Controller.swift

class ViewController: UIViewController {

    var model: Model = Model()

    override func loadView() {
        super.loadView()

        let button = UIButton(frame: CGRect(x: 60, y: 100, width: 200, height: 100))
        button.setTitle("Start Fitbit Auth", for: UIControl.State())
        button.backgroundColor = .red
        button.addTarget(self, action: #selector(authDidTap), for: .touchUpInside)

        view.addSubview(button)
    }

    // MARK: - Actions
    @objc private func authDidTap() {
        if let context = UIApplication.shared.keyWindow {
            model.contextProvider = AuthContextProvider(context)
        }
        model.auth { authCode, error in
            if error != nil {
                print("Auth flow finished with error \(String(describing: error))")
            } else {
                print("Your auth code is \(String(describing: authCode))")

                DispatchQueue.main.async {
                    let alert = UIAlertController(title: "Success", message: "auth code is \(String(describing: authCode))", preferredStyle: .alert)
                    alert.addAction(UIAlertAction(title: "OK", style: .default))
                    self.present(alert, animated: true)
                }
            }
        }
    }

}