StickyHeaderFlowLayout.h

//
//  StickyHeaderLayout.h
//  Wombat
//
//  Created by Todd Laney on 1/9/13.
//  Copyright (c) 2013 ToddLa. All rights reserved.
//
//  Modified from http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using THANKS!
//

#import <UIKit/UIKit.h>

@interface StickyHeaderLayout : UICollectionViewFlowLayout

@end

StickyHeaderFlowLayout.m

//
//  StickyHeaderFlowLayout.m
//  Wombat
//
//  Created by Todd Laney on 1/9/13.
//  Copyright (c) 2013 ToddLa. All rights reserved.
//
//  Modified from http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using THANKS!
//

#import "StickyHeaderFlowLayout.h"

@implementation StickyHeaderFlowLayout

- (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];
        [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;
        UIEdgeInsets const contentEdgeInsets = cv.contentInset;        
        CGPoint const contentOffset = CGPointMake(cv.contentOffset.x, cv.contentOffset.y + contentEdgeInsets.top
                                                  );

        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;
}

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

@end