swift-composable-architecture-mirror/Examples/CaseStudies/UIKitCaseStudies/CounterViewController.swift

import ComposableArchitecture
import UIKit

@Reducer
struct Counter {
  @ObservableState
  struct State: Equatable, Identifiable {
    let id = UUID()
    var count = 0
  }

  enum Action {
    case decrementButtonTapped
    case incrementButtonTapped
  }

  var body: some Reducer<State, Action> {
    Reduce { state, action in
      switch action {
      case .decrementButtonTapped:
        state.count -= 1
        return .none
      case .incrementButtonTapped:
        state.count += 1
        return .none
      }
    }
  }
}

final class CounterViewController: UIViewController {
  private let store: StoreOf<Counter>

  init(store: StoreOf<Counter>) {
    self.store = store
    super.init(nibName: nil, bundle: nil)
  }

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

  override func viewDidLoad() {
    super.viewDidLoad()

    view.backgroundColor = .systemBackground

    let decrementButton = UIButton(type: .system)
    decrementButton.addTarget(self, action: #selector(decrementButtonTapped), for: .touchUpInside)
    decrementButton.setTitle("−", for: .normal)

    let countLabel = UILabel()
    countLabel.font = .monospacedDigitSystemFont(ofSize: 17, weight: .regular)

    let incrementButton = UIButton(type: .system)
    incrementButton.addTarget(self, action: #selector(incrementButtonTapped), for: .touchUpInside)
    incrementButton.setTitle("+", for: .normal)

    let rootStackView = UIStackView(arrangedSubviews: [
      decrementButton,
      countLabel,
      incrementButton,
    ])
    rootStackView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(rootStackView)

    NSLayoutConstraint.activate([
      rootStackView.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor),
      rootStackView.centerYAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerYAnchor),
    ])

    observe { [weak self] in
      guard let self else { return }
      countLabel.text = "\(store.count)"
    }
  }

  @objc private func decrementButtonTapped() {
    store.send(.decrementButtonTapped)
  }

  @objc private func incrementButtonTapped() {
    store.send(.incrementButtonTapped)
  }
}

#Preview {
  CounterViewController(
    store: Store(initialState: Counter.State()) {
      Counter()
    }
  )
}