android的Jetpack的navigation原理详解

还没有用过navigation的小伙伴赶紧来用它，实在是太方便了，·再也不用写supportFragmentManager.beginTransaction().add()和supportFragmentManager.beginTransaction().replace()来切换布局了，只要将所有的需要跳转的Frament全部放到布局里面就好了，navigation和flutter的路由差不多，它只不过是将frament装到了作为一个个的跳转点而已。我觉着它最大的优势是可以通过android studio清晰的看到每个界面的跳转点。如图所示：

不用运行就可以看见每个模块的界面跳转，是不是很炫。大家可以参考一下官方文档android导航。

用起来也比较简单首先在build.gradle中加入

 implementation "androidx.navigation:navigation-fragment-ktx:2.1.0"
    implementation "androidx.navigation:navigation-ui-ktx:2.1.0"

引入支持库之后，第二步在你的模块主布局中加入需要动态变化的Frament

对，就是上面这个东西，注意一点最终实例化的Fragment是androidx.navigation.fragment.NavHostFragment类，实例化的Frament是扩展库中的类，注意 app:navGraph="@navigation/mobile_navigation" 这个属性是真正的填充Fragment的类。第三步，在res下新建navigation文件夹，xml内容如下：

其中app:startDestination="@+id/home_dest"属性用来声明第一个显示的Fragment，argument标签用来声明传递的属性，action标签是用来跳转的，这和activity的action有异曲同工之妙，跳转到新的Fragment的时候调用Navigation.findNavController(view).navigate()，回到上一个Fragment的时候调Navigation.findNavController(view).navigateUp()

方法。好了，这就是Navigation的基本使用了，大家可以在官网下载demo看一下，接下来来探究一下navigation的原理。

前面介绍了navigation使用的入口就是

androidx.navigation.fragment.NavHostFragment这个类

public class NavHostFragment extends Fragment implements NavHost

NavHostFragment 就是一个Fragment并实现了NavHost接口，这个接口只有一个方法

public interface NavHost {

    /**
     * Returns the {@link NavController navigation controller} for this navigation host.
     *
     * @return this host's navigation controller
     */
    @NonNull
    NavController getNavController();
}

在它的onCreateView中，它只是填充了简单的帧布局FrameLayout，它的用处就是最终用来替换布局的

  public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        FrameLayout frameLayout = new FrameLayout(inflater.getContext());
        // When added via XML, this has no effect (since this FrameLayout is given the ID
        // automatically), but this ensures that the View exists as part of this Fragment's View
        // hierarchy in cases where the NavHostFragment is added programmatically as is required
        // for child fragment transactions
        frameLayout.setId(getId());
        return frameLayout;
    }

onCreateView执行完以后会执行onViewCreated，看一下

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);
        // 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) {
            View rootView = (View) view.getParent();
            if (rootView.getId() == getId()) {
                Navigation.setViewNavController(rootView, mNavController);
            }
        }
    }

很简单就是为当前Fragment设置当前的根view设置一个tag，这个tag的值是NavHostController，这个类实现了对Fragment的控制的封装。而onInflate是在fragment标签被解析的时候调用的，它早于fragment的任何生命周期方法

  public void onInflate(@NonNull Context context, @NonNull AttributeSet attrs,
            @Nullable Bundle savedInstanceState) {
        super.onInflate(context, attrs, savedInstanceState);

        final TypedArray navHost = context.obtainStyledAttributes(attrs, R.styleable.NavHost);
        final int graphId = navHost.getResourceId(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();
    }

这里根据属性解析出app：navGraph引用的xml的id

然后在onCreate方法通过  mNavController.setGraph(mGraphId)方法解析出来第一个需要显示的Fragment

 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;
                requireFragmentManager().beginTransaction()
                        .setPrimaryNavigationFragment(this)
                        .commit();
            }
        }

        if (navState != null) {
            // Navigation controller state overrides arguments
            mNavController.restoreState(navState);
        }
        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);
            }
        }
    }

继续看

  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);
    }

如果mGraph 不为null就证明栈中已经存放了当前的Fragment，直接让它出栈就好了，接着看onGraphCreated方法

 private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
        if (mNavigatorStateToRestore != null) {
            ArrayList 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 (mBackStackUUIDsToRestore != null) {
            for (int index = 0; index < mBackStackUUIDsToRestore.length; index++) {
                UUID uuid = UUID.fromString(mBackStackUUIDsToRestore[index]);
                int destinationId = mBackStackIdsToRestore[index];
                Bundle args = (Bundle) mBackStackArgsToRestore[index];
                NavDestination node = findDestination(destinationId);
                if (node == null) {
                    throw new IllegalStateException("unknown destination during restore: "
                            + mContext.getResources().getResourceName(destinationId));
                }
                if (args != null) {
                    args.setClassLoader(mContext.getClassLoader());
                }
                mBackStack.add(new NavBackStackEntry(uuid, node, args, mViewModel));
            }
            updateOnBackPressedCallbackEnabled();
            mBackStackUUIDsToRestore = null;
            mBackStackIdsToRestore = null;
            mBackStackArgsToRestore = 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
                navigate(mGraph, startDestinationArgs, null, null);
            }
        }
    }

和流程有关的方法是navigate，继续进入

    private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
            @Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
        boolean popped = false;
        if (navOptions != null) {
            if (navOptions.getPopUpTo() != -1) {
                popped = popBackStackInternal(navOptions.getPopUpTo(),
                        navOptions.isPopUpToInclusive());
            }
        }
        Navigator navigator = mNavigatorProvider.getNavigator(
                node.getNavigatorName());
        Bundle finalArgs = node.addInDefaultArgs(args);
        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
                }
            }
            // The mGraph should always be on the back stack after you navigate()
            if (mBackStack.isEmpty()) {
                mBackStack.add(new NavBackStackEntry(mGraph, finalArgs, mViewModel));
            }
            // Now ensure all intermediate NavGraphs are put on the back stack
            // to ensure that global actions work.
            ArrayDeque hierarchy = new ArrayDeque<>();
            NavDestination destination = newDest;
            while (destination != null && findDestination(destination.getId()) == null) {
                NavGraph parent = destination.getParent();
                if (parent != null) {
                    hierarchy.addFirst(new NavBackStackEntry(parent, finalArgs, mViewModel));
                }
                destination = parent;
            }
            mBackStack.addAll(hierarchy);
            // And finally, add the new destination with its default args
            NavBackStackEntry newBackStackEntry = new NavBackStackEntry(newDest,
                    newDest.addInDefaultArgs(finalArgs), mViewModel);
            mBackStack.add(newBackStackEntry);
        }
        updateOnBackPressedCallbackEnabled();
        if (popped || newDest != null) {
            dispatchOnDestinationChanged();
        }
    }

这个方法有点长，大部分代码都是保存当前需要显示的Fragment类的封装类NavDestination ，保存到栈中。

navigator.navigate(node, finalArgs,
                navOptions, navigatorExtras);

而navigator.navigate是真正实现Fragment切换的，真正实现类是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;
        }
        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 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;
        }
    }

这个方法最重要的就是这个代码   ft.replace(mContainerId, frag)，ft是FragmentTransaction 类，而mContainerId就是NavHostFragment的布局View的id如下：

 public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        FrameLayout frameLayout = new FrameLayout(inflater.getContext());
        // When added via XML, this has no effect (since this FrameLayout is given the ID
        // automatically), but this ensures that the View exists as part of this Fragment's View
        // hierarchy in cases where the NavHostFragment is added programmatically as is required
        // for child fragment transactions
        frameLayout.setId(getId());
        return frameLayout;
    }

所以Frament的切换都是通过NavHostFragment的FrameLayout 来切换的