用 Tag、 反射原理 和 findFragmentByTag 少寫代碼實現Tab 切換時實例化、顯示、隱藏 Fragment

在一些場景裏， TabLayout 會對應一組Fragment，而後經過 onTabSelected(TabLayout.Tab tab) 來觸發Tab所對應Fragment的 add() (實例化) show() 與 hide() ，那麼如何儘可能少寫代碼來實現這個場景呢？

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個人解決方案是用 Tag、 反射原理 以及 FragmentManager 的 findFragmentByTag(String tag) 方法

1. 將Fragment的類的限定名做爲 Tag 附加到 Tab 上

2. onTabSelected(TabLayout.Tab tab) 時獲取 Tab 對應的 Tag
   這時得到的就是Fragment的限定名

3. 利用反射經過限定名實例化Fragment並 add()

4. show() 對應的Fragment

5. hide() 也是一樣的道理

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代碼片斷：

tabLayout = (TabLayout) findViewById(R.id.tab_layout);
            final TabLayout.Tab tab1 = tabLayout
                    .newTab()
                    .setText(R.string.text_1)
                    .setTag(Tab1Fragment.class.getName()); //用Fragment的限定名做爲Tag
            TabLayout.Tab tab2 = tabLayout
                    .newTab()
                    .setText(R.string.text_2)
                    .setTag(Tab2Fragment.class.getName());
            TabLayout.Tab tab3 = tabLayout
                    .newTab()
                    .setText(R.string.text_3)
                    .setTag(Tab3Fragment.class.getName());
            tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
                @Override
                public void onTabSelected(TabLayout.Tab tab) {
                    FragmentUtils.showBeforeAddByTag(SomeActivity.this, R.id.frame_layout_fragment_container, (String) tab.getTag()); //用Tab的Tag來進一步完成Fragment的add show hide
                }
                @Override
                public void onTabUnselected(TabLayout.Tab tab) {
                    FragmentUtils.hideByTag(SomeActivity.this, (String) tab.getTag());
                }
                @Override
                public void onTabReselected(TabLayout.Tab tab) {
                }
            });

FragmentUtils代碼片斷：

//經過反射實例化Fragment  
    @Nullable
    public static Fragment reflectionNewInstance(@NonNull String fragmentClassName, Object... args) {
        Class<?>[] parameterTypes;
        if (args == null) {
            parameterTypes = new Class[0];
        } else {
            parameterTypes = new Class[args.length];
            for (int i = 0; i < args.length; i++) {
                parameterTypes[i] = args[i].getClass();
            }
        }
        try {
            Method method = Class.forName(fragmentClassName).getMethod("newInstance", parameterTypes);
            return (Fragment) method.invoke(null, args);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
    }
    
    //添加Fragment
    public static Fragment addByTag(FragmentActivity fragmentActivity, int containerResourceId, String tag, Bundle arguments) {
        FragmentManager fragmentManager = fragmentActivity.getSupportFragmentManager();
        Fragment fragment = reflectionNewInstance(tag);
        fragment.setArguments(arguments);
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(containerResourceId, fragment, tag);
        fragmentTransaction.commit();
        return fragment;
    }

    //顯示Fragment
    public static void showByTag(FragmentActivity fragmentActivity, String tag) {
        FragmentManager fragmentManager = fragmentActivity.getSupportFragmentManager();
        Fragment fragment = fragmentManager.findFragmentByTag(tag); //經過Tab上的tag查找Fragment實例
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.show(fragment);
        fragmentTransaction.commit();
    }
    
    //隱藏Fragment
    public static void hideByTag(FragmentActivity fragmentActivity, String tag) {
        FragmentManager fragmentManager = fragmentActivity.getSupportFragmentManager();
        Fragment fragment = fragmentManager.findFragmentByTag(tag);
        if (fragment != null) {
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.hide(fragment);
            fragmentTransaction.commit();
        }
    }