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)
	}
}