Question

我正尝试通过以下两种方式之一打开应用程序：

如果用户没有保存UserDefaults，则打开WelcomeViewController
如果用户保存了UserDefaults，则打开一个MenuContainerViewController作为主页

在第2步中，如果保存了UserDefaults，那么我需要使用Firebase登录用户，该用户可以通过具有完成处理程序的函数来使用。如果是第2步，我想在完成块内打开MenuContainerViewController，而不会打扰用户界面。

这是我当前拥有的代码：

func application(_ application: UIApplication,
                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    self.window = UIWindow(frame: UIScreen.main.bounds)

    FirebaseApp.configure()

    guard
        let email = UserDefaults.standard.string(forKey: "email"),
        let password = UserDefaults.standard.string(forKey: "password")
    else {
        // User has no defaults, open welcome screen
        let welcomeViewController = WelcomeViewController()
        self.window?.rootViewController = welcomeViewController
        self.window?.makeKeyAndVisible()
        return true
    }

    // User has defaults saved locally, open home screen of app
    let authentificator = Authentificator()
    authentificator.login(with: email, password) { result, _ in
        if result {
            let menuContainerViewController = MenuContainerViewController()
            self.window?.rootViewController = menuContainerViewController
            self.window?.makeKeyAndVisible()
        }
    }

    return true
}

这是当前用户界面的视频，当我需要运行完成处理程序时，过渡到应用程序的过程并不顺利（有一秒钟短暂的黑屏）。

请帮助我弄清楚如何使应用程序顺利启动。

Answer 1

在Firebase应用程序中，我不得不同样地处理各种情况。我通常要做的是制作一个InitialViewController。无论如何，这都是始终加载的视图控制器。最初，此视图控制器已设置为与启动屏幕完全一样无缝显示。

这是InitialViewController在界面构建器中的样子：

这是我的启动屏幕：

因此，当我说它们看起来完全相同时，是指它们看起来完全相同。 InitialViewController的唯一目的是处理异步检查并决定下一步要做什么，所有这些看起来都像启动屏幕一样。您甚至可以在两个视图控制器之间复制/粘贴界面构建器元素。

因此，在此InitialViewController中，您需要在viewDidAppear()中进行身份验证检查。如果用户已登录，我们将对主视图控制器执行segue。如果没有，我们会为用户的入职元素设置动画。展示我的意思的gif相当大（在尺寸方面和在数据方面），因此加载它们可能需要一些时间。您可以在下面找到每一个：

User previously logged in.

User not previously logged in.

这是我在InitialViewController中执行检查的方式：

@IBOutlet var loginButton: UIButton!
@IBOutlet var signupButton: UIButton!
@IBOutlet var stackView: UIStackView!
@IBOutlet var stackViewVerticalCenterConstraint: NSLayoutConstraint!

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    //When the view appears, we want to check to see if the user is logged in.
    //Remember, the interface builder is set up so that this view controller **initially** looks identical to the launch screen
    //This gives the effect that the authentication check is occurring before the app actually finishes launching
    checkLoginStatus()
}

func checkLoginStatus() {
    //If the user was previously logged in, go ahead and segue to the main app without making them login again

    guard
        let email = UserDefaults.standard.string(forKey: "email"),
        let password = UserDefaults.standard.string(forKey: "password")
    else {
        // User has no defaults, animate onboarding elements into place
        presentElements()
        return
    }

    let authentificator = Authentificator()
    authentificator.login(with: email, password) { result, _ in
        if result {
            //User is authenticated, perform the segue to the first view controller you want the user to see when they are logged in
            self.performSegue(withIdentifier: "SkipLogin", sender: self)
        }
    }
}

func presentElements() {

    //This is the part where the illusion comes into play
    //The storyboard elements, like the login and signup buttons were always here, they were just hidden
    //Now, we are going to animate the onboarding UI elements into place
    //If this function is never called, then the user will be unaware that the launchscreen was even replaced with this view controller that handles the authentication check for us

    //Make buttons visible, but...
    loginButton.isHidden = false
    signupButton.isHidden = false

    //...set their alpha to 0
    loginButton.alpha = 0
    signupButton.alpha = 0

    //Calculate distance to slide up
    //(stackView is the stack view that holds our elements like loginButton and signupButton. It is invisible, but it contains these buttons.)
    //(stackViewVerticalCenterConstraint is the NSLayoutConstraint that determines our stackView's vertical position)
    self.stackViewVerticalCenterConstraint.constant = (view.frame.height / 2) + (stackView.frame.height / 2)

    //After half a second, we are going to animate the UI elements into place
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
        UIView.animate(withDuration: 0.75) {
            self.loginButton.alpha = 1
            self.signupButton.alpha = 1

            //Create correct vertical position for stackView
            self.stackViewVerticalCenterConstraint.constant = (self.view.frame.height - self.navigationController!.navigationBar.frame.size.height - self.signupButton.frame.maxY - (self.stackView.frame.size.height / 2)) / 3
            self.view.layoutIfNeeded()
        }
    }   
}

在应用启动之前执行完成处理程序

1 个答案: