实现UICollectionView的section header悬停效果.效果和UITableView的plain模式下的header section效果相同,支持指定某一个或者多个sction header的悬停
关键在于设置下面的这个属性
@property (nonatomic,strong) NSArray<NSNumber *> *stickySections;
将我们的UICollectionView的布局替换为YLStickyHeaderFlowLayout即可
- (NSArray *) layoutAttributesForElementsInRect:(CGRect)rect {
if (self.disableStickyFlow) {
return [super layoutAttributesForElementsInRect:rect];
}
NSMutableArray<UICollectionViewLayoutAttributes *> *allItems ;
//collectionView中的item(包括cell和header、footer这些)的《结构信息》.关键!!!!cell,header,footer都是利用这个数组的,在这个中,原来创建的section等,要按顺序存放到数组中来!
NSArray *originalAttributes = [super layoutAttributesForElementsInRect:rect];
//allItems = (NSMutableArray *)originalAttributes ;
allItems = [originalAttributes mutableCopy];//实际上layoutAttributesForElementsInRect返回的是NSMutableArray,所以,可以直接强转
NSMutableDictionary *headers = [[NSMutableDictionary alloc] init];//存放每个section的header
NSMutableDictionary<NSNumber *,UICollectionViewLayoutAttributes *> *lastCells = [[NSMutableDictionary alloc] init];//存放的是每个section的最后一个cell
[allItems enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
NSIndexPath *indexPath = [obj indexPath];
BOOL isHeader = [[obj representedElementKind] isEqualToString:UICollectionElementKindSectionHeader];
BOOL isFooter = [[obj representedElementKind] isEqualToString:UICollectionElementKindSectionFooter];
if (isHeader) {
headers[@(indexPath.section)] = obj;
} else if (isFooter) {
// 不处理
} else {
//其实用这两句也是可以的
//NSNumber *sectionObj = @(indexPath.section);
//lastCells[sectionObj] = obj;
UICollectionViewLayoutAttributes *currentAttribute = lastCells[@(indexPath.section)];
// 确保取到的是section中最后一个cell
if ( ! currentAttribute || indexPath.row > currentAttribute.indexPath.row) {
[lastCells setObject:obj forKey:@(indexPath.section)];
}
}
//如果按照正常情况下,header离开屏幕被系统回收,而header的层次关系又与cell相等,如果不去理会,会出现cell在header上面的情况
//通过打印可以知道cell的层次关系zIndex数值为0,我们可以将header的zIndex设置成1,如果不放心,也可以将它设置成非常大,这里随便填了个1024
if (isHeader) {
obj.zIndex = 1024;
} else {
// For iOS 7.0, the cell zIndex should be above sticky section header
obj.zIndex = 1;
}
}];
[lastCells enumerateKeysAndObjectsUsingBlock:^(NSNumber * _Nonnull key, UICollectionViewLayoutAttributes * _Nonnull obj, BOOL * _Nonnull stop) {
NSIndexPath *indexPath = obj.indexPath;
NSNumber *indexPathKey = @(indexPath.section);
UICollectionViewLayoutAttributes *header = headers[indexPathKey];
if ( ! header) {
// CollectionView自动将不再bounds内的headers移除了,所以,Headers可能为nil.这种情况下我们需要重新将其加回来 automatically removes headers not in bounds
header = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
atIndexPath:[NSIndexPath indexPathForItem:0 inSection:indexPath.section]];
if (!CGSizeEqualToSize(CGSizeZero, header.frame.size)) {
[allItems addObject:header];
}
}
if (!CGSizeEqualToSize(CGSizeZero, header.frame.size)) {
[self updateHeaderAttributes:header lastCellAttributes:lastCells[indexPathKey]];
}
}];
return allItems;
}