UICollectionViewCell change cell size while scrolling

I'm trying to create a UICollectionView where the UICollectionViewCell is getting scaled down when "leaving" the visible area at the top or bottom. And getting scaled up to normal size while "entering" the visible area.

I've been trying some scale/animation code in: scrollViewDidScroll() , but I can't seem to get it right.

My complete function looks like this:

 func scrollViewDidScroll(scrollView: UIScrollView) {

    var arr = colView.indexPathsForVisibleItems()

    for indexPath in arr{

        var cell = colView.cellForItemAtIndexPath(indexPath as! NSIndexPath)!
        var pos = colView.convertRect(cell.frame, toView: self.view)

        if pos.origin.y < 50 && pos.origin.y >= 0{

            cell.hidden = false

            UIView.animateWithDuration(0.5, animations: { () -> Void in

                cell.transform = CGAffineTransformMakeScale(0.02 * pos.origin.y, 0.02 * pos.origin.y)

            })

        }else if pos.origin.y == 50{

            UIView.animateWithDuration(0.5, animations: { () -> Void in

                cell.transform = CGAffineTransformMakeScale(1, 1)
            })

        }
    }
}

Is this in some way the right approach, or is there another better way?