Paged UICollectionViewFlowLayout with snapping and centering for arbitrary items

UICollectionViewFlowLayout arbitrary paging

- (CGSize)collectionViewContentSize
{
    CGSize size = [super collectionViewContentSize];
    CGSize cvSize = self.collectionView.frame.size;
    CGSize newSize = CGSizeMake(size.width, size.height + ((cvSize.height/2) - (self.itemSize.height/2)));
    return newSize;
}

// Vertical scrolling
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    CGFloat offSetAdjustment = MAXFLOAT;
    CGFloat verticalCenter = (CGFloat)(proposedContentOffset.y + (self.collectionView.bounds.size.height / 2.0));
    
    CGRect targetRect = CGRectMake(0.0, proposedContentOffset.y, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
    
    NSArray *array = [self layoutAttributesForElementsInRect:targetRect];
    
    UICollectionViewLayoutAttributes *currentAttributes;
    
    for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
        if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
            CGFloat itemVerticalCenter = layoutAttributes.center.y;
            if (ABS(itemVerticalCenter - verticalCenter) < ABS(offSetAdjustment)) {
                currentAttributes = layoutAttributes;
                offSetAdjustment = itemVerticalCenter - verticalCenter;
            }
        }
    }
    
    CGFloat nextOffset  = proposedContentOffset.y + offSetAdjustment;
    proposedContentOffset.y = nextOffset;
    CGFloat deltaY = proposedContentOffset.y - self.collectionView.contentOffset.y;
    CGFloat velY = velocity.y;
    
    // detection form  gist.github.com/rkeniger/7687301
    // based on http://stackoverflow.com/a/14291208/740949
    if (deltaY == 0.0 || velY == 0 || (velY > 0.0 && deltaY > 0.0) || (velY < 0.0 && deltaY < 0.0)) {
        
    } else if (velocity.y > 0.0) {
        for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
            if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell)  {
                CGFloat itemVerticalCenter = layoutAttributes.center.y;
                if (itemVerticalCenter > proposedContentOffset.y) {
                    proposedContentOffset.y = nextOffset + (currentAttributes.frame.size.height / 2) + (layoutAttributes.frame.size.height / 2);
                    break;
                }
            }
        }
    } else if (velocity.y< 0.0) {
        for (UICollectionViewLayoutAttributes *layoutAttributes in array)  {
            if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
                CGFloat itemVerticalCenter = layoutAttributes.center.y;
                if (itemVerticalCenter > proposedContentOffset.y) {
                    proposedContentOffset.y = nextOffset - ((currentAttributes.frame.size.height / 2) + (layoutAttributes.frame.size.height / 2));
                    break;
                }
            }
        }
    }
    
    proposedContentOffset.x = 0.0;
    
    return proposedContentOffset;
}

// Horizontal scrolling
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    CGFloat offSetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = (CGFloat)(proposedContentOffset.x + (self.collectionView.bounds.size.width / 2.0));

    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

    NSArray *array = [self layoutAttributesForElementsInRect:targetRect];

    UICollectionViewLayoutAttributes *currentAttributes;

    for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
        if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
            CGFloat itemHorizontalCenter = layoutAttributes.center.x;
            if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offSetAdjustment)) {
                currentAttributes = layoutAttributes;
                offSetAdjustment = itemHorizontalCenter - horizontalCenter;
            }
        }
    }

    CGFloat nextOffset = proposedContentOffset.x + offSetAdjustment;

    proposedContentOffset.x = nextOffset;
    CGFloat deltaX = proposedContentOffset.x - self.collectionView.contentOffset.x;
    CGFloat velX = velocity.x;

    // detection form  gist.github.com/rkeniger/7687301
    // based on http://stackoverflow.com/a/14291208/740949
    if (deltaX == 0.0 || velX == 0 || (velX > 0.0 && deltaX > 0.0) || (velX < 0.0 && deltaX < 0.0)) {

    } else if (velocity.x > 0.0) {
        for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
            if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
                CGFloat itemHorizontalCenter = layoutAttributes.center.x;
                if (itemHorizontalCenter > proposedContentOffset.x) {
                    proposedContentOffset.x = nextOffset + (currentAttributes.frame.size.width / 2) + (layoutAttributes.frame.size.width / 2);
                    break;
                }
            }
        }
    } else if (velocity.x < 0.0) {
        for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
            if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
                CGFloat itemHorizontalCenter = layoutAttributes.center.x;
                if (itemHorizontalCenter > proposedContentOffset.x) {
                    proposedContentOffset.x = nextOffset - ((currentAttributes.frame.size.width / 2) + (layoutAttributes.frame.size.width / 2));
                    break;
                }
            }
        }
    }

    proposedContentOffset.y = 0.0;

    return proposedContentOffset;
}}