ios - 为什么在从另一个页面传递值之前，UILable 不会显示？

Question

我是 swift 新手，当我尝试构建应用程序时遇到了一些问题。当我尝试将值从 View A 传递到 View B(但应用程序将首先访问 View B)时，B 的标签将不会显示。但是，如果我从 View B 转到 View A 并更新 View A 中的值，然后返回 View B，则 View B 中的值将会更新。为什么会出现这种情况？

还有一个问题，从 View A 和 B 更改的值在注销/返回到另一个 View 后将不会保留。即使我注销/切换到另一个 View ，如何保留更改？

谢谢大家!这是我的代码。

类 View B

@IBOutlet weak var num_1: UILabel!
@IBOutlet weak var num_2: UILabel!

var labeltext = String()

override func viewDidLoad() {
    super.viewDidLoad()
    num_1.text = labeltext
}

@IBAction func topic1(sender: AnyObject) {
    self.performSegueWithIdentifier("contentView", sender: self)
}

类 View A

override func viewDidAppear(animated: Bool)
{

let isUserLoggedIn = NSUserDefaults.standardUserDefaults().boolForKey("isUserLoggedIn");

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


}


@IBAction func logoutButtonTapped(sender: AnyObject) {

    NSUserDefaults.standardUserDefaults().setBool(false,forKey:"isUserLoggedIn");
    NSUserDefaults.standardUserDefaults().synchronize();

    let loginViewController = self.storyboard!.instantiateViewControllerWithIdentifier("loginView") as! LoginViewController
    let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate

    appDelegate.window?.rootViewController = loginViewController
    appDelegate.window?.makeKeyAndVisible()

  //  self.performSegueWithIdentifier("loginView", sender: self);


}

@IBOutlet weak var voteNum1: UILabel!
@IBOutlet weak var voteNum2: UILabel!
@IBOutlet weak var centerCircle: UIImageView!

var num1 = 0.0
var num2 = 0.0
var sum = 0.0
var a = 0.0
var b = 0.0
let circleShape = CAShapeLayer()

@IBAction func vote1(sender: AnyObject) {
    num1 = Double(voteNum1.text!)!
    self.voteNum1.text = String(num1 + 1)
    a = num1 + 1

}

@IBAction func vote2(sender: AnyObject) {
    num2 = Double(voteNum2.text!)!
    self.voteNum2.text = String(num2 + 1)
    b = num2 + 1
}

@IBAction func calculate(sender: AnyObject) {
    sum = Double(a/(b+a))
    circleShape.strokeEnd = CGFloat(sum)
}

override func viewDidLoad() {
    super.viewDidLoad()
    // round view
    let roundView = UIView(frame: CGRectMake(85, 100, 150, 150))
    roundView.backgroundColor = UIColor.whiteColor()
    roundView.layer.cornerRadius = roundView.frame.size.width / 2

    // bezier path
    let circlePath = UIBezierPath(arcCenter: CGPoint (x: roundView.frame.size.width / 2, y: roundView.frame.size.height / 2),
                                  radius: roundView.frame.size.width / 2.2 ,
                                  startAngle: CGFloat(-0.5 * M_PI),
                                  endAngle: CGFloat(1.5 * M_PI),
                                  clockwise: true)
    // circle shape
    circleShape.path = circlePath.CGPath
    circleShape.strokeColor = UIColor.blackColor().CGColor
    circleShape.fillColor = UIColor.clearColor().CGColor
    circleShape.lineWidth = 14
    // set start and end values
    circleShape.strokeStart = 0.0
    circleShape.strokeEnd = CGFloat(sum)

    // add sublayer
    roundView.layer.addSublayer(circleShape)
    // add subview
    self.view.addSubview(roundView)
    self.view.insertSubview(roundView, belowSubview: centerCircle)

}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    var DestViewController : View B = segue.destinationViewController as! View B

    DestViewController.labeltext = voteNum1.text!
}

@IBAction func back(sender: AnyObject) {
        self.performSegueWithIdentifier("back", sender: self)
}

Answer 1

您 View 中的线b；

num_1.text = labeltext

每次都实例化一个新字符串。

换行

var labeltext = String()

到

var labelText : String? = nil

要在注销后保留数据，您需要在应用程序中开发“持久性”。本质上，您需要在应用程序中嵌入持久的数据存储。

流行的做法是；

• 核心数据
• 领域*

* https://realm.io

• https://realm.io/docs/swift/latest/ <- 有关如何使用 swift 通过 Realm 向应用程序添加持久性的教程。