CalendarView.swift

import UIKit

class ViewController: UIViewController {
    var yearView: UIView?
    var currentYear: Int = 2020 {
        didSet {
            createYearView()
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .systemBackground

        navigationItem.leftBarButtonItem = UIBarButtonItem(title: "-", style: .plain, target: self, action: #selector(minus))
        navigationItem.rightBarButtonItem = UIBarButtonItem(title: "+", style: .plain, target: self, action: #selector(add))

        createYearView()
    }

    func createYearView() {
        yearView?.removeFromSuperview()

        let yearView = YearView(year: currentYear)
        view.addSubview(yearView)
        yearView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            yearView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            yearView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
            yearView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            yearView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor)
        ])

        self.yearView = yearView
    }

    @objc private func minus() {
        currentYear -= 1
    }

    @objc private func add() {
        currentYear += 1
    }
}

class TitleGridView: UIView {
    private let titleLabel = UILabel()
    private let items: [[UIView]]

    let rows: Int
    let columns: Int
    var interval: CGPoint = .zero
    var spaceUnderTitle: CGFloat = 0

    init(rows: Int, columns: Int, titleConfiguration: (UILabel) -> Void, itemCreator: (Int, Int) -> UIView) {
        self.rows = rows
        self.columns = columns

        items = (0..<rows).map { row in
            (0..<columns).map { column in
                itemCreator(row, column)
            }
        }

        super.init(frame: .zero)

        titleConfiguration(titleLabel)

        addSubview(titleLabel)
        items.forEach { $0.forEach { addSubview($0) } }
    }

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

    override func layoutSubviews() {
        titleLabel.frame = CGRect(x: 0, y: 0, width: frame.width, height: titleLabel.intrinsicContentSize.height)

        let firstRowY = titleLabel.frame.maxY + spaceUnderTitle
        let size = CGSize(
            width: (frame.width - interval.x * CGFloat(columns - 1)) / CGFloat(columns),
            height: (frame.height - firstRowY - interval.y * CGFloat(rows - 1)) / CGFloat(rows)
        )

        for (row, aRow) in items.enumerated() {
            for (column, item) in aRow.enumerated() {
                item.frame = CGRect(
                    x: CGFloat(column) * (size.width + interval.x),
                    y: CGFloat(row) * (size.height + interval.y) + firstRowY,
                    width: size.width,
                    height: size.height
                )
            }
        }
    }
}

class YearView: TitleGridView {
    init(year: Int) {
        let titleConfiguration = { (label: UILabel) in
            label.font = .preferredFont(forTextStyle: .largeTitle)
            label.text = String(year)
        }

        let itemCreator = { (row: Int, column: Int) in
            MonthInYearView(year: year, month: row * 3 + column + 1)
        }

        super.init(rows: 4, columns: 3, titleConfiguration: titleConfiguration, itemCreator: itemCreator)

        interval = CGPoint(x: 8, y: 4)
        spaceUnderTitle = 4
    }

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

class MonthInYearView: TitleGridView {
    init(year: Int, month: Int) {
        let calendar = Calendar.current
        let timeZone = TimeZone.current

        let titleConfiguration = { (label: UILabel) in
            label.font = .preferredFont(forTextStyle: .title2)
            label.text = calendar.monthSymbols[month - 1]
        }

        let components = DateComponents(calendar: calendar, timeZone: timeZone, year: year, month: month)
        let date = calendar.date(from: components)!
        let firstWeekday = calendar.dateComponents(in: timeZone, from: date).weekday! - 1
        let range = calendar.range(of: .day, in: .month, for: date)!
        let itemCreator = { (row: Int, column: Int) -> UILabel in
            let label = UILabel()
            label.textAlignment = .center
            label.font = .systemFont(ofSize: 10)
            let day = row * 7 + column + 1 - firstWeekday
            if range.contains(day) {
                label.text = String(day)
            }
            return label
        }

        super.init(rows: 6, columns: 7, titleConfiguration: titleConfiguration, itemCreator: itemCreator)

        interval = CGPoint(x: 4, y: 3)
        spaceUnderTitle = 4
    }

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