Android知识总结

一、使用 navigation 导航 Fragment 分析

1.1、Fragment

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

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

1.2、Navigation.findNavController

    public static NavController findNavController(@NonNull View view) {
        //根据 view 获取导航控制器
        NavController navController = findViewNavController(view);
        if (navController == null) {
            throw new IllegalStateException("View " + view + " does not have a NavController set");
        }
        return navController;
    }

    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;
    }

   private static NavController getViewNavController(@NonNull View view) {
        Object tag = view.getTag(R.id.nav_controller_view_tag);
        NavController controller = null;
        if (tag instanceof WeakReference) {
            controller = ((WeakReference<NavController>) tag).get();
        } else if (tag instanceof NavController) {
            controller = (NavController) tag;
        }
        return controller;
    }

1.3、 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);
}

final Deque<NavBackStackEntry> mBackStack = new ArrayDeque<>();

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();
        }
         // 获取下一个目的地的 id号信息等
        destId = navAction.getDestinationId();
        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");
    }
     
    // 检查健壮性
    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);
}

1.4、 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());
        }
    }
    //这里又回到了，最初开始的代码
    Navigator<NavDestination> 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)) {
            while (!mBackStack.isEmpty()
                    && mBackStack.peekLast().getDestination() instanceof FloatingWindow
                    && popBackStackInternal(
                            mBackStack.peekLast().getDestination().getId(), true)) {
                // Keep popping
            }
        }

        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();
    }
}

接下来的分析建上节Navigation(一)FragmentNavigator导航页面分析

二、绑定底部导航栏 BottomNavigationView

• 入口

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_navigation_main);
    BottomNavigationView mBottomNavigationView = findViewById(R.id.nav_view);

    NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);
    NavController navController = navHostFragment.getNavController();
    //绑定底部导航栏
    NavigationUI.setupWithNavController(mBottomNavigationView, navController);
}

进入NavigationUI#setupWithNavController方法

    public static void setupWithNavController(
            @NonNull final BottomNavigationView bottomNavigationView,
            @NonNull final NavController navController) {
        bottomNavigationView.setOnNavigationItemSelectedListener(
                new BottomNavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                        //通过 item 获取导航
                        return onNavDestinationSelected(item, navController);
                    }
                });
        //底部导航进行弱引用防止内存泄露
        final WeakReference<BottomNavigationView> weakReference =
                new WeakReference<>(bottomNavigationView);
        navController.addOnDestinationChangedListener(
                new NavController.OnDestinationChangedListener() {
                    @Override
                    public void onDestinationChanged(@NonNull NavController controller,
                            @NonNull NavDestination destination, @Nullable Bundle arguments) {
                        BottomNavigationView view = weakReference.get();
                        if (view == null) {
                            navController.removeOnDestinationChangedListener(this);
                            return;
                        }
                        Menu menu = view.getMenu();
                        //遍历导航对应的item
                        for (int h = 0, size = menu.size(); h < size; h++) {
                            MenuItem item = menu.getItem(h);
                            //根据 id 判断fragment对应的 item
                            if (matchDestination(destination, item.getItemId())) {
                                //设置状态
                                item.setChecked(true);
                            }
                        }
                    }
                });
    }

• bottomNavigationView 监听流程

    public static boolean onNavDestinationSelected(@NonNull MenuItem item,
            @NonNull NavController navController) {
        NavOptions.Builder builder = new NavOptions.Builder()
                .setLaunchSingleTop(true);
        //判断导航目是否是 ActivityNavigator 导航
        if (navController.getCurrentDestination().getParent().findNode(item.getItemId())
                instanceof ActivityNavigator.Destination) {
            builder.setEnterAnim(R.anim.nav_default_enter_anim)
                    .setExitAnim(R.anim.nav_default_exit_anim)
                    .setPopEnterAnim(R.anim.nav_default_pop_enter_anim)
                    .setPopExitAnim(R.anim.nav_default_pop_exit_anim);

        } else {
            builder.setEnterAnim(R.animator.nav_default_enter_anim)
                    .setExitAnim(R.animator.nav_default_exit_anim)
                    .setPopEnterAnim(R.animator.nav_default_pop_enter_anim)
                    .setPopExitAnim(R.animator.nav_default_pop_exit_anim);
        }
        if ((item.getOrder() & Menu.CATEGORY_SECONDARY) == 0) {
            builder.setPopUpTo(findStartDestination(navController.getGraph()).getId(), false);
        }
        NavOptions options = builder.build();
        try {
            //调用 NavController#navigate 进行实际的导航，到指定的fragment
            navController.navigate(item.getItemId(), null, options);
            return true;
        } catch (IllegalArgumentException e) {
            return false;
        }
    }

NavController#getCurrentDestination

    public NavDestination getCurrentDestination() {
        NavBackStackEntry entry = getCurrentBackStackEntry();
        return entry != null ? entry.getDestination() : null;
    }

NavGraph#findNode 获取导航目的

    final SparseArrayCompat<NavDestination> mNodes = new SparseArrayCompat<>();

    public final NavDestination findNode(@IdRes int resid) {
        return findNode(resid, true);
    }

    final NavDestination findNode(@IdRes int resid, boolean searchParents) {
        NavDestination destination = mNodes.get(resid);
        // 返回导航目的
        return destination != null
                ? destination
                : searchParents && getParent() != null ? getParent().findNode(resid) : null;
    }

• navController 监听流程

    static boolean matchDestination(@NonNull NavDestination destination,
            @IdRes int destId) {
        NavDestination currentDestination = destination;
        while (currentDestination.getId() != destId && currentDestination.getParent() != null) {
            currentDestination = currentDestination.getParent();
        }
        return currentDestination.getId() == destId;
    }

流程归总