ViewModelAPI with Views

ViewModel.swift

import UIKit

class View: UIView {
  init() {
    super.init(frame: .zero)
  }

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

protocol ViewModelAPI {
  associatedtype ViewType: View

  var view: ViewType! { get }
  func sizeThatFits(_ size: CGSize) -> CGSize
}

class ViewModel<V: View>: ViewModelAPI {
  var view: V!

  func sizeThatFits(_ size: CGSize) -> CGSize {
    return .zero
  }
}

protocol RootViewModelAPI: ViewModelAPI {
  var title: String { get }
}

final class RootViewModel: ViewModel<RootView>, RootViewModelAPI {
  let title: String

  init(title: String) {
    self.title = title
    super.init()
    view = ViewType(viewModel: self)
  }
}

final class RootView: View {
  var viewModel: any RootViewModelAPI

  init(viewModel: any RootViewModelAPI) {
    self.viewModel = viewModel
    super.init()
  }

  override func layoutSubviews() {
    super.layoutSubviews()
    
    print(viewModel.title)
  }
}

final class VMViewController<VM: ViewModelAPI>: UIViewController {
  private let viewModel: VM

  init(viewModel: VM) {
    self.viewModel = viewModel
    super.init(nibName: nil, bundle: nil)
  }
  
  required init?(coder: NSCoder) { fatalError("init(coder:) has not been implemented") }

  override func viewDidLoad() {
    super.viewDidLoad()

    view.addSubview(viewModel.view)
  }
}

enum Controller {
  func start() {
    let viewController = VMViewController(viewModel: RootViewModel(title: "Hello, world!"))
  }
}