Center the cell of a collectionView everytime the scrollview ends

Question

I have a collectionView and I have an extension for its scrollview that helps me to get my cell 20pixels(the minimum space from the next cell) right of the screen like this:

now I want to change my extension to center it to the container like this:

my extension is this: (if I can do with any other way feel free to tell it)

extension MyProject : UIScrollViewDelegate
{
    func scrollViewWillEndDragging(scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>)
    {
        let layout = self.collectionView?.collectionViewLayout as! UICollectionViewFlowLayout
        let cellWidthIncludingSpacing = layout.itemSize.width + layout.minimumLineSpacing

        var offset = targetContentOffset.memory
        let index = (offset.x + scrollView.contentInset.left) / cellWidthIncludingSpacing
        let roundedIndex = round(index)

        offset = CGPoint(x: roundedIndex * cellWidthIncludingSpacing - scrollView.contentInset.left, y: -scrollView.contentInset.top)
        targetContentOffset.memory = offset
    }
}

Answer 1

I'd say

offset = CGPoint(
  x: roundedIndex * cellWidthIncludingSpacing - scrollView.contentInset.left + cellWithIncludingSpacing / 2 - scrollView.bounds.size.width / 2,
  y: -scrollView.contentInset.top
)

or, to consider contentInset better

let visibleWidth = scrollView.bounds.size.width
  - scrollView.contentInset.left
  - scrollView.contentInset.right
offset = CGPoint(
  x: roundedIndex * cellWidthIncludingSpacing
    - scrollView.contentInset.left
    + layout.itemSize.width / 2
    - visibleWidth / 2,
  y: -scrollView.contentInset.top
)