Android-Jetpack笔记--Navigation源码

           这一节主要是了解一下Navigation的源码，主要涉及如下类: 1 NavHostFragment：它相当于是Fragment和NavController的纽带，并提供导航的容器布局；2 NavController 导航组件的核心，用它来加载xml中Fragment节点并转化成NavDestination,再通过navigate(）方法切换NavDestionation,以实现Fragment的切换。NavigationUI,它为各个View设置接口监听，将View的UI状态和NavController中的Fragment做了绑定。

       从xml布局入手:

说明:引用androidx库中NavHostFragment,既承载导航界面，又管理控制导航行为，看看它在创建时做了什么:

 @CallSuper
    @Override
    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) {
            requireFragmentManager().beginTransaction()
                    .setPrimaryNavigationFragment(this)
                    .commit();
        }
    }

     在onAttach生命周期 通过defaultNavHost 条件判读 开启事务将它自己设置成了PrimaryFragment，

 @CallSuper
    @Override
    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) {
            // 如果存在之前Fragment,恢复状态
            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);
            }
        }
    }
	
	 protected void onCreateNavController(@NonNull NavController navController) {
        navController.getNavigatorProvider().addNavigator(
                new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
        navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
    }
	
	 protected Navigator createFragmentNavigator() {
        return new FragmentNavigator(requireContext(), getChildFragmentManager(), getId());
    }

          创建了一个NavController，并且通过navController 创建一个FragmentNavigator, 这个类继承Navigator, 它为每个Navigation设置策略，Fragment之间的导航切换是通过它来实现的; 然后看一下NavController设置setGraph(),在 xml里面定义的navGraph，导航布局里面的Fragment及action跳转等信息；它是如何将Fragment添加进去的 NavController.setGraph()

NavController.java 

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

//恢复状态
    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 (mGraph != null && mBackStack.isEmpty()) {
            boolean deepLinked = !mDeepLinkHandled && mActivity != null
                    && handleDeepLink(mActivity.getIntent());
            if (!deepLinked) {              
                // 如果没有深层链接  导航到目标Fragment 
                navigate(mGraph, startDestinationArgs, null, null);
            }
        }
		
	}
	

如果设置的graph不为null，它执行了popBackStackInternal

 boolean popBackStackInternal(@IdRes int destinationId, boolean inclusive) {
        if (mBackStack.isEmpty()) {
            // Nothing to pop if the back stack is empty
            return false;
        }
        ArrayList popOperations = new ArrayList<>();
        Iterator iterator = mBackStack.descendingIterator();
        boolean foundDestination = false;
        while (iterator.hasNext()) {
            NavDestination destination = iterator.next().getDestination();
            Navigator navigator = mNavigatorProvider.getNavigator(
                    destination.getNavigatorName());
            if (inclusive || destination.getId() != destinationId) {
                popOperations.add(navigator);
            }
            if (destination.getId() == destinationId) {
                foundDestination = true;
                break;
            }
        }
		...
		 boolean popped = false;
        for (Navigator navigator : popOperations) {
            if (navigator.popBackStack()) {
                NavBackStackEntry entry = mBackStack.removeLast();
                if (mViewModel != null) {
                    mViewModel.clear(entry.mId);
                }
                popped = true;
            } else {
                // The pop did not complete successfully, so stop immediately
                break;
            }
        }
		updateOnBackPressedCallbackEnabled();
        return popped;
	}

说明:remove之前所有的navigator,mBackStack是在哪里初始化的 如下:

private final Deque mBackStack = new ArrayDeque<>();
	
	   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);
            // 添加新的Destination
            NavBackStackEntry newBackStackEntry = new NavBackStackEntry(newDest,
                    newDest.addInDefaultArgs(finalArgs), mViewModel);
            mBackStack.add(newBackStackEntry);
        }
        updateOnBackPressedCallbackEnabled();
        if (popped || newDest != null) {
            dispatchOnDestinationChanged();
        }
    }

这个方法的调用是在 手动切换fragment时 调用:

findNavConroller().navigate(R.id.bottomNavFragment);

//Dispatch changes to all OnDestinationChangedListeners.
	 private boolean dispatchOnDestinationChanged() {
        // We never want to leave NavGraphs on the top of the stack
        //noinspection StatementWithEmptyBody
        while (!mBackStack.isEmpty()
                && mBackStack.peekLast().getDestination() instanceof NavGraph
                && popBackStackInternal(mBackStack.peekLast().getDestination().getId(), true)) {
            // Keep popping
        }
        if (!mBackStack.isEmpty()) {
            NavBackStackEntry backStackEntry = mBackStack.peekLast();
            for (OnDestinationChangedListener listener :
                    mOnDestinationChangedListeners) {
                listener.onDestinationChanged(this, backStackEntry.getDestination(),
                        backStackEntry.getArguments());
            }
            return true;
        }
        return false;
    }

说明: 分发目标界面切换。看一下这个接口的实现类，AbstractAppBarOnDestinationChangedListener

 @Override
    public void onDestinationChanged(@NonNull NavController controller,
            @NonNull NavDestination destination, @Nullable Bundle arguments) {
        if (destination instanceof FloatingWindow) {
            return;
        }
        DrawerLayout drawerLayout = mDrawerLayoutWeakReference != null
                ? mDrawerLayoutWeakReference.get()
                : null;
        if (mDrawerLayoutWeakReference != null && drawerLayout == null) {
            controller.removeOnDestinationChangedListener(this);
            return;
        }
        CharSequence label = destination.getLabel();
        if (!TextUtils.isEmpty(label)) {
            // Fill in the data pattern with the args to build a valid URI
            StringBuffer title = new StringBuffer();
            Pattern fillInPattern = Pattern.compile("\\{(.+?)\\}");
            Matcher matcher = fillInPattern.matcher(label);
            while (matcher.find()) {
                String argName = matcher.group(1);
                if (arguments != null && arguments.containsKey(argName)) {
                    matcher.appendReplacement(title, "");
                    //noinspection ConstantConditions
                    title.append(arguments.get(argName).toString());
                } else {
                    throw new IllegalArgumentException("Could not find " + argName + " in "
                            + arguments + " to fill label " + label);
                }
            }
            matcher.appendTail(title);
            setTitle(title);
        }
        boolean isTopLevelDestination = NavigationUI.matchDestinations(destination,
                mTopLevelDestinations);
        if (drawerLayout == null && isTopLevelDestination) {
            setNavigationIcon(null, 0);//设置icon
        } else {
		    //设置返回箭头
            setActionBarUpIndicator(drawerLayout != null && isTopLevelDestination);
        }
    }

到此 大体能知道切换Fragment的流程: 1 将目标Fragment放到栈顶  2 分发目标Fragment状态 3 设置 ActionBar  4 设置setNavigationIcon

看一下Navigation.findNavController(this,R.id.fragment_home)

 @NonNull
    public static NavController findNavController(@NonNull Activity activity, @IdRes int viewId) {
        View view = ActivityCompat.requireViewById(activity, viewId);
		//通过递归View层级 来获取Controller
        NavController navController = findViewNavController(view);
        if (navController == null) {
            throw new IllegalStateException("Activity " + activity
                    + " does not have a NavController set on " + viewId);
        }
        return navController;
    }
	
	@Nullable
    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;
    }

什么时候setViewNavController的？NavHostFragment 初始化的时候 ：

NavHostFragment.java 

  @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);
        // 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);
            }
        }
    }

在视图创建的时候设置的，简单的一个navigation源码分析到此完毕。