当collectionViewCell未显示时，调用cellForItemAtIndexPath返回nil

解决方法是：先让cell可见，刷新后再获取：

// 记录原indexPath
NSIndexPath *orginIndexPath = indexPath;

// 滚动collectionView到需要获取的那个indexPath对应的cell
[collectionView scrollToItemAtIndexPath:indexPath atScrollPosition:0 animated:NO];

UICollectionViewCell *endView = [collectionView cellForItemAtIndexPath:indexPath];
// 当获取不到时，刷新下再获取
 if (!endView) {
      [collectionView layoutIfNeeded];
      endView = [collectionView cellForItemAtIndexPath:indexPath];
                                                    }
                                                    
if(!endView) {
    [collectionView reloadData];
     [collectionView layoutIfNeeded];
     endView = [collectionView cellForItemAtIndexPath:indexPath];
                                                    }
                                                    
// 获取完成后再回到原位置
[collectionView scrollToItemAtIndexPath:orginIndexPath atScrollPosition:0 animated:NO];