TableViewWithDiffable.swift

import UIKit

class ViewController: UIViewController {
    let tableView: UITableView = {
        let tableView = UITableView()
        tableView.translatesAutoresizingMaskIntoConstraints = false
        tableView.register(ItemCell.self, forCellReuseIdentifier: "ItemCell")
        return tableView
    }()
    lazy var tableViewDataSource = TableViewDataSource(tableView: tableView)

    override func viewDidLoad() {
        super.viewDidLoad()
        setup()
    }
    
    func setup() {
        view.addSubview(tableView)
        NSLayoutConstraint.activate([
            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)
        ])
        let item = UIBarButtonItem(image: .add, style: .plain, target: self, action: #selector(addItem))
        navigationItem.rightBarButtonItem = item
        tableViewDataSource.snapshotData(items: (1...2).map{ Item(text: "Demo \($0)") })
    }
    
    @objc
    func addItem() {
        let item = Item(text: "demo")
        tableViewDataSource.snapshotData(items: [item])
    }
}

class ItemCell: UITableViewCell {
    override var reuseIdentifier: String? {
        "ItemCell"
    }
}

struct Item: Hashable, Identifiable {
    let id: UUID = UUID()
    let text: String
}

class TableViewDataSource {
    enum Section {
        case main
    }
    typealias DataSource = UITableViewDiffableDataSource<Section, Item>
    typealias Snapshot = NSDiffableDataSourceSnapshot<Section, Item>
    
    private let tableView: UITableView
    var snapshot = Snapshot()
    lazy var dataSource = DataSource(tableView: tableView) { tableView, indexPath, itemIdentifier in
        let cell = tableView.dequeueReusableCell(withIdentifier: "ItemCell", for: indexPath)
        var config = cell.defaultContentConfiguration()
        config.text = itemIdentifier.text
        cell.contentConfiguration = config
        return cell
    }
    
    init(tableView: UITableView) {
        self.tableView = tableView
        snapshot.appendSections([.main])
    }
    
    func snapshotData(items: [Item]) {
        snapshot.appendItems(items, toSection: .main)
        dataSource.apply(snapshot)
    }
}