ios - 带有粘性 header 的 UICollectionView

Question

I found a blog on how to make sticky headers效果很好。唯一的问题是我认为它没有考虑 sectionInserts。

这是它的预期外观:

我有我的插页:

collectionViewFlowLayout.sectionInset = UIEdgeInsetsMake(16, 16, 16, 16);

使用粘性标题，它向下移动 16 个像素:

我尝试修改原始代码，我认为问题出在最后一部分:

layoutAttributes.frame = (CGRect){
    .origin = CGPointMake(origin.x, origin.y),
    .size = layoutAttributes.frame.size

如果我将它更改为 origin.y - 16，标题将从正确的位置开始，但是当向上推时，头部的 16 个像素会离开屏幕:

我不确定如何让它考虑到 sectionInsects。有人可以帮忙吗？

这是博客中的完整代码:

- (NSArray *) layoutAttributesForElementsInRect:(CGRect)rect {

    NSMutableArray *answer = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
    UICollectionView * const cv = self.collectionView;
    CGPoint const contentOffset = cv.contentOffset;

    NSMutableIndexSet *missingSections = [NSMutableIndexSet indexSet];
    for (UICollectionViewLayoutAttributes *layoutAttributes in answer) {
        if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
            [missingSections addIndex:layoutAttributes.indexPath.section];
        }
    }
    for (UICollectionViewLayoutAttributes *layoutAttributes in answer) {
        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
            [missingSections removeIndex:layoutAttributes.indexPath.section];
        }
    }

    [missingSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {

        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];

        UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];

        [answer addObject:layoutAttributes];
    }];

    for (UICollectionViewLayoutAttributes *layoutAttributes in answer) {

        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {

            NSInteger section = layoutAttributes.indexPath.section;
            NSInteger numberOfItemsInSection = [cv numberOfItemsInSection:section];

            NSIndexPath *firstCellIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
            NSIndexPath *lastCellIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];

            UICollectionViewLayoutAttributes *firstCellAttrs = [self layoutAttributesForItemAtIndexPath:firstCellIndexPath];
            UICollectionViewLayoutAttributes *lastCellAttrs = [self layoutAttributesForItemAtIndexPath:lastCellIndexPath];

            CGFloat headerHeight = CGRectGetHeight(layoutAttributes.frame);
            CGPoint origin = layoutAttributes.frame.origin;
            origin.y = MIN(
                MAX(
                    contentOffset.y,
                    (CGRectGetMinY(firstCellAttrs.frame) - headerHeight)
                ),
                (CGRectGetMaxY(lastCellAttrs.frame) - headerHeight)
            );

            layoutAttributes.zIndex = 1024;
            layoutAttributes.frame = (CGRect){
                .origin = origin,
                .size = layoutAttributes.frame.size
            };
        }
    }

    return answer;
}

Answer 1

适用于 iOS 9 + 的最简单解决方案，因为它不需要编写 UICollectionViewFlowLayout 的子类。

在带有 collectionView 的 viewController 的 viewDidLoad 中使用以下代码:

let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout // casting is required because UICollectionViewLayout doesn't offer header pin. Its feature of UICollectionViewFlowLayout
layout?.sectionHeadersPinToVisibleBounds = true