Question

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

经过大量测试后，我确定上述过程正常。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，我也实施了数据源方法'viewForSupplementaryElementOfKind'，显示一些单元格标题（也基于对核心数据的更改），但这不能正常工作。

出于某种原因，似乎当核心数据发生变化后调用'reloadItemsAtIndexPaths'时，'viewForSupplementaryElementOfKind'没有被调用，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，然后调用'viewForSupplementaryElementOfKind'，我能够根据核心数据的变化看到补充标题视图中的更新。在初始创建UICollectionView时，也会成功调用此方法。

我有一个自定义布局，我正在使用我的集合视图，所以问题可能就在那里？我希望有人能发现我的错误。

以下是用于创建UICollectionView，其布局，单元格及其标题视图的代码：

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"];
        }

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;
    }

单元格的数据源方法（在CD更新时调用）

        - (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;
        }

补充标题视图的数据源方法（未在CD更新时调用）

        - (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，我确实看到了为自定义布局类IS中的补充视图创建属性的方法。如果有人能指出任何可能的原因，那可能是发生了，我很感激！

Answer 1

reloadItemsAtIndexPaths仅用于重新加载单元格。您可以使用reloadSections:重新加载该部分来重新加载补充视图。如果您仍然遇到问题，请在问题的NSFetchedResultsControllerDelegate实施中加入。

未在CoreData Change上调用UICollectionView DataSource方法

1 个答案: