import RxSwift

struct LoginViewModel {

    var username = Variable<String>("")
    var password = Variable<String>("")

    var isValid : Observable<Bool>{
        return Observable.combineLatest(self.username.asObservable(), self.password.asObservable())
        { (username, password) in
            return username.characters.count > 0
                && password.characters.count > 0
        }
    }
}

import RxSwift
import RxCocoa
import UIKit

class LoginViewController: UIViewController {
    var usernameTextField: UITextField!
    var passwordTextField: UITextField!
    var confirmButton: UIButton!

    var viewModel = LoginViewModel()

    var disposeBag = DisposeBag()

    override func viewDidLoad() {
        usernameTextField.rx.text.bindTo(viewModel.username).addTo(disposeBag)
        passwordTextField.rx.text.bindTo(viewModel.password).addTo(disposeBag)

        //from the viewModel
        viewModel.rx.isValid.map { $0 }
            .bindTo(confirmButton.rx.isEnabled)
    }
}

let _ = viewModel.username.asObservable()
            .map { $0 }
            .bind(to: usernameTextField.rx.text)