setRetainInstance（true），但UI不出現

Question

我試圖創建一個圖片庫，使用this tutorial作爲指南，進行了一些調整。

因此，這裏是我的代碼片段部分：

public static class PlaceholderFragment extends Fragment { 

    private static final String TAG = "com.example.imageencryptor.RetainedFragment"; 

    LruCache<String, Bitmap> cache; 

    public PlaceholderFragment() { 

     initCache(); 

    } 

    public void initCache() { 
     final int maxMemory = (int) (Runtime.getRuntime().maxMemory()/1024); 
     final int cacheSize = maxMemory/8; 
     cache = new LruCache<String, Bitmap>(cacheSize) { 
      @Override 
      protected int sizeOf(String key, Bitmap bitmap) { 
       // The cache size will be measured in kilobytes rather than 
       // number of items. 
       return ImageLoader.getSizeInBytes(bitmap)/1024; 
      } 
     }; 

    } 

    public static PlaceholderFragment findOrCreateRetainFragment(
      FragmentManager fm) { 
     PlaceholderFragment fragment = (PlaceholderFragment) fm 
       .findFragmentByTag(TAG); 
     if (fragment == null) { 
      fragment = new PlaceholderFragment(); 
      fm.beginTransaction().add(fragment, TAG).commit(); 
     } 
     return fragment; 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 
     View rootView = inflater.inflate(R.layout.fragment_main, container, 
       false); 

     GridView imageGrid = (GridView) rootView 
       .findViewById(R.id.gridview); 

     ImageLoader loader = new ImageLoader(getActivity(), cache); 
     imageGrid.setAdapter(new ImageAdapter(getActivity(), loader)); 

     return rootView; 
    } 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setRetainInstance(true); 
    } 

} 

我活動的onCreate方法是這樣的：

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    PlaceholderFragment.findOrCreateRetainFragment(getSupportFragmentManager()); 
    } 

不過，我結束了在啓動一個空白的UI。沒有網格視圖。這是爲什麼？

在另一方面，如果我的onCreate改變這個它會工作：

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

    PlaceholderFragment pFragment = PlaceholderFragment 
      .findOrCreateRetainFragment(getSupportFragmentManager()); 


    if (savedInstanceState == null) { 
    getSupportFragmentManager().beginTransaction() 
    .add(R.id.container, pFragment,PlaceholderFragment.TAG).commit(); 
    } 

} 

但不喜歡它，一遍又一遍，只要添加多個相同的片段用相同的標籤，給FragmentManager屏幕方向改變？

Answer 1

我猜這個UI是不會顯示的，因爲你在你的界面中輸入FragmentTransaction，名字是findOrCreateRetainFragment。此接口用於將數據從Activity傳遞到Fragment動態添加，即使片段以某種方式由Android重新創建，該接口也可用。你需要做的FragmentTransaction在你的活動如下：

// Init your fragment 
PlaceholderFragment pFragment = (PlaceholderFragment) 
    getSupportFragmentManager().findFragmentByTag(PlaceholderFragment.TAG); 

// If it is null: the fragment is not created yet - create it 
if(pFragment == null) { 
    // send data to interface - which will create a new one 
    PlaceholderFragment pFragment = 
      new PlaceholderFragment().instanceRetainFragment(value); 
    // add the new fragment 
    getSupportFragmentManager().beginTransaction() 
      .add(R.id.container, pFragment, PlaceholderFragment.TAG).commit(); 
} 
// else: avoid to create the fragment again 

，然後在片段中的界面可能是這樣的：

public static PlaceholderFragment instanceRetainFragment(int value) { 
    // create a new fragment 
    PlaceholderFragment mFragment = new PlaceholderFragment(); 
    // create a bundle to store data 
    Bundle args = new Bundle(); 
    // insert data 
    args.putInt("mValue", value); 
    // attach data 
    mFragment.setArguments(args); 
    // return new fragment in the activity 
    return mFragment; 
} 

您還可以創建一個接口，而不必存儲數據和剛剛呼叫第一行和最後一行。但是，存儲和接收活動中的一些信息可能非常有用（例如：a Boolean來過濾您的GridView，Integer只顯示一個類別等）。
所以，主要的問題是在Fragment中調用FragmentManager而不是Activity。這可能是「不顯示UI」的原因。讓我知道這個是否奏效。