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protocol ViewModelType {

    associatedtype Input: InputType
    associatedtype Output

    func transform(input: Input) -> Output
}

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protocol InputType {

    associatedtype FromView
    associatedtype FromController

    var fromView: FromView { get }
    var fromController: FromController { get }

    init(fromView: FromView, fromController: FromController)
}

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import RxSwift
import RxCocoa

class BaseViewModel<NavigationStep, FromView, Output>: ViewModelType {
    let bag = DisposeBag()
    // Small type wrapping a RxSwift PublishSubject, that we notify about navigation, passing
    // a contextual `NavigationStep` which typically is an enum, modelling all possible
    // outgoing navigation steps we can take from this scene.
    let navigator = Stepper<NavigationStep>()

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// The cases should be named as if they were prefixed with the words
// "user intends to" or "user did"
enum AuthenticateNavigationStep {
    case signIn
    case signUp
}

final class AuthenticateViewModel: BaseViewModel<
    AuthenticateNavigationStep,
    AuthenticateViewModel.InputFromView,

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private extension AuthenticationCoordinator {

    func toAuthentication() {
        let viewModel = AuthenticateViewModel()

        push(scene: Authenticate.self, viewModel: viewModel) { [unowned self] userIntendsTo in
            switch userIntendsTo {
            case .signUp: self.toSignIn()
            case .signIn: self.toSignUp()
            }

Sajjon

import RxSwift

class SceneController<View>: UIViewController where View: UIView & ViewModelled {
    typealias ViewModel = View.ViewModel

    private let bag = DisposeBag()
    private let viewModel: ViewModel

    // We omit some additional "AbstractTarget" wrappers taking these `PublishSubjects` as arguments

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/* This ⬇ single line ⬇ constitutes a fully working ViewController. Clean code 👌🏽 */
typealias Authenticate = SceneController<AuthenticateView>

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final class SignInView: UIView {

    // Some loading view, it might contain a `UIActivityIndicatorView`
    // it has reactive binding for `isLoading`, but starts invisible
    private lazy var loadingView = LoadingView()

    // In fact these TextFields are some subclass capable of displaying validation errors
    private lazy var usernameField = UITextField()
    private lazy var passwordField = UITextField()
    private lazy var confirmPasswordField = UITextField()

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protocol AuthenticationUseCase: AnyObject {
    func signIn(request: SignInRequest) -> Observable<Void>
}

struct SignInRequest {
    let username: String
    let password: String
}

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enum Validation<Validated> {
    case valid(Validated)
    case invalid(errorMessage: String)
    var asValidated: Validated? {
        switch self {
        case .valid(let validated): return validated
        default: return nil
        }
    }