Android旋转：为什么片段不能被TAG恢复？

Question

我有一个名为MainActivity的活动，里面有一个名为“HomeFragment”的片段。我想在屏幕旋转后恢复片段，但有些东西似乎丢失了。 我采取了以下过程：

1. 节省使用getSupportFragmentManager().putFragment(...)片段中onSaveInstanceState
2. 尝试通过fragmentManager.findFragmentByTag的onCreate期间恢复片段（检查包后不为空）

然而findFragmentByTag返回null。

这里是代码片段里面MainActivity：

protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     // Set up the action bar to show a dropdown list. 
     final ActionBar actionBar = getSupportActionBar(); 
     actionBar.setDisplayShowTitleEnabled(false); 
     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST); 
     setupNavigationSpinner(actionBar); 
     FragmentManager fragmentManager = getSupportFragmentManager(); 

     if (savedInstanceState != null) { 

      if (fragmentManager.findFragmentByTag("frag") != null) { 
       // findFragmentByTag always return null 
       homeFragment = (HomeFragment) fragmentManager 
         .findFragmentByTag(HomeFragment.ARG_ITEM_ID); 
       contentFragment = homeFragment; 
      }else{ 
       homeFragment = new HomeFragment(); 
       switchContent(homeFragment, HomeFragment.ARG_ITEM_ID); 
      } 
     } else { 
      homeFragment = new HomeFragment(); 
      switchContent(homeFragment, HomeFragment.ARG_ITEM_ID); 
     } 

} 


@Override 
public void onSaveInstanceState(Bundle outState) { 
    getSupportFragmentManager().putFragment(outState, "frag",homeFragment); 
} 

public void switchContent(Fragment fragment, String tag) { 
    FragmentManager fragmentManager = getSupportFragmentManager(); 
    while (fragmentManager.popBackStackImmediate()) 
     ; 

    if (fragment != null) { 
     FragmentTransaction transaction = fragmentManager 
       .beginTransaction(); 
     transaction.replace(R.id.content_frame, fragment, tag); 
     // Only ArticlDetailFragment is added to the back stack. 
     if (!(fragment instanceof Fragment)) { 
      transaction.addToBackStack(tag); 
     } 
     transaction.commit(); 
     contentFragment = fragment; 
    } 
} 

Answer 1

没有理由叫

getSupportFragmentManager().putFragment(outState, "frag",homeFragment); 

一个Fragment状态将自动恢复。问题是你没有叫你的onSaveInstanceState超级。它应该只是看起来像这样：

@Override 
protected void onSaveInstanceState(Bundle outState) { 
    super.onSaveInstanceState(outState); 
} 

此外，你设置的标签作为HomeFragment.ARG_ITEM_ID然后找"frag"。 HomeFragment.ARG_ITEM_ID == "frag"？