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！
答案 0 :(得分：0)
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。