Fragment 基础使用及重叠问题

Posted 2020-11-10 随易来了

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

1 FragmentTransaction transaction = mBaseActivity.getSupportFragmentManager().beginTransaction();
2 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

 1     private void switchFragment(Fragment from, Fragment to) {
 2         if (mCurrent != to) {
 3             mCurrent = to;
 4             FragmentTransaction transaction = mBaseActivity.getSupportFragmentManager().beginTransaction();
 5             // 判断目标fragment是否被add过
 6             if (!to.isAdded()) {
 7                 transaction.hide(from).add(R.id.fl_container, to，to.getClass().getName()).commit();
 8             } else {
 9                 transaction.hide(from).show(to).commit();
10             }
11         }
12     }

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

1    private static final String SAVED_CURRENT_ID = "currentId";
2     private int currentIndex;
3     @Override
4     public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
5         outState.putInt(SAVED_CURRENT_ID, currentIndex);//保存当前显示fragment索引
6         super.onSaveInstanceState(outState, outPersistentState);
7     }

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

 1     @Override
 2     protected void onCreate(Bundle savedInstanceState) {
 3         super.onCreate(savedInstanceState);
 4         setContentView(R.layout.activity_main);
 5         //重启时防止fragment重叠
 6         if (savedInstanceState != null) {
 7             currentIndex = savedInstanceState.getInt(SAVED_CURRENT_ID, 0);
 8             homeFragment = (HomeFragment) getSupportFragmentManager().findFragmentByTag(HomeFragment.class.getName());
 9             newsFragment = (NewsFragment) getSupportFragmentManager().findFragmentByTag(NewsFragment.class.getName());
10             findFragment = (FindFragment) getSupportFragmentManager().findFragmentByTag(FindFragment.class.getName());
11             adminFragment = (AdminFragment) getSupportFragmentManager().findFragmentByTag(AdminFragment.class.getName());
12         } else {
13             initFragment();
14         }
15         intView();
16     }
17 
18     @Override
19     protected void intView() {
20         setTitle("主页");
23         radioGroup = (RadioGroup) findViewById(R.id.rg_main_bottom);
24         radioGroup.setOnCheckedChangeListener(occl);
25         ((RadioButton) radioGroup.getChildAt(currentIndex)).setChecked(true);
26     }

最后，恢复选中fragment

 1     private RadioGroup.OnCheckedChangeListener occl = new RadioGroup.OnCheckedChangeListener() {
 2 
 3         @Override
 4         public void onCheckedChanged(RadioGroup group, int checkedId) {
 5             switch (checkedId) {
 6                 case R.id.rb_home:
 7                     setTitle("首页");
 8                     currentIndex = 0;
 9                     if (homeFragment == null) {
10                         homeFragment = HomeFragment.newInstance(null, null);
11                     }
12                     switchFragment(mBackHandledFragment, homeFragment);
13                     break;
14                 case R.id.rb_news:
15                     setTitle("新闻");
16                     currentIndex = 1;
17                     if (newsFragment == null) {
18                         newsFragment = NewsFragment.newInstance(null, null);
19                     }
20                     switchFragment(mBackHandledFragment, newsFragment);
21                     break;
22                 case R.id.rb_find:
23                     showTabSelectView();
24                     currentIndex = 2;
25                     if (findFragment == null) {
26                         findFragment = FindFragment.newInstance(null, null);
27                     }
28                     switchFragment(mBackHandledFragment, findFragment);
29                     break;
30                 case R.id.rb_admin:
31                     setTitle("我的");
32                     currentIndex = 3;
33                     if (adminFragment == null) {
34                         adminFragment = AdminFragment.newInstance(null, null);
35                     }
36                     switchFragment(mBackHandledFragment, adminFragment);
37                     break;
38             }
39         }
40     };
41 
43     private void switchFragment(Fragment from, Fragment to) {
44         FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
45         if (null == from) {
46             if (mBackHandledFragment != to) {
47                 mBackHandledFragment = (BackHandledFragment) to;
48                 if (!to.isAdded()) {
49                     transaction.add(R.id.frameLayout_container, to, to.getClass().getName()).commit();
50                 } else {
51                     transaction.show(to).commit();
52                 }
53             }
54         } else {
55             if (mBackHandledFragment != to) {
56                 mBackHandledFragment = (BackHandledFragment) to;
57                 // 判断目标fragment是否被add过
58                 if (!to.isAdded()) {
59                     transaction.hide(from).add(R.id.frameLayout_container, to, to.getClass().getName()).commit();
60                 } else {
61                     transaction.hide(from).show(to).commit();
62                 }
63             }
64         }
65     }