UICollectionView中只有一个粘性标头

Question

我正在尝试在一个中实现一个粘性标头UICollectionView.

我的粘性标题行为与您可以看到的通常情况略有不同,例如UITableView.我在集合视图中有3个标题,我希望其中只有一个是粘性的,并在滚动内容时粘在顶部.

我的代码工作得很好.但是,当我向下滚动时,粘性标题会在某个时刻突然消失.向后滚动会使标题再次出现.我究竟做错了什么？

我附加了自定义布局的实现.它是的子类UICollectionViewFlowLayout.

@implementation CustomFlowLayout


- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray *attributes = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
    CGPoint const contentOffset = self.collectionView.contentOffset;

    for (UICollectionViewLayoutAttributes *layoutAttributes in attributes)
    {
        // Adjust the sticky header frame.
        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader] &&
            layoutAttributes.indexPath.section == SectionWithStickyHeadeIndex)
        {
            NSInteger numberOfItemsInSection = [self.collectionView numberOfItemsInSection:SectionWithStickyHeadeIndex];
            NSIndexPath *firstObjectIndexPath = [NSIndexPath indexPathForItem:0
                                                                    inSection:SectionWithStickyHeadeIndex];
            UICollectionViewLayoutAttributes *firstObjectAttrs;

            if (numberOfItemsInSection > 0)
            {
                firstObjectAttrs = [self layoutAttributesForItemAtIndexPath:firstObjectIndexPath];
            }
            else
            {
                firstObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
                                                                        atIndexPath:firstObjectIndexPath];
            }

            CGPoint origin = layoutAttributes.frame.origin;

            // Adjust the header origin so it sticks to the top.
            origin.y = MAX(contentOffset.y + self.collectionView.contentInset.top,
                           CGRectGetMinY(firstObjectAttrs.frame) - CGRectGetHeight(layoutAttributes.frame));

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

            break;
        }
    }

    return attributes;
}


- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBound
{
    return YES;
}


@end

Answer 1

我对此并不是百分之百确定,但看起来就像你向下滚动得足够远,标题的原始位置不再位于rect参数内.这导致标题的布局属性不包含在循环中attributes迭代的数组中for,导致布局位置不再调整到屏幕顶部的"粘性"位置.

尝试在for循环之前添加这些行,以便将粘滞标题的布局属性添加到attributes数组中(如果它们尚不存在):

NSIndexPath *stickyHeaderIndexPath = [NSIndexPath indexPathForItem:0 inSection:SectionWithStickyHeaderIndex];
UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
                                                                                          atIndexPath:stickyHeaderIndexPath];
if (![attributes containsObject:layoutAttributes])
{
    [attributes addObject:layoutAttributes];
}