UICollectionView Section Header Stick to a certain Y

Question

I am trying to replicate a scenario where I have a collection view all the way to the top, a title view on top of the first row of the collection view, then a header section view (for indexPath.section == 1) that sticks to the title view without overlapping or going underneath it.

I've implemented a custom layout, to make the header sticky. But I can't make it stick to the title view (so after its' origin.y+height), it sticks to the top of the collection view.

Here's my IB structure:

with the custom layout linked up :

Here's the ideal scenario:

Before scrolling :

After scrolling :

Here's my code :

    @implementation LLCollectionCustomFlowLayout

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

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

    NSMutableArray *answer = [[super layoutAttributesForElementsInRect:rect] mutableCopy];

    NSMutableIndexSet *missingSections = [NSMutableIndexSet indexSet];
    for (NSUInteger idx=0; idx<[answer count]; idx++) {
        UICollectionViewLayoutAttributes *layoutAttributes = answer[idx];

        if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
            [missingSections addIndex:layoutAttributes.indexPath.section];  // remember that we need to layout header for this section
        }
        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
            [answer removeObjectAtIndex:idx];  // remove layout of header done by our super, we will do it right later
            idx--;
        }
    }

    // layout all headers needed for the rect using self code
    [missingSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];
        UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];
        if (layoutAttributes != nil) {
            [answer addObject:layoutAttributes];
        }
    }];

    return answer;
}


- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *attributes = [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        UICollectionView * const cv = self.collectionView;
        CGPoint const contentOffset = cv.contentOffset;
        CGPoint nextHeaderOrigin = CGPointMake(INFINITY, INFINITY);

        if (indexPath.section+1 < [cv numberOfSections]) {
            UICollectionViewLayoutAttributes *nextHeaderAttributes = [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:[NSIndexPath indexPathForItem:0 inSection:indexPath.section+1]];
            nextHeaderOrigin = nextHeaderAttributes.frame.origin;
        }

        CGRect frame = attributes.frame;
        if (self.scrollDirection == UICollectionViewScrollDirectionVertical) {

            frame.origin.y = MIN(MAX(contentOffset.y, frame.origin.y), nextHeaderOrigin.y - CGRectGetHeight(frame));


        }
        else { // UICollectionViewScrollDirectionHorizontal
            frame.origin.x = MIN(MAX(contentOffset.x, frame.origin.x), nextHeaderOrigin.x - CGRectGetWidth(frame));
        }
        attributes.zIndex = 1024;
        attributes.frame = frame;
    }
    return attributes;
}

- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    return attributes;
}
- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *attributes = [self layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    return attributes;
}

@end

Answer 1

The issue is here:

frame.origin.y = MIN(MAX(contentOffset.y, frame.origin.y), nextHeaderOrigin.y - CGRectGetHeight(frame));

When the header is the last one, nextHeaderOrigin = CGPointMake(INFINITY, INFINITY). That is why frame.origin.y will be MAX(contentOffset.y, frame.origin.y) and stick to the top of the collection view. You should update layoutAttributesForSupplementaryViewOfKind in the next way:

- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *attributes = [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        // for the header in the section #0, we have "sticky" behaviour by default
        if (indexPath.section == 0) {
            return attributes; 
        }
        CGPoint contentOffset = self.collectionView.contentOffset;
        UICollectionViewLayoutAttributes *prevHeaderAttributes = [self layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:[NSIndexPath indexPathForItem:0 inSection:indexPath.section - 1]];
        CGFloat prevHeaderMaxY = CGRectGetMaxY(prevHeaderAttributes.frame); // The maximum Y value from the previous header should be the position we use to "stick" the current one
        CGFloat prevHeaderMaxX = CGRectGetMaxX(prevHeaderAttributes.frame);
        CGRect frame = attributes.frame;
        if (self.scrollDirection == UICollectionViewScrollDirectionVertical) {
            frame.origin.y = MAX(frame.origin.y, prevHeaderMaxY);
        }
        else { // UICollectionViewScrollDirectionHorizontal
            frame.origin.x = MAX(frame.origin.x, prevHeaderMaxX);
        }
        attributes.frame = frame;
    }
    return attributes;
}