FlowLayoutWithStickyHeaders

FlowLayoutWithStickyHeaders.h

//
//  FlowLayoutWithStickyHeaders.h
//  From: http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using
//
//  Created by Tim Heuer on 7/3/14.
//

#import <UIKit/UIKit.h>

@interface FlowLayoutWithStickyHeaders : UICollectionViewFlowLayout

@end

FlowLayoutWithStickyHeaders.m

//
//  FlowLayoutWithStickyHeaders.m
//  From: http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using
//
//  Created by Tim Heuer on 7/3/14.
//

#import "FlowLayoutWithStickyHeaders.h"

@implementation FlowLayoutWithStickyHeaders

- (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 *firstCellIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
            NSIndexPath *lastCellIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];
            
            UICollectionViewLayoutAttributes *firstCellAttrs = [self layoutAttributesForItemAtIndexPath:firstCellIndexPath];
            UICollectionViewLayoutAttributes *lastCellAttrs = [self layoutAttributesForItemAtIndexPath:lastCellIndexPath];
            
            CGFloat headerHeight = CGRectGetHeight(layoutAttributes.frame);
            CGPoint origin = layoutAttributes.frame.origin;
            origin.y = MIN(
                           MAX(
                               contentOffset.y,
                               (CGRectGetMinY(firstCellAttrs.frame) - headerHeight)
                               ),
                           (CGRectGetMaxY(lastCellAttrs.frame) - headerHeight)
                           );
            
            layoutAttributes.zIndex = 1024;
            layoutAttributes.frame = (CGRect){
                .origin = origin,
                .size = layoutAttributes.frame.size
            };
            
        }
        
    }
    
    return answer;
    
}

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

@end