CollectionCellCustomLayoutSample.m

@interface MFLMHorizontalFlowLayout()
@property (nonatomic, strong) NSDictionary *layoutInfo;

@end

@implementation MFLMHorizontalFlowLayout

- (void)prepareLayout {
    self.itemSize = CGSizeMake((ScreenRect.size.width - 45) / 3, (ScreenRect.size.width - 45) / 3);
    
    NSMutableDictionary *cellLayoutInfo = [NSMutableDictionary dictionary];
    
    NSInteger sectionCount = [self.collectionView numberOfSections];
    for (int section = 0; section < sectionCount; section++) {
        for (int item = 0; item < [self.collectionView numberOfItemsInSection:section]; item++) {
            NSIndexPath *indexPath = [NSIndexPath indexPathForItem:item inSection:section];
            UICollectionViewLayoutAttributes *itemAttributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
            itemAttributes.frame = [self frameForAlbumPhotoAtIndexPath:indexPath];
            cellLayoutInfo[indexPath] = itemAttributes;
        }
    }
    
    self.layoutInfo = cellLayoutInfo;
}

- (CGSize)collectionViewContentSize {
    CGSize boundsSize = self.collectionView.bounds.size;
    NSInteger numberOfPages = [self.collectionView numberOfSections];
    
    CGSize size = CGSizeMake(boundsSize.width * numberOfPages, boundsSize.height);
    
    return size;
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSMutableArray *allAttributes = [NSMutableArray new];
    
    [self.layoutInfo enumerateKeysAndObjectsUsingBlock:^(NSIndexPath *indexPath, UICollectionViewLayoutAttributes *attributes, BOOL *innerStop) {
        if (CGRectIntersectsRect(rect, attributes.frame)) {
            [allAttributes addObject:attributes];
        }
    }];
    
    return allAttributes;
}

- (UICollectionViewLayoutAttributes*)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    return self.layoutInfo[indexPath];
}

- (CGRect)frameForAlbumPhotoAtIndexPath:(NSIndexPath *)indexPath {
    CGSize itemSize = self.itemSize;
    
    if (itemSize.width == 0 || itemSize.height == 0) {
        return CGRectZero;
    }
    
    NSInteger item = indexPath.item;
    
    CGRect bounds = self.collectionView.bounds;
    
    NSInteger rowPosition = (item / 3) % 3;
    NSInteger columnPosition = item % 3;
    CGRect frame = CGRectZero;
    frame.origin.x = indexPath.section * bounds.size.width + 15 + columnPosition * itemSize.width +  (15 / 2) * columnPosition;
    frame.origin.y = rowPosition * itemSize.height + 15 + (15 / 2) * rowPosition;
    frame.size = self.itemSize;

    return frame;
}

@end