Updated Booking / Calendar cell

class BookingController: UIViewController, UITableViewDelegate, UITableViewDataSource {

    // NEVER do something like this!!!
    //  create a UITableViewCell subclass
    //let calendarCell = UITableViewCell()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .gray // Manager.Colors.dark

        if let img = UIImage(named: "Background Image") {
            addBackgroundImage(image: img)
        }
        setupTableContent()
    }

    private func addBackgroundImage(image: UIImage) {
        let backgroundImage = UIImageView(image: image)
        backgroundImage.frame = view.bounds
        backgroundImage.alpha = 0.5
        view.addSubview(backgroundImage)
    }

    override var preferredStatusBarStyle: UIStatusBarStyle { .lightContent }

    private func setupTableContent() {
        let tableView = UITableView()
        tableView.backgroundColor = .clear
        tableView.delegate = self
        tableView.dataSource = self
        tableView.translatesAutoresizingMaskIntoConstraints =  false
        tableView.estimatedRowHeight = 80
        tableView.rowHeight = UITableView.automaticDimension
        view.addSubview(tableView)

        // register CalendarCell class
        tableView.register(CalendarCell.self, forCellReuseIdentifier: "calendarCell")

        [tableView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
         tableView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
         tableView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
         tableView.bottomAnchor.constraint(equalTo: view.bottomAnchor)].forEach({$0.isActive = true})
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int { 1 }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        // dequeue a CalendarCell and set its Data
        let cell = tableView.dequeueReusableCell(withIdentifier: "calendarCell", for: indexPath) as! CalendarCell
        cell.initData()
        return cell
    }

    func tableView(_ tableView: UITableView, viewForFooterInSection section: Int) -> UIView? { UIView() }
}

// custom table view cell
class CalendarCell: UITableViewCell {

    let calendarView = CalendarView()

    // we will set the .constant on this constraint to set the height of the
    //  calendarView (and thus the height of the collection view)
    var calendarViewHeightConstraint: NSLayoutConstraint!

    override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
        super.init(style: style, reuseIdentifier: reuseIdentifier)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() -> Void {

        calendarView.translatesAutoresizingMaskIntoConstraints = false
        contentView.addSubview(calendarView)

        calendarView.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
        calendarView.leadingAnchor.constraint(equalTo: contentView.leadingAnchor).isActive = true
        calendarView.trailingAnchor.constraint(equalTo: contentView.trailingAnchor).isActive = true

        // bottom anchor needs to have Priority less than required to avoid auto-layout complaints
        let c = calendarView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor)
        c.priority = .defaultHigh
        c.isActive = true

        // initial height constant doesn't matter... it will be changed when we set the data
        calendarViewHeightConstraint = calendarView.heightAnchor.constraint(equalToConstant: 10.0)
        calendarViewHeightConstraint.isActive = true

    }

    var m: Int = 0 // tiene traccia del mese
    var y: Int = 0 // tiene traccia dell'anno
    var totalCellsNeeded: Int = 0 // number of cell

    func initData() {
        let calendar = Calendar.current

        m = Calendar.current.component(.month, from: Date())
        y = Calendar.current.component(.year, from: Date())

        let dateComponents = DateComponents(year: y, month: m)

        guard let startDate = calendar.date(from: dateComponents) else { fatalError("Something is wrong with the date!") }
        guard let range = calendar.range(of: .day, in: .month, for: startDate) else { fatalError("Something is wrong with the date!") }

        let numberOfDaysInMonth = range.count
        let startDayOfWeek = Calendar.current.component(.weekday, from: startDate)

        totalCellsNeeded = numberOfDaysInMonth + (startDayOfWeek - 1)

        let numRows = Int(ceil(Double(totalCellsNeeded) / Double(7)))

        // we now know the Height needed for the collection view
        //  you said your calendar cell height is 75, so...
        //  cvHeight = numRows * 75

        calendarViewHeightConstraint.constant = CGFloat(numRows * 75)

        // give these values to the calendarView to be used
        //  for the collection view data
        calendarView.startDayOfWeek = startDayOfWeek
        calendarView.totalCellsNeeded = totalCellsNeeded


    }

}

// CALENDAR VIEW

class CalendarView: UIView, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {

    // these are set by the cell class
    var startDayOfWeek: Int = 0
    var totalCellsNeeded: Int = 0 // number of cell

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .red

        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: UICollectionViewFlowLayout())
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.backgroundColor = .clear
        addSubview(collectionView)

        collectionView.register(DaysCell.self, forCellWithReuseIdentifier: "cellDay")

        collectionView.topAnchor.constraint(equalTo: topAnchor).isActive = true
        collectionView.leadingAnchor.constraint(equalTo: leadingAnchor).isActive = true
        collectionView.trailingAnchor.constraint(equalTo: trailingAnchor).isActive = true
        collectionView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true

        // no height anchor
        //collectionView.heightAnchor.constraint(equalToConstant: 200).isActive = true

    }

    required init?(coder: NSCoder) { fatalError("init(coder:) has not been implemented") }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        totalCellsNeeded
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cellDay", for: indexPath) as! DaysCell

        // startDayOfWeek is 1-based
        cell.populateCell(indexPath.item - (startDayOfWeek - 1))

        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        .init(width: collectionView.frame.width / 7, height: 75)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat { 0 }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat { 0 }
}

// Day CELL

class DaysCell: UICollectionViewCell {
    private let dayLbl = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)

        dayLbl.translatesAutoresizingMaskIntoConstraints = false
        addSubview(dayLbl)

        dayLbl.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true
        dayLbl.centerXAnchor.constraint(equalTo: centerXAnchor).isActive = true
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func populateCell(_ i: Int) -> Void {
        if i >= 0 {
            dayLbl.text = "\(i + 1)"
        }
    }
}