UICollectionFlowLayout - Cell Spacing in a UICollectionView Flow Layout.

GiftCardFlowLayout.m

//
//  GiftCardFlowLayout.m
//  ID.me Marketplace
//
//  Created by Arthur Sabintsev on 9/7/14.
//  Copyright (c) 2014 ID.me, Inc. All rights reserved.
//

#import "GiftCardFlowLayout.h"

@interface  GiftCardFlowLayout ()

@property (nonatomic, assign) CGFloat maximumSpacing;

@end

@implementation GiftCardFlowLayout

- (instancetype)init
{
    self = [super init];
    if (self) {
        _maximumSpacing = 100.0f/2.0f;
    }
    
    return self;
}

// http://stackoverflow.com/a/19429189/814861
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray *layoutAttributes = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
    NSUInteger attirbuteCount = [layoutAttributes count];

    for (NSUInteger i = 1; i < attirbuteCount; ++i) {
    
        UICollectionViewLayoutAttributes *currentLayoutAttributes = layoutAttributes[i];
        UICollectionViewLayoutAttributes *previousLayoutAttributes = layoutAttributes[i - 1];
        NSInteger origin = CGRectGetMaxX(previousLayoutAttributes.frame);
        
        CGFloat sum = origin + _maximumSpacing + CGRectGetWidth([currentLayoutAttributes frame]);
        if (sum < self.collectionViewContentSize.width ) {
            CGRect frame = [currentLayoutAttributes frame];
            frame.origin.x = origin + _maximumSpacing;
            currentLayoutAttributes.frame = frame;
        }
    }
    
    return layoutAttributes;
}

- (CGSize)collectionViewContentSize
{
    NSUInteger count = [self.collectionView.dataSource collectionView:self.collectionView numberOfItemsInSection:0];
    UICollectionViewFlowLayout *layout = (UICollectionViewFlowLayout *)[self.collectionView collectionViewLayout];
    
    CGFloat sizeFromItems = count * (layout.itemSize.width + _maximumSpacing);
    CGFloat missingRightInsetSize = layout.sectionInset.right/2.0f;
    CGFloat newContentSizeWidth = sizeFromItems + missingRightInsetSize;
    
    return CGSizeMake(newContentSizeWidth, self.collectionView.frame.size.height);
}

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

@end