Sticky Headers for UICollectionView using UICollectionViewFlowLayout that acts like Photo.app collection, taking care of edgeInsets

Springboard.h

//
//  SpringboardLayout.h
//
//  Created by valv0 on 11/10/14.
//  Copyright (c) 2014 sofapps. All rights reserved.
//


#import <UIKit/UIKit.h>

@interface SpringboardLayout : UICollectionViewFlowLayout

@end

Springboard.m

//
//  SpringboardLayout.m
//
//  Created by valv0 on 11/10/14.
//  Copyright (c) 2014 sofapps. All rights reserved.
//

#import "SpringboardLayout.h"

@implementation SpringboardLayout

- (id)init
{
    if (self = [super init])
    {
        self.headerReferenceSize = CGSizeMake(0, 50);
        self.footerReferenceSize = CGSizeMake(0, 0);
        self.sectionInset = UIEdgeInsetsMake(10, 10, 80, 10);
        self.scrollDirection = UICollectionViewScrollDirectionVertical;
        self.minimumInteritemSpacing = 10;
        self.minimumLineSpacing = 10;
        if(IS_IPHONE_6 || IS_IPHONE_6PLUS) {
            self.itemSize = CGSizeMake(100, 128);
        } else {
            self.itemSize = CGSizeMake(80, 108);
        }
    }
    return self;
}

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

- (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];
        } else 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;
           
            
            origin.y = MIN(MAX(contentOffset.y + cv.contentInset.top,
                               (CGRectGetMinY(firstObjectAttrs.frame) - topHeaderHeight - self.sectionInset.top))
                           ,(CGRectGetMaxY(lastObjectAttrs.frame) - bottomHeaderHeight + self.sectionInset.bottom));
            layoutAttributes.zIndex = 1024;
            layoutAttributes.frame = (CGRect){
                .origin = origin,
                .size = layoutAttributes.frame.size
            };
        }
    }
    return answer;
}

@end