Collectionview paging by cell Swift

Question

I'm trying implement pagination by cell and not by screen in Swift. I found the solution here : targetContentOffsetForProposedContentOffset:withScrollingVelocity without subclassing UICollectionViewFlowLayout

Here is my understanding and conversion to Swift.

My subclass:

class ChannelFlowLayout: UICollectionViewFlowLayout {

func pageWidth () -> CGFloat {
    return itemSize.width + minimumLineSpacing
}

func flickVelocity () -> CGFloat {
    return 0.3
}

override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

    var rawPageValue: CGFloat = collectionView!.contentOffset.x / pageWidth()

    var currentPage: CGFloat = (velocity.x > 0.0) ? floor(rawPageValue) : ceil(rawPageValue)
    var nextPage: CGFloat = (velocity.x > 0.0) ? ceil(rawPageValue) : floor(rawPageValue)

    var pannedLessThanAPage : Bool = fabs(1 + currentPage - rawPageValue) > 0.5
    var flicked : Bool = fabs(velocity.x) > flickVelocity()

    var targetContentOffset: CGPoint
    if pannedLessThanAPage && flicked {
        targetContentOffset = CGPoint(x: nextPage * pageWidth(), y: proposedContentOffset.y)
    } else {
        targetContentOffset = CGPoint(x: round(rawPageValue) * pageWidth(), y: proposedContentOffset.y)
    }
    return proposedContentOffset
}   
}

In my viewController:

override func viewDidLoad() {
    super.viewDidLoad()

    collectionView.dataSource = self
    collectionView.delegate = self
    registerNibs()

    var layout = ChannelFlowLayout()
    collectionView.pagingEnabled = false
    layout.scrollDirection = UICollectionViewScrollDirection.Horizontal
    collectionView.collectionViewLayout = layout
}

What am I missing? Thanks so much!