@main
class AppDelegate: UIResponder, UIApplicationDelegate {

  var window: UIWindow?

  let engines = FlutterEngineGroup(name: "multiple-flutters", project: nil)

  // MARK: - UIApplicationDelegate

  func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    window = UIWindow(frame: UIScreen.main.bounds)

    let mainViewController = ListViewController()
    let navigationController = UINavigationController(rootViewController: mainViewController)

    navigationController.navigationBar.isTranslucent = false

    if let window = window {
      window.rootViewController = navigationController
      window.makeKeyAndVisible()
    }

    return true
  }
}

private let reuseIdentifier = "CollectionViewCell"

class ListViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {
  init() {
    super.init(collectionViewLayout: UICollectionViewFlowLayout())
  }

  required init?(coder: NSCoder) {
    super.init(coder: coder)
  }

  override func viewDidLoad() {
    super.viewDidLoad()

    self.collectionView!.register(
      CollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
  }

  // MARK: UICollectionViewDataSource

  override func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
  }

  override func collectionView(
    _ collectionView: UICollectionView, numberOfItemsInSection section: Int
  ) -> Int {
    return 5
  }

  override func collectionView(
    _ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath
  ) -> UICollectionViewCell {
    guard
      let cell = collectionView.dequeueReusableCell(
        withReuseIdentifier: reuseIdentifier, for: indexPath) as? CollectionViewCell
    else {
      fatalError("Unable to dequeue CollectionViewCell")
    }
    cell.initializeFlutterEmbedding(parent: self)
    return cell
  }

  // MARK: UICollectionViewDelegateFlowLayout

  public func collectionView(
    _ collectionView: UICollectionView,
    layout collectionViewLayout: UICollectionViewLayout,
    sizeForItemAt indexPath: IndexPath
  ) -> CGSize {
    return CGSize(width: collectionView.bounds.width, height: 300)
  }
}

class CollectionViewCell: UICollectionViewCell {
  @IBOutlet weak var myLabel: UILabel!
  weak var parentViewController: UIViewController? = nil

  var engine: FlutterEngine?
  var flutterViewController: FlutterViewController?

  @available(*, unavailable)
  required init?(coder aDecoder: NSCoder) {
    preconditionFailure("init(coder:) has not been implemented")
  }

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

  public func initializeFlutterEmbedding(parent: UIViewController) {
    guard engine == nil else {
      return
    }

    let appDelegate: AppDelegate = UIApplication.shared.delegate as! AppDelegate
    engine = appDelegate.engines.makeEngine(with: nil)
    flutterViewController = FlutterViewController(engine: engine!, nibName: nil, bundle: nil)

    guard let flutterViewController, let engine else {
      fatalError("Failed to initialize Gen UI")
    }

    let contentView = self.contentView
    parent.addChild(flutterViewController)
    contentView.addSubview(flutterViewController.view)
    flutterViewController.view.frame = contentView.bounds
    flutterViewController.didMove(toParent: parent)
  }
}