RxViewModel.swift

protocol ViewModelInputs {
  var buttonTapped: AnyObserver<Void> { get }
}

protocol ViewModelOutputs {
  var showLoginErrorMessage: Driver<String> { get }
}

protocol ViewModelIO {
  var inputs: ViewModelInputs { get }
  var outputs: ViewModelOutputs { get }
}

protocol ViewModelNavigation {
  var loginSuccessful: Observable<User> { get }
}

final class ViewModel: ViewModelIO, ViewModelInputs, ViewModelOutputs, ViewModelNavigation {
  // IO
  var inputs: ViewModelInputs { return self }
  var outputs: ViewModelOutputs { return self }
  
  // Inputs
  let buttonTapped: AnyObserver<Void>
  
  // Outputs
  let showLoginErrorMessage: Driver<String>
  
  // Navigation
  var navigation: ViewModelNavigation { return self }
  let loginSuccessful: Observable<User>
  
  init(api: LoginAPI) {
    let buttonTappedSubject = PublishSubject<Void>()
    self.buttonTapped = buttonTappedSubject.asObserver()
    
    let result: Observable<Result<User, LoginError>> = buttonTappedSubject
    	.flatMapLatest { _ in return api.login() }
    
    self.loginSuccessful = result.value
    	.unwrap()
    
    self.showLoginErrorMessage = result.error
    	.unwrap()
    	.map("You didn't say the magic word!")
    	.asDriver(onErrorDriveWith: .empty())
  }
}