iOSでMVVM（バインディングを使わない） ref: https://qiita.com/masamichiueta/items/3d09b98b80762bd6f248

**ViewController.swift

class **ViewController: UIViewController {

    var viewModel: **ViewModel!

    ...
    override func viewDidLoad() {
        super.viewDidLoad()

        self.viewModel.stateDidUpdate = { [unowned self] state in
            switch state {
            case .state1:
                ....
            case .state2:
                ....
            }
        }
    }
}

**ViewModel.swift

class **ViewModel: ViewModel {

    var stateDidUpdate: ((**ViewModelState) -> Void)?

    ...

    func updateToState1() {
        ....
        self.stateDidUpdate?(.state1)
    }
}

file0.swift

enum **ViewModelState {
    case state1, state2, ...
}

ViewModel.swift

protocol ViewModel {
    associatedtype State
    var stateDidUpdate: ((State) -> Void)? { get set }
}