swift - 在应用程序启动之前执行完成处理程序

Question

我正尝试通过以下两种方式之一打开应用:

1. 如果用户没有保存UserDefaults，则打开一个WelcomeViewController
2. 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，我通过一个带有完成处理程序的函数。如果第 2 步是这种情况，我想在完成 block 中打开 MenuContainerViewController，而不会出现任何 UI 问题。

这是我目前的代码:

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 UI 的视频，当我需要运行完成处理程序时，过渡到应用程序时并不平滑(有一秒钟黑屏)。

请帮我弄清楚如何让应用程序顺利启动。

Answer 1

我不得不在我的 Firebase 应用程序中处理类似的情况。我通常做的是制作一个 InitialViewController。这是始终加载的 View Controller ，无论如何。这个 View Controller 最初被设置为无缝地看起来与启动屏幕完全一样。

这就是 InitialViewController 在界面构建器中的样子:

这是我的启动屏幕的样子:

所以当我说它们看起来完全一样时，我的意思是它们看起来完全一样。此 InitialViewController 的唯一目的是处理此异步 检查并决定下一步要做什么，同时看起来像启动屏幕。您甚至可以在两个 View Controller 之间复制/粘贴界面构建器元素。

因此，在此 InitialViewController 中，您在 viewDidAppear() 中进行身份验证检查。如果用户已登录，我们将对主视图 Controller 执行 segue。如果没有，我们将动画化用户入职元素到位。演示我的意思的 gif 非常大(维度方面和数据方面)，因此它们可能需要一些时间才能加载。您可以在下面找到每一个:

User previously logged in.

User not previously logged in.

这就是我在 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()
        }
    }   
}