Scrolling item on a horizontal UICollectionView, stopped item on center of screen.

CenterItemInCollectionView.playground

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

class WLCollectionCell: UICollectionViewCell {
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }
    
    func setup() {
        self.layer.borderWidth  = 1.0
        self.layer.borderColor  = UIColor.red.cgColor
        self.layer.cornerRadius = 5.0
        self.backgroundColor    = UIColor.white
    }
    
}

class WLCollectionViewLayout: UICollectionViewFlowLayout {
    
    var previousOffset: CGFloat    = 0
    var currentPage: Int           = 0
    
    override func prepare() {
    }
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

        guard let collectionView = self.collectionView else {
            return CGPoint.zero
        }
        
        guard let itemsCount = collectionView.dataSource?.collectionView(collectionView, numberOfItemsInSection: 0) else {
            return CGPoint.zero
        }
        
        if ((previousOffset > collectionView.contentOffset.x) && (velocity.x < 0)) {
            currentPage = max(currentPage - 1, 0)
        } else if ((previousOffset < collectionView.contentOffset.x) && (velocity.x > 0.0)) {
            currentPage = min(currentPage + 1, itemsCount - 1);
        }

        let itemEdgeOffset:CGFloat = (collectionView.frame.width - itemSize.width -  minimumLineSpacing * 2) / 2
        let updatedOffset: CGFloat = (itemSize.width + minimumLineSpacing) * CGFloat(currentPage) - (itemEdgeOffset + minimumLineSpacing);
        
        previousOffset = updatedOffset;
        
        return CGPoint(x: updatedOffset, y: proposedContentOffset.y);
    }
}

class RootVC: UIViewController, UICollectionViewDataSource {
    
    var collectionView: UICollectionView?
    var collectionLayout: WLCollectionViewLayout = WLCollectionViewLayout()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        /*
         *
            |<-------     view width     -------->|
            ---------------------------------------
            |                                     |
            |<-w0->|<ws>|<--   wi  -->|<ws>|<-w0->|
     ---------------    ---------------    ---------------
     |             |    |             |    |             |
     |             |    |             |    |             |
     ---------------    ---------------    ---------------
            |                                     |
            |                                     |
            ---------------------------------------
         * 
         * w0: itemEdgeOffset
         * ws: space
         * wi: itemWidth
         * in this example: ws == w0
         */
 
        let space: CGFloat                  = 30
        
        self.view                           = UIView(frame: CGRect(x: 0, y: 0, width: 400, height: 600))
        self.view.backgroundColor           = UIColor.blue
        
        let collectionFrame                 = CGRect(x: 0, y: 60, width: view.frame.width, height: 200)
        
        self.collectionView                 = UICollectionView(frame: collectionFrame, collectionViewLayout: collectionLayout)
        collectionView?.register(WLCollectionCell.self, forCellWithReuseIdentifier: "CELL")
        collectionView?.backgroundColor     = self.view.backgroundColor
        
        collectionView?.dataSource          = self
        collectionView?.isPagingEnabled     = false
        collectionView?.contentInset        = UIEdgeInsets(top: 0, left: space, bottom: 0, right: space)
        
        collectionLayout.scrollDirection    = .horizontal
        
        let itemWidth                       = view.frame.width - space * 4 // w0 == ws
        collectionLayout.itemSize           = CGSize(width: itemWidth, height: 180)
        collectionLayout.minimumLineSpacing = space
        
        
        self.view.addSubview(self.collectionView!)
    }
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 4
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        if let cell: WLCollectionCell = collectionView.dequeueReusableCell(withReuseIdentifier: "CELL", for: indexPath) as? WLCollectionCell {
            
            return cell
        }
        
        return UICollectionViewCell()
    }
}

let vc = RootVC()
PlaygroundPage.current.liveView = vc.view