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class SingleLabelCell: UICollectionViewCell {
    static let identifier: String = "singleLabelCell"
    
    let theLabel: UILabel = {
        let v = UILabel()
        v.font = .systemFont(ofSize: 14.0, weight: .light)
        v.textAlignment = .center
        v.numberOfLines = 0
        v.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
        v.layer.cornerRadius = 12
        v.clipsToBounds = true
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() -> Void {
        contentView.addSubview(theLabel)
        let g = contentView
        NSLayoutConstraint.activate([
            theLabel.topAnchor.constraint(equalTo: g.topAnchor, constant: 4.0),
            theLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 4.0),
            theLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -4.0),
            theLabel.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -4.0),
        ])
        
        // during development, so we can see the cell framing
        contentView.backgroundColor = .systemYellow
    }
}
 
class SampleViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
    
    var collectionCarousel: UICollectionView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBackground
        
        // horizontal flow layout
        let fl = UICollectionViewFlowLayout()
        fl.scrollDirection = .horizontal
        fl.minimumLineSpacing = 8
        fl.minimumInteritemSpacing = 8
        fl.itemSize = .init(width: 80.0, height: 40.0)
        collectionCarousel = UICollectionView(frame: .zero, collectionViewLayout: fl)
        
        collectionCarousel.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(collectionCarousel)
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            // constrain collectionView to Top/Leading/Trailing of safe-area
            collectionCarousel.topAnchor.constraint(equalTo: g.topAnchor, constant: 0.0),
            collectionCarousel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 0.0),
            collectionCarousel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: 0.0),
            collectionCarousel.heightAnchor.constraint(equalToConstant: 50.0),
        ])
        
        collectionCarousel.register(SingleLabelCell.self, forCellWithReuseIdentifier: SingleLabelCell.identifier)
        collectionCarousel.dataSource = self
        collectionCarousel.delegate = self
        
        // so we can see the collection view framing
        collectionCarousel.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
        
    }
    
    var bFirstLayout: Bool = true
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        // if this is the first layout of this controller,
        //  center section 8 horizontally (but not animated)
        if bFirstLayout {
            bFirstLayout = false
            selectItemAndScroll(8, animated: false)
        }
    }
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 11
    }
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 1
    }
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let c = collectionView.dequeueReusableCell(withReuseIdentifier: SingleLabelCell.identifier, for: indexPath) as! SingleLabelCell
        c.theLabel.text = "\(indexPath.section)"
        return c
    }
    
    func selectItemAndScroll(_ setIndex: Int = 0, animated: Bool ) {
        let startIndexPath = IndexPath(item: 0, section: setIndex)
        collectionCarousel.selectItem(at: startIndexPath,
                                      animated: animated,
                                      scrollPosition: .centeredHorizontally)
    }
}