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我有一个 UICollectionView，其内容由 NSFetchedResultsController 提供。每当更新核心数据时，都会通过 reloadItemsAtIndexPaths: 方法调用以更新集合视图中更改的项目。

经过大量测试，我确定上述过程正常工作。我对核心数据进行了更新，最终调用了数据源方法“cellForItemAtIndexPath”并按预期更新了单元格。但是，我还实现了数据源方法“viewForSupplementaryElementOfKind”，以显示一些单元格标题（也基于对核心数据的更改），但这无法正常工作。

出于某种原因，似乎在核心数据更改后调用“reloadItemsAtIndexPaths”时，不会调用“viewForSupplementaryElementOfKind”，我无法弄清楚为什么会出现这种情况。但是，一旦我开始滚动集合视图，就会调用“viewForSupplementaryElementOfKind”，并且我可以根据核心数据更改在补充标题视图中看到更新。在初始创建 UICollectionView 时也会成功调用此方法。

我有一个与我的集合视图一起使用的自定义布局，所以问题可能出在那儿？我希望有人能发现我的错误。

下面是创建 UICollectionView、它的布局、单元格和它的标题视图的代码：

  1. 创建 UICollectionView

            - (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. 补充标题视图的数据源方法（在 CD 更新中不调用）

            - (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，似乎没有调用补充标题视图的数据源方法。但是，我确实看到在自定义布局类中为补充视图创建属性的方法被调用了。如果有人能指出发生这种情况的任何可能原因，我将不胜感激！

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1 回答 1

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reloadItemsAtIndexPaths仅用于重新加载单元格。您可以通过使用 重新加载部分来重新加载补充视图reloadSections:。如果您仍然有问题，请在问题中包含您的NSFetchedResultsControllerDelegate实施。

于 2013-10-16T17:35:38.693 回答