Sticky Headers at the top of a UICollectionView! -- // -- http://stackoverflow.com/questions/13511733/how-to-make-supplementary-view-float-in-uicollectionview-as-section-headers-do-i -- // -- still needs work around contentInsets.bottom and oddly-sized footers.

CustomCollectionFlowLayout.h

//
//  CustomCollectionFlowLayout.h
//  evilapples
//
//  http://stackoverflow.com/questions/13511733/how-to-make-supplementary-view-float-in-uicollectionview-as-section-headers-do-i
//
//

#import <UIKit/UIKit.h>

@interface CustomCollectionFlowLayout : UICollectionViewFlowLayout

@end

CustomCollectionFlowLayout.m

//
//  CustomCollectionFlowLayout.m
//  evilapples
//
//  http://stackoverflow.com/questions/13511733/how-to-make-supplementary-view-float-in-uicollectionview-as-section-headers-do-i
//
//

#import "CustomCollectionFlowLayout.h"

@implementation CustomCollectionFlowLayout

- (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 *firstObjectIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
            NSIndexPath *lastObjectIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];
            
            BOOL cellsExist;
            UICollectionViewLayoutAttributes *firstObjectAttrs;
            UICollectionViewLayoutAttributes *lastObjectAttrs;
            
            if (numberOfItemsInSection > 0) { // use cell data if items exist
                cellsExist = YES;
                firstObjectAttrs = [self layoutAttributesForItemAtIndexPath:firstObjectIndexPath];
                lastObjectAttrs = [self layoutAttributesForItemAtIndexPath:lastObjectIndexPath];
            } else { // else use the header and footer
                cellsExist = NO;
                firstObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
                                                                        atIndexPath:firstObjectIndexPath];
                lastObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionFooter
                                                                       atIndexPath:lastObjectIndexPath];
                
            }
            
            CGFloat topHeaderHeight = (cellsExist) ? CGRectGetHeight(layoutAttributes.frame) : 0;
            CGFloat bottomHeaderHeight = CGRectGetHeight(layoutAttributes.frame);
            CGRect frameWithEdgeInsets = UIEdgeInsetsInsetRect(layoutAttributes.frame,
                                                               cv.contentInset);
            
            CGPoint origin = frameWithEdgeInsets.origin;
            
            // we don't want header be positioned out of CV bounds, we we check if lastObjectAttrs exists, else section header origin will be negative
            if (lastObjectAttrs) {
                origin.y = MIN(
                               MAX(
                                   contentOffset.y + cv.contentInset.top,
                                   (CGRectGetMinY(firstObjectAttrs.frame) - topHeaderHeight)
                                   ),
                               (CGRectGetMaxY(lastObjectAttrs.frame) - bottomHeaderHeight)
                               );
            }
            
            
            layoutAttributes.zIndex = 1024;
            layoutAttributes.frame = (CGRect){
                .origin = origin,
                .size = layoutAttributes.frame.size
            };
            
        }
        
    }
    
    return answer;
    
}

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

@end