Yet another GitHub sign in screen using MVVM

AppDelegate.swift

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        window = UIWindow(frame: UIScreen.mainScreen().bounds)
        
        let viewModel = LoginViewModel(provider: GitHubProvider)
        let loginViewController = R.storyboard.main.loginViewController!
        loginViewController.viewModel = viewModel
        
        if let window = window {
            window.rootViewController = loginViewController
            window.makeKeyAndVisible()
        }
        return true
    }

LoginViewController.swift

//
//  LoginViewController.swift
//  Origin
//
//  Created by krawiecp on 06/01/2016.
//  Copyright © 2016 tailec. All rights reserved.
//

import UIKit
import RxSwift
import RxCocoa

class LoginViewController: UIViewController {

    @IBOutlet weak var usernameTextField: UITextField!
    @IBOutlet weak var passwordTextField: UITextField!
    @IBOutlet weak var loginButton: UIButton!
    
    var viewModel: LoginViewModel!
    
    private let disposeBag = DisposeBag()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        bindToRx()
        
        let tapBackground = UITapGestureRecognizer(target: self, action: Selector("dismissKeyboard"))
        view.addGestureRecognizer(tapBackground)
    }
    
    private func bindToRx() {
        _ = usernameTextField.rx_text.bindTo(viewModel.username)
        _ = passwordTextField.rx_text.bindTo(viewModel.password)
        _ = loginButton.rx_tap.bindTo(viewModel.loginTaps)

        viewModel.loginEnabled.bindTo(loginButton.rx_enabled).addDisposableTo(disposeBag)
        
        viewModel.loginExecuting.subscribeNext { executing in
            UIApplication.sharedApplication().networkActivityIndicatorVisible = executing
        }
        .addDisposableTo(disposeBag)
        
        viewModel.loginFinished.subscribeNext { loginResult in
            switch loginResult {
                case .Failed(let message):
                    let alert = UIAlertController(title: "Oops!", message:message, preferredStyle: .Alert)
                    alert.addAction(UIAlertAction(title: "OK", style: .Default) { _ in })
                    self.presentViewController(alert, animated: true, completion: nil)
                case .OK:
                    self.dismissViewControllerAnimated(true, completion: nil)
            }
        }
        .addDisposableTo(disposeBag)
    }
    
    func dismissKeyboard() {
        view.endEditing(true)
    }
}

LoginViewModel.swift

//
//  LoginViewModel.swift
//  Origin
//
//  Created by krawiecp on 06/01/2016.
//  Copyright © 2016 tailec. All rights reserved.
//

import Foundation
import RxSwift
import RxCocoa
import RxViewModel
import Moya

class LoginViewModel: RxViewModel {
    // Input
    var username = Variable("")
    var password = Variable("")
    var loginTaps = PublishSubject<Void>()
    
    // Output
    let loginEnabled: Observable<Bool>
    let loginFinished: Observable<LoginResult>
    let loginExecuting: Observable<Bool>
    
    // Private
    private let provider: RxMoyaProvider<GitHub>
    
    init(provider: RxMoyaProvider<GitHub>) {
        self.provider = provider
        
        let activityIndicator = ActivityIndicator()
        loginExecuting = activityIndicator
            .asObservable()
            .shareReplay(1)
            .observeOn(MainScheduler.instance)

        loginEnabled = Observable.combineLatest(username.asObservable(), password.asObservable())
            { $0.characters.count > 0 && $1.characters.count > 6 }.debug()
            .shareReplay(1)
            .observeOn(MainScheduler.instance)
    
        let usernameAndPassword = Observable.combineLatest(username.asObservable(), password.asObservable())
            { ($0, $1) }
        
        loginFinished = loginTaps
            .withLatestFrom(usernameAndPassword)
            .flatMapLatest { (username, password) -> Observable<LoginResult>  in
                provider.request(GitHub.Token(username: username, password: password))
                    .trackActivity(activityIndicator)
                    .mapJSON()
                    .doOn(onNext: { json in
                        var appToken = AppToken()
                        appToken.token = json["token"] as? String
                    })
                    .map { json in
                        if let message = json["message"] as? String {
                            return LoginResult.Failed(message: message)
                        } else {
                            return LoginResult.OK
                        }
                }
                .shareReplay(1)
                .observeOn(MainScheduler.instance)
        }
        super.init()
    }
}

LoginViewModelSpec.swift

import Quick
import Nimble
import RxBlocking
import RxTests
@testable import Origin

class LoginViewModelSpec: QuickSpec {
    override func spec() {
        describe("view model with") {
            var viewModel: LoginViewModel!
            beforeEach {
                viewModel = LoginViewModel(provider: GitHubProvider)
            }
            context("valid credentials") {
                
                beforeEach {
                    viewModel.username.value = "johny"
                    viewModel.password.value = "yellowpanties"
                }
                it("enables login") {
                    expect(try! viewModel.loginEnabled.toBlocking().first()).toEventually(equal(true))
                }
            }
            
            context("invalid credentials") {
                
                beforeEach {
                    viewModel.username.value = ""
                    viewModel.password.value = ""
                }
                it("disables login") {
                    expect(try! viewModel.loginEnabled.toBlocking().first()).toEventually(equal(false))
                }
            }
        }
    }
}