associatedtype.swift

protocol ViewModelType {
  associatedtype Model
  associatedtype Input

  init(model: Model, input: Input)
}

class ViewModel: ViewModelType {

  required init(model: Model, input: Input) {
    /// bind input
  }

  struct Model {
    let userId: String
  }

  struct Input {
    let buttonTap: Observable<Void>
  }

}

let model = ViewModel.Model(userId: "123")