Layout that applies a rotational transform on scrolling

AnimatedLayout.swift

    override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes?
    {
        guard let collectionView = collectionView else { return nil }
        
        let attributes = OnboardingTutorialAttributes(forCellWithIndexPath: indexPath)
        
        // Bounds independant of scroll offset
        let bounds = CGRect(origin: CGPoint.zero, size: collectionView.frame.size)
        var frame : CGRect = CGRect.zero
        
        // Get the offset
        let offset = delegate?.cellPhoneOffset ?? Defaults.PhoneOffset
        
        // Size comes from the delegate
        frame.size = delegate?.cellPhoneSize ?? Defaults.PhoneSize
        
        // Position centered horizontally
        frame.origin.x = bounds.size.width * (CGFloat(indexPath.item) + 0.5) - frame.size.width/2.0 + offset.horizontal
        
        // Position centered vertically too
        frame.origin.y = (bounds.size.height - frame.size.height)/2.0 + offset.vertical + 32
        
        // Animate transformation
        var transform = CGAffineTransformIdentity
        
        if (appliesTransform)
        {
            // Normalized delta
            // = 0 when the view is fully visible
            let delta = (collectionView.bounds.origin.x - bounds.size.width * CGFloat(indexPath.item)) / frame.size.width
            
            transform = CGAffineTransformMakeRotation(-delta * CGFloat(M_PI_2) / 8.0)
        }
        
        attributes.frame     = frame
        attributes.transform = transform
        attributes.zIndex    = ZIndex.Phone.rawValue
        
        return attributes
    }