屏幕旋转后不添加Android片段

Question

我使用没有任何布局的片段（它在后台下载文件）。 我在第一次创建活动时添加一次，假设它将在屏幕方向后再次附加，但它不是！
活动的onCreate：

if (savedInstanceState == null) { 
    getSupportFragmentManager().beginTransaction() 
      .add(new FragmentDownloadApp(), FragmentDownloadApp.TAG) 
      .commit(); 
} else { 
    if (savedInstanceState.containsKey(STATE_SELECTED_NAVIGATION_ITEM)) { 

片段onActivityCreated：

@Override 
public void onActivityCreated(Bundle savedInstanceState) { 
    super.onActivityCreated(savedInstanceState); 
    // this is really important in order to save the state across screen 
    // configuration changes for example 
    setRetainInstance(true); 

    // ---- magic lines starting here ----- 
    // call this to re-connect with an existing 
    // loader (after screen configuration changes for e.g!) 
    LoaderManager lm = getLoaderManager(); 
    if (lm.getLoader(LOADER_DOWNLOAD_APP) != null) { 
     lm.initLoader(LOADER_DOWNLOAD_APP, null, mDownLoader); 
    } 
    // ----- end magic lines ----- 
} 

和错误：

public void onPositiveClick() { 
      FragmentDownloadApp fragment = (FragmentDownloadApp) getSupportFragmentManager() 
        .findFragmentByTag(FragmentDownloadApp.TAG); 
      fragment.startDownload(mAppToDownload); 
     } 

这里出现NullPointerException异常！

Answer 1

找到答案：

@Override 
public void onSaveInstanceState(Bundle outState) { 
    super.onSaveInstanceState(outState); // NEED THIS!