UICollectionViewCompositionalLayout month calendar example

MonthViewController.swift

//
//  MonthViewController.swift
//  Calendar
//
//  Created by Alexander T on 22.04.2023.
//

import UIKit

// MARK: - View Controller

final class MonthViewController: UIViewController {
    
    // MARK: - Private variables
    
    private let days = [
        [                     1,  2],
        [ 3,  4,  5,  6,  7,  8,  9],
        [10, 11, 12, 13, 14, 15, 16],
        [17, 18, 19, 20, 21, 22, 23],
        [24, 25, 26, 27, 28, 29, 30],
        [31]
    ]
    
    // MARK: - Outlets
    
    @IBOutlet private var collectionView: UICollectionView!

    // MARK: - Lifecycle
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        setupCollectionViewLayout()
    }
    
    // MARK: - Private methods
    
    private func setupCollectionViewLayout() {
        
        collectionView.collectionViewLayout = UICollectionViewCompositionalLayout { section, env in
            let dayItemSize = NSCollectionLayoutSize(
                widthDimension: .fractionalWidth(1.0 / CGFloat(self.days[section].count)),
                heightDimension: .fractionalHeight(1.0)
            )
            let dayItem = NSCollectionLayoutItem(layoutSize: dayItemSize)
            
            let weekGroupSize = NSCollectionLayoutSize(
                widthDimension: .fractionalWidth(CGFloat(self.days[section].count) / 7),
                heightDimension: .absolute(40.0)
            )
            let weekGroup = NSCollectionLayoutGroup.horizontal(
                layoutSize: weekGroupSize,
                subitems: [dayItem]
            )
            weekGroup.edgeSpacing = NSCollectionLayoutEdgeSpacing(
                leading: section == .zero ? .flexible(.zero) : .fixed(.zero),
                top: .fixed(.zero),
                trailing: section == .zero ? .fixed(.zero) : .flexible(.zero),
                bottom: .fixed(.zero)
            )
            
            return NSCollectionLayoutSection(group: weekGroup)
        }
        
    }
    
}

// MARK: - Day Cell

final class DayCell: UICollectionViewCell {
    
    @IBOutlet var dayLabel: UILabel!
    
}

// MARK: - Collection View DataSource

extension MonthViewController: UICollectionViewDataSource {
    
    func numberOfSections(
        in collectionView: UICollectionView
    ) -> Int {
        days.count
    }
    
    func collectionView(
        _ collectionView: UICollectionView,
        numberOfItemsInSection section: Int
    ) -> Int {
        days[section].count
    }
    
    func collectionView(
        _ collectionView: UICollectionView,
        cellForItemAt indexPath: IndexPath
    ) -> UICollectionViewCell {
        guard let cell = collectionView.dequeueReusableCell(
            withReuseIdentifier: "DayCell",
            for: indexPath
        ) as? DayCell else {
            return UICollectionViewCell()
        }
        
        cell.dayLabel.text = String(days[indexPath.section][indexPath.row])
        
        return  cell
    }
    
}