为什么这个变量不在这个函数中打印？

Question

当 ViewController 加载时，会打印正确的值。但是当尝试使用 UIButton 打印相同的值时，打印的是 nil

视图控制器

var userEmail: String?

func userLoggedIn(data: String) {

    userEmail = data //sent from delagate
    print(userEmail) // successfully printed
}

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "loginView" {
        let loginViewController: LoginViewController = segue.destination as! LoginViewController
        loginViewController.delegate = self
    }
}

override func viewDidAppear(_ animated: Bool) {

   let isUserLoggedIn = UserDefaults.bool(UserDefaults.standard)(forKey: "isUserLoggedIn")

    if(!isUserLoggedIn) {
        self.performSegue(withIdentifier: "loginView", sender: self);
    }   
}

@IBAction func createCommunityTapped(_ sender: AnyObject) {

    let createCommunityController = self.storyboard?.instantiateViewController(withIdentifier: "CreateNewCommunity") as! CreateNewCommunity

    print ("now here ", userEmail) // prints nil
    createCommunityController.myEmail = userEmail
}

userEmail 值是通过这段代码从 LoginViewController 传递过来的：

if(returnValue != "error") {

     self.delegate?.userLoggedIn(data: userEmail! )                
     UserDefaults.set(UserDefaults.standard)(true, forKey: "isUserLoggedIn");

     let mainPage = self.storyboard?.instantiateViewController(withIdentifier: "ViewController")
     let mainPageNav = UINavigationController(rootViewController: mainPage!)
     let appDelegate = UIApplication.shared.delegate
     appDelegate?.window??.rootViewController = mainPageNav 

     self.dismiss(animated: true, completion: nil)                
}

AppDelegate：

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.

    let mainStoryboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)        
    let isUserLoggedIn = UserDefaults.bool(UserDefaults.standard)(forKey: "isUserLoggedIn")

    if(!isUserLoggedIn) {

        let loginViewController = mainStoryboard.instantiateViewController(withIdentifier: "LoginViewController")            
        self.window?.rootViewController = loginViewController
        window!.makeKeyAndVisible()

    } else {

        let protectedPage = mainStoryboard.instantiateViewController(withIdentifier: "ViewController")            
        self.window?.rootViewController = protectedPage
        window!.makeKeyAndVisible()            
    }
    return true
}

Answer 1

问题似乎在于您的登录视图控制器。您将userEmail 传递给ViewController 的第一个实例，然后创建ViewController 的新实例并将其设置为根，从而释放第一个实例。第一步应该是删除此代码：

let mainPage = self.storyboard?.instantiateViewController(withIdentifier: "ViewController")
let mainPageNav = UINavigationController(rootViewController: mainPage!)
let appDelegate = UIApplication.shared.delegate
appDelegate?.window??.rootViewController = mainPageNav 

在您的 AppDelegate 中，您应该将根视图控制器设置为 ViewController，如果用户未登录，则在其顶部模态显示 LoginViewController。确保将 LoginViewController's 委托设置为 @987654328 @。那么你的登录逻辑可以是：

if(returnValue != "error") {
    self.delegate?.userLoggedIn(data: userEmail!)
    UserDefaults.standard.set(true, forKey: "isUserLoggedIn")

    self.dismiss(animated: true, completion: nil)                
}