Why is my UILabel variable always nil?

问题

I am simply trying to connect a variable with a UILabel in Swift. I have looked at many tutorials but I keep getting errors. The main cause seems to be that @IBOutlet weak var str = UILabel! is always nil. When I try to assign a value to it, the console prints a fatal error: unexpectedly found nil while unwrapping an Optional value and inside the coding interface, the line self.stringLabel.text = self.string is highlighted with the error Thread 1: EXC_BAD_INSTRUCTION(code=EXC_I386_INVOP,subcode=0x0. Here is my code:

@IBOutlet weak var stringLabel: UILabel!
var string: String!

override func viewDidLoad() {
    self.stringLabel.text = self.string
    super.viewDidLoad()
}

@IBAction func buttonClick(sender: UIButton) {
    self.stringLabel.text = "Button clicked!"
}

I have tried different positioning of the .self and .text around the variables and have had no success. Am I on the right track? And, if so, what amI doing wrong? If I'm completely wrong, how does one set a label's text to that of a variable? Thank you very much in advance.

EDIT: I have connected the label in the storyboard to the @IBOutlet

回答1:

Cocoa will take care of your UILabel if you have it connected within interface builder (make sure that's the case) but your string object is uninitialized, you have to allocate it with var string: String = "" or if you want to use an optional var string: String? = ""

You can do this in the global scope but I would recommend that all of your view controllers have an init() method where these sorts of things are assigned.

Edit: I tried reproducing the problem, but I got an expected result. Here is my code for the view controller

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var testLabel: UILabel!
    var string: String!

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)

        string = "Hello World"
    }

    override func viewDidLoad() {
        self.testLabel.text = self.string
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

回答2:

The issue is that the variable string is nil.

An implicit unwrapped optional (!) must be non-nil when being used.

Try something real:

var string = "Hello there"

回答3:

The stringLabel property is NOT connected to Interface Builder

You must connect the UILabel object in Interface Builder to the property in your Swift code.

In Xcode open side by side the xib/storyboard representing your UI and the Swift View Controller
Keep ctrl pressed on the keyboard
Move the pointer over the UILabel object in Interface Builder
Press the pointer button
Drag the pointer from the UILabel (Interface Builder) to the UILabel! part in your declaration @IBOutlet weak var stringLabel: UILabel! in your Swift source code

That's it.

来源：https://stackoverflow.com/questions/35417025/why-is-my-uilabel-variable-always-nil

标签

ios

swift

uilabel