当我将support-v4版本从23.1.1升级到23.4.0时，我注意到了这个问题：基本上，在过去返回true的情况下，isAdded（）总是返回false。
我有一个活动（FragmentActivity），它有一个包含Fragments的ViewPager。每个片段在启动时都会在onCreate（）中启动异步任务以下载一些图像;为了提高效率，在回调中，我正在检查isAdded（）以确保在继续处理之前附加了Fragment。
如果我包含support-v4库的23.1.1版，我的代码将按预期工作。但是，当我更新到23.4.0时，isAdded（）似乎几乎总是返回false，这甚至不允许当前片段完成处理异步结果。
注意：如果我翻阅相册并不重要 - 每次调用isAdded（）似乎都会返回false。
下面的相关代码（注意：此示例简化了一些代码）：
// note FetchableListener implements onFetchableUpdate()
public class CameraAlbumItemFragment
implements Fetchable.FetchableListener
{
private static final String CAMERA_ALBUM_ITEM_FRAGMENT_CAMERA_KEY = "camera" ;
// Member Variables
//
@Nullable private Camera m_camera ;
@Nullable private ArrayList<CameraViewImageDownloadResult> m_imageDownloads;
public static CameraAlbumItemFragment newInstance ( @NotNull Camera camera )
{
final CameraAlbumItemFragment fragment = new CameraAlbumItemFragment();
fragment.setRetainInstance( true );
final Bundle bundle = new Bundle( 1 );
bundle.putParcelable( CAMERA_ALBUM_ITEM_FRAGMENT_CAMERA_KEY, camera );
fragment.setArguments( bundle );
return fragment;
}
@Override
public void onCreate ( @Nullable Bundle savedInstanceState )
{
super.onCreate( savedInstanceState );
m_camera = getArguments().getParcelable( CAMERA_ALBUM_ITEM_FRAGMENT_CAMERA_KEY );
// If the images have not been downloaded, then start background
// tasks to retrieve them. Not likely, but make sure our camera is not null
//
if ( m_camera != null && m_imageDownloads == null )
{
// This will start an async task that will call onFetchableUpdate() when it receives a response from the server
m_camera.updateNonCurrentViews( getActivity(), this );
}
}
/** The Fragment's UI */
@Override
public View onCreateView ( @NotNull LayoutInflater inflater,
@Nullable ViewGroup container,
@Nullable Bundle savedInstanceState )
{
final View view = inflater.inflate( R.layout.camera_album_item, container, false );
// Set image if already downloaded
//
// Set the on click listener for the cycle image button
// This has to be done here instead of using the android:onClick attribute in the layout
// file because this is a fragment, not an activity
//
final ImageView cameraImageView = (ImageView) view.findViewById( R.id.camera_image_view );
return view;
}
/**
* Add an image to the list of downloaded images. Display or hide the cycle images button based on
* the number of retrieved images
*
* @param bitmap An image retrieved by a background process
*/
public void addImage ( @Nullable final Bitmap bitmap, @NotNull final String viewName )
{
assert m_imageDownloads != null;
m_imageDownloads.add( new CameraViewImageDownloadResult( bitmap, viewName ) );
}
@Override
public void onFetchableUpdate ( @NotNull Fetchable fetchable, @Nullable Object data )
{
//*********************************************************
// NOTE: It is the call here to isAdded() that is returning false nearly
// every time in support-v4:23.4.0 but not in 23.1.1
//**********************************************************
if ( fetchable == m_camera && isAdded() )
{
final List<CameraView> cameraViews = m_camera.getViews();
m_imageDownloads = new ArrayList<>( cameraViews.size() );
// Download camera images
for ( CameraView cameraView : cameraViews )
{
if ( cameraView.isCurrent() )
{
final String imageURL = cameraView.getCameraURL();
if ( imageURL != null )
{
new GetCameraImageAsyncTask( this, cameraView.getName() ).execute( imageURL );
}
else
{
Log.e( LOG_TAG, "No valid image URL for " + cameraView.getName() ) ;
addImage( null, cameraView.getName() );
}
}
else
{
addImage( null, cameraView.getName() );
}
}
// We don't need to maintain the observer reference anymore
m_camera.removeListener( this );
}
}
/**
* Get the image view for displaying a camera view
*
* @return The camera image view
*/
@Nullable
private ImageView getCameraImageView ()
{
final View v = getView();
if ( v != null )
{
return (ImageView)v.findViewById( R.id.camera_image_view );
}
else
{
return null;
}
}
}
包含ViewPager的活动（FragmentActivity）
public class CameraAlbumActivity
extends FragmentActivity
{
// Intent Data Key
//
public final static String CAMERA_ALBUM_SELECTED_ID_KEY = "selectedId" ;
private static final String LOG_TAG = "CameraAlbumActivity" ;
@Override
protected void onCreate ( @Nullable Bundle savedInstanceState )
{
super.onCreate( savedInstanceState );
final Intent intent = getIntent();
final Object sharedData = SharedDataWrapper.getInstance().getData();
CameraCollection cameraCollection ;
if ( sharedData != null && sharedData instanceof CameraCollection )
{
cameraCollection = ( CameraCollection ) sharedData;
}
else
{
// just create an empty collection
cameraCollection = new CameraCollection() ;
}
// Load view
setContentView( R.layout.album );
// Get references to buttons
//
m_previousButton = (ImageView)findViewById( R.id.album_previous_btn );
m_nextButton = (ImageView)findViewById( R.id.album_next_btn );
// Configure view pager
//
m_viewPager = (ViewPager)findViewById( R.id.album_view_pager );
final CameraAlbumPagerAdapter adapter = new CameraAlbumPagerAdapter( getSupportFragmentManager(), cameraCollection );
m_viewPager.setAdapter( adapter );
m_viewPager.addOnPageChangeListener( new OnCyclingContentAlbumViewScrollListener( this, adapter ) );
// Set the selected item
int selectedId = intent.getIntExtra( CAMERA_ALBUM_SELECTED_ID_KEY, -1 );
if( selectedId == -1 )
{
return;
}
List<Camera> models = cameraCollection.getAllModels();
for ( int i = 0 ; i < models.size() ; i++ )
{
Camera camera = models.get( i );
if ( selectedId == camera.getId() )
{
m_viewPager.setCurrentItem( i, false );
break;
}
}
}
/**
* OnPageChangeListeners should be removed to prevent memory leaks
*/
@Override
public void onDestroy()
{
m_viewPager.clearOnPageChangeListeners() ;
super.onDestroy() ;
}
/**
* Scroll one item to the right, if possible
*
* @param v the view triggering the event
*/
public void scrollToNext ( @SuppressWarnings("UnusedParameters") View v )
{
int currentIndex = m_viewPager.getCurrentItem();
if( currentIndex < m_viewPager.getAdapter().getCount() - 1 )
{
m_viewPager.setCurrentItem( currentIndex + 1, true );
}
}
/**
* Scroll one item to the left, if possible
*
* @param v the view triggering the event
*/
public void scrollToPrevious ( @SuppressWarnings("UnusedParameters") View v )
{
int currentIndex = m_viewPager.getCurrentItem();
if( currentIndex > 0 )
{
m_viewPager.setCurrentItem( currentIndex - 1, true );
}
}
/**
* Set the visibility of the previous and next buttons based on view pager contents and position
*/
public void setPreviousAndNextButtonVisibility ()
{
final int position = m_viewPager.getCurrentItem();
m_previousButton.setVisibility( position == 0 ? View.INVISIBLE : View.VISIBLE );
m_nextButton.setVisibility( position < m_viewPager.getAdapter().getCount() - 1 ? View.VISIBLE : View.INVISIBLE );
}
/**
* @return The item fragment which is currently displayed
*/
@Nullable
public Fragment getCurrentItemFragment ()
{
int currentItem = m_viewPager.getCurrentItem();
ModelCollectionAlbumPagerAdapter adapter = (ModelCollectionAlbumPagerAdapter)m_viewPager.getAdapter();
return adapter.getRegisteredFragment( currentItem );
}
}
我不确定这个版本的支持库是否存在问题（希望如此），或者我的代码中的某些内容不正确，最终会出现在这个最新版本中。正如我所提到的，如果我只是在我的gradle文件中交换版本，上面的代码在第23.1.1节中按预期工作，但是当我更改为23.4.0时，它会失败。
思考？建议？
谢谢！
答案 0 :(得分：1)
经过进一步调查后，对支持库的更新揭示了现有代码中的一个缺陷。请注意，异步任务的开始在onCreate（）中开始。如果异步任务在onCreateView（）完成之前完成，则当前的Fragment.isAdded（）调用将返回false。
无论出于何种原因，使用较旧的支持库，这种情况都没有发生（或者如果是这样，很少我没有观察到它）。对新支持库的更新相当一致地触发了这种情况。
修复是将异步任务的开始移动到onActivityCreated（），当然，在添加了Fragment之后调用它。
答案 1 :(得分：0)
在isAdded（）之前调用此方法为我解决了这个问题。
getSupportFragmentManager().executePendingTransactions();