Fragment 基础使用及重叠问题

一 基本使用

Fragment依附于Activity使用，方面我们在一个页面里面切换显示多屏内容。

Activity管理Fragment有两种方式，通过FragmentTransacation这个类来管理fragment的显示、隐藏

1 replace方法

 FragmentTransaction transaction = mBaseActivity.getSupportFragmentManager().beginTransaction();
 transaction.replace(R.id.fl_container, mCurrent).commit();

使用这种方法，可以避免出现fragment重叠问题。但每次执行replace时，fragment都会重新走一遍生命周期方法，会造成重复加载数据。不推荐使用。

2 hide，show，add方法

     private void switchFragment(Fragment from, Fragment to) {
         if (mCurrent != to) {
             mCurrent = to;
             FragmentTransaction transaction = mBaseActivity.getSupportFragmentManager().beginTransaction();
             // 判断目标fragment是否被add过
             if (!to.isAdded()) {
                 transaction.hide(from).add(R.id.fl_container, to，to.getClass().getName()).commit();
             } else {
                 transaction.hide(from).show(to).commit();
             }
         }
     }

使用这种方法，避免了重新加载数据问题，但是会出现fragment重叠问题。

原因：当内存不足时，系统会回收宿主Activity，而Fragment实例并没有被一并回收，activity被回收时会主动调用onSaveInstanceState方法，保存视图层（View Hierarchy）及一些activity的数据。

二 重叠问题

通过onCreate中的参数savedInstanceState恢复了之前的fragment。此时的FragmentTransaction中的相当于又再次add了fragment进去的，之前保存的fragment也还在。hide()和show()方法对之前保存的fragment已经失效了。之前显示的fragment一直显示，所以出现了重叠的现象。解决方法：

首先，activity被系统收回，触发onSaveInstanceState(Bundle outState)方法，在outstate里，系统自动保存了fragment数据，手动保存当前显示的fragment索引

    private static final String SAVED_CURRENT_ID = "currentId";
     private int currentIndex;
     @Override
     public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
         outState.putInt(SAVED_CURRENT_ID, currentIndex);//保存当前显示fragment索引
         super.onSaveInstanceState(outState, outPersistentState);
     }

其次，当重新进入activity时，走生命周期方法oncreae(Bundle savedInstanceState),saveInstanceState 包含了fragment数据，以及其他一些view的状态，和我们手动保存的数据，从bundle里取出fragment对象，避免重新实例化fragment，造成视图重叠

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         //重启时防止fragment重叠
         if (savedInstanceState != null) {
             currentIndex = savedInstanceState.getInt(SAVED_CURRENT_ID, 0);
             homeFragment = (HomeFragment) getSupportFragmentManager().findFragmentByTag(HomeFragment.class.getName());
             newsFragment = (NewsFragment) getSupportFragmentManager().findFragmentByTag(NewsFragment.class.getName());
             findFragment = (FindFragment) getSupportFragmentManager().findFragmentByTag(FindFragment.class.getName());
             adminFragment = (AdminFragment) getSupportFragmentManager().findFragmentByTag(AdminFragment.class.getName());
         } else {
             initFragment();
         }
         intView();
     }

     @Override
     protected void intView() {
         setTitle("主页");
         radioGroup = (RadioGroup) findViewById(R.id.rg_main_bottom);
         radioGroup.setOnCheckedChangeListener(occl);
         ((RadioButton) radioGroup.getChildAt(currentIndex)).setChecked(true);
     }

最后，恢复选中fragment

     private RadioGroup.OnCheckedChangeListener occl = new RadioGroup.OnCheckedChangeListener() {

         @Override
         public void onCheckedChanged(RadioGroup group, int checkedId) {
             switch (checkedId) {
                 case R.id.rb_home:
                     setTitle("首页");
                     currentIndex = 0;
                     if (homeFragment == null) {
                         homeFragment = HomeFragment.newInstance(null, null);
                     }
                     switchFragment(mBackHandledFragment, homeFragment);
                     break;
                 case R.id.rb_news:
                     setTitle("新闻");
                     currentIndex = 1;
                     if (newsFragment == null) {
                         newsFragment = NewsFragment.newInstance(null, null);
                     }
                     switchFragment(mBackHandledFragment, newsFragment);
                     break;
                 case R.id.rb_find:
                     showTabSelectView();
                     currentIndex = 2;
                     if (findFragment == null) {
                         findFragment = FindFragment.newInstance(null, null);
                     }
                     switchFragment(mBackHandledFragment, findFragment);
                     break;
                 case R.id.rb_admin:
                     setTitle("我的");
                     currentIndex = 3;
                     if (adminFragment == null) {
                         adminFragment = AdminFragment.newInstance(null, null);
                     }
                     switchFragment(mBackHandledFragment, adminFragment);
                     break;
             }
         }
     };

     private void switchFragment(Fragment from, Fragment to) {
         FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
         if (null == from) {
             if (mBackHandledFragment != to) {
                 mBackHandledFragment = (BackHandledFragment) to;
                 if (!to.isAdded()) {
                     transaction.add(R.id.frameLayout_container, to, to.getClass().getName()).commit();
                 } else {
                     transaction.show(to).commit();
                 }
             }
         } else {
             if (mBackHandledFragment != to) {
                 mBackHandledFragment = (BackHandledFragment) to;
                 // 判断目标fragment是否被add过
                 if (!to.isAdded()) {
                     transaction.hide(from).add(R.id.frameLayout_container, to, to.getClass().getName()).commit();
                 } else {
                     transaction.hide(from).show(to).commit();
                 }
             }
         }
     }