中，通过反射实例化目的Fragment，然后replace原来的Fragment,并commit，这样目的Fragment就显示出来了。

导航到其它Fragment

通过Navigation.findNavController(v).navigate(resId)可以导航到指定的Fragment

public static NavController findNavController(@NonNull View view) {
        NavController navController = findViewNavController(view);
        if (navController == null) {
            throw new IllegalStateException("View " + view + " does not have a NavController set");
        }
        return navController;
    }

然后调用findViewNavController

private static NavController findViewNavController(@NonNull View view) {
        while (view != null) {
            NavController controller = getViewNavController(view);
            if (controller != null) {
                return controller;
            }
            ViewParent parent = view.getParent();
            view = parent instanceof View ? (View) parent : null;
        }
        return null;
    }

这里通过子View往父View不停查找NavController，这个NavController在前面onCreate的时候已经附加到了view上。

找到NavController后，调用navigate。这个过程和前面第一次导航到起始Fragment是一样的流程，这里不再分析。

其实这里只是比较粗的一个梳理，涉及很多细节并没有具体去看，暂且先有一个流程的印象吧。

附上一张时序图