UICollectionView 数据源方法未在 CoreData Change 上调用

Question

我有一个UICollectionView，其内容由NSFetchedResultsController提供。每当更新核心数据时，都会调用一个调用以通过 reloadItemsAtIndexPaths： 方法更新集合视图中已更改的项。

经过大量的测试，我确定上述过程工作正常。我对核心数据进行更新，最终调用数据源方法"cellForItemAtIndexPath"，并按预期更新单元格。 但是，我也实现了数据源方法"viewForSupplementaryElementOfKind"，以显示一些单元格标题（也基于对核心数据的更改），这无法正常工作。

出于某种原因，似乎在核心数据更改后调用"reloadItemsAtIndexPaths"时，"viewForSupplementaryElementOfKind"不会被调用，我无法弄清楚为什么会这样。但是，一旦我开始滚动集合视图，然后调用"viewForSupplementaryElementOfKind"，我能够看到补充标题视图中的更新，正如我根据核心数据更改所期望的那样。此方法也会在初始创建 UICollectionView 时成功调用。

我

有一个自定义布局，我正在与我的集合视图一起使用，所以问题可能出在那里？我希望有人能发现我的错误。

以下是用于创建 UICollectionView、其布局、单元格和标题视图的代码：

1. 创建 UI 链接视图

           - (void)createCollectionView
           {
               _layout1 = [[BigLayout alloc] init]; //custom layout
               [_layout1 setScrollDirection:UICollectionViewScrollDirectionHorizontal];
               _collectionView = [[UICollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:_layout1];
               _collectionView.translatesAutoresizingMaskIntoConstraints = NO;
               _collectionView.backgroundColor = [UIColor clearColor];
               _collectionView.dataSource = self;
               _collectionView.delegate = self;
               [self.view addSubview:_collectionView];
               //set up constraints, add them to view...
               [self.collectionView registerClass:[InboxCell class]
                       forCellWithReuseIdentifier:@"inbox"];
               [self.collectionView registerClass:[UICollectionReusableView class]
                       forSupplementaryViewOfKind:UICollectionElementKindSectionHeader
                              withReuseIdentifier:@"header"];
           }
   

2. 创建自定义布局

       @implementation BigLayout
       -(id)init
       {
           self = [super init];
           if (self) {
               self.itemSize = CGSizeMake(330, 588);
               self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
               self.headerReferenceSize = CGSizeMake(13, 13);
           }
           return self;
       }
       -(void)prepareLayout
       {
           [super prepareLayout];
           _cellCount = [[self collectionView] numberOfItemsInSection:0];
       }
       - (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path
       {
           UICollectionViewLayoutAttributes* attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:path];
           attributes.size = self.itemSize;
           attributes.center = CGPointMake(path.item * (self.itemSize.width + 20) + self.itemSize.width/2.0 + 20, self.collectionView.center.y);
           return attributes;
       }
       - (CGSize)collectionViewContentSize
       {
           return CGSizeMake(((self.itemSize.width + 20) * _cellCount) + 80, [self collectionView].height);
       }
       -(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
       {
           NSMutableArray* attributes = [NSMutableArray array];
           for (NSInteger i=0 ; i < self.cellCount; i++) {
               NSIndexPath* indexPath = [NSIndexPath indexPathForItem:i inSection:0];
               UICollectionViewLayoutAttributes *attr = [self layoutAttributesForItemAtIndexPath:indexPath];
               UICollectionViewLayoutAttributes *hattr = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];
               if (CGRectIntersectsRect(attr.frame, rect)) {
                   [attributes addObject:attr];
                   [attributes addObject:hattr];
               }
           }
           return attributes;
       }
       - (void)prepareForCollectionViewUpdates:(NSArray *)updateItems
       {
       }
       - (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
       {
           UICollectionViewLayoutAttributes* attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
           attributes.alpha = 1.0;
           return attributes;
       }
       - (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
       {
           UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader withIndexPath:indexPath];
           attributes.size = CGSizeMake(13, 13);
           attributes.center = CGPointMake(indexPath.item * (self.itemSize.width + 20) + self.itemSize.width/2.0 + 20 + self.collectionView.left, self.collectionView.height == 768 ? 75 : 200);
           attributes.alpha = 1.0f;
           return attributes;
       }
   

3. 单元格的数据源方法（在 CD 更新中调用）

           - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
           {
               //this method gets called on reload items
               InboxCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"inbox" forIndexPath:indexPath];
               Email *email = [self.fetchedResultsController objectAtIndexPath:indexPath];
               //do stuff with the email in the cell
               return cell;
           }
   

4. 补充标头视图的数据源方法（在 CD 更新时不调用）

           - (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView
                      viewForSupplementaryElementOfKind:(NSString *)kind
                                            atIndexPath:(NSIndexPath *)indexPath
           {
               //this method does not get called on reload items, only gets called initially and on scroll
               UICollectionReusableView *reusableView;
                if (kind == UICollectionElementKindSectionHeader) { //specify in case we add a footer later
                    UICollectionReusableView *unreadEmailImageIdentifier = [collectionView dequeueReusableSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:@"header" forIndexPath:indexPath];
                    Email *email = [self.fetchedResultsController objectAtIndexPath:indexPath];
                    UIImageView *unreadEmailDot = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"dot.png"]];
                    [unreadEmailImageIdentifier addSubview:unreadEmailDot];
                    unreadEmailImageIdentifier.hidden = YES;
                    if (email.isUnread == YES || email.isUnread == 1) {
                        unreadEmailImageIdentifier.hidden = NO;
                    }
                    reusableView = unreadEmailImageIdentifier;
                }
               return reusableView;
           }
   

简而言之，当我尝试重新加载集合视图或集合视图的组件时，似乎不会调用补充标题视图的数据源方法。但是，我确实看到调用了在自定义布局类中为补充视图创建属性的方法。如果有人可以指出任何可能的原因，这可能是发生，我将不胜感激！

Answer 1

reloadItemsAtIndexPaths仅用于重新加载单元格。您可以通过使用 reloadSections: 重新加载部分来重新加载补充视图。如果您仍然遇到问题，请在问题中包含您在NSFetchedResultsControllerDelegate实现中的问题。