2020-3-24 EShop代码分析（1）-main activity

// 当前正在显示的Fragment
private Fragment mCurrentFragment;
// 主页面只负责四个Fragment的切换, 没有业务逻辑的处理.

//四个Fragment的监听点击事件
@Override public void onTabSelected(@IdRes int tabId) {
         switch (tabId) {
             case R.id.tab_home:
                 if (mHomeFragment == null) mHomeFragment = HomeFragment.newInstance();
                 switchFragment(mHomeFragment);
                 break;
             case R.id.tab_category:
                 if (mCategoryFragment == null) mCategoryFragment = CategoryFragment.newInstance();
                 switchFragment(mCategoryFragment);
                 break;
             case R.id.tab_cart:
                 if (mCartFragment == null) mCartFragment = CartFragment.newInstance();
                 switchFragment(mCartFragment);
                 break;
             case R.id.tab_mine:
                 if (mMineFragment == null) mMineFragment = MineFragment.newInstance();
                 switchFragment(mMineFragment);
                 break;
             default:
                 throw new UnsupportedOperationException("Illegal branch!");
         }
     }
  // 如果不是在HomeFragment, 则按返回键回到HomeFragment
@Override public void onBackPressed() {
         if (mCurrentFragment != mHomeFragment) {
             bottomBar.selectTabWithId(R.id.tab_home);
             return;
         }
     // 将Activity所在的Task移到后台, 而不是finish此Activity
         moveTaskToBack(true);
     }
@Subscribe(sticky = true, threadMode = ThreadMode.MAIN)
     public void onEvent(CartEvent event) {
         // 如果购物车数据发生变更, 需要更新BottomBar的角标
         if (UserManager.getInstance().hasCart()) {
             int total = UserManager.getInstance().getCartBill().getGoodsCount();
             bottomBar.getTabAtPosition(2).setBadgeCount(total);
         } else {
             bottomBar.getTabAtPosition(2).removeBadge();
         }

    }

     /**
      * 首页4个Fragment切换, 使用hide和show, 而不是replace.
      *
      * @param target 要显示的目标Fragment.
      */
     private void switchFragment(Fragment target) {
         if (mCurrentFragment == target) return;

        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

        if (mCurrentFragment != null) {
             // 隐藏当前正在显示的Fragment
             transaction.hide(mCurrentFragment);
         }

        if (target.isAdded()) {
             // 如果目标Fragment已经添加过, 就显示它
             transaction.show(target);
         } else {
             // 否则直接添加该Fragment
             transaction.add(R.id.layout_container, target, target.getClass().getName());
         }

        transaction.commit();

        mCurrentFragment = target;
     }

    // 找回FragmentManager中存储的Fragment
     private void retrieveFragments() {
         FragmentManager manager = getSupportFragmentManager();
         mHomeFragment = (HomeFragment) manager.findFragmentByTag(HomeFragment.class.getName());
         mCategoryFragment = (CategoryFragment) manager
                 .findFragmentByTag(CategoryFragment.class.getName());
         mCartFragment = (CartFragment) manager.findFragmentByTag(CartFragment.class.getName());
         mMineFragment = (MineFragment) manager.findFragmentByTag(MineFragment.class.getName());
     }