我不明白为什么这个if语句在Swift中不起作用

Question

My app involves two UILabels labelDisplay and labelStatus , one UIButton , buttonAddOne , and a variable numbers . I can't understand why the labelStatus isn't being updated when numbers reaches 5.

import UIKit

var numbers:Int = 0

class ViewController: UIViewController {

    // Declaration of the two labels
    @IBOutlet weak var labelDisplay: UILabel!
    @IBOutlet weak var labelStatus: UILabel!

    // Code for the "Add 1" button
    @IBAction func buttonAddOne(sender: AnyObject) {
        numbers = numbers + 1
        labelDisplay.text = "\(numbers)"
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        if numbers == 5 {
            labelStatus.text = "Numbers variable is equal to 5! Hurray!"
        }
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}

Answer 1

I can't understand why the labelStatus isn't being updated when "numbers" reaches 5

What would update it? The code inside viewDidLoad ? But viewDidLoad is called once, early, when the view controller gets its view; it will never be called again, so that code will never run again.

If you want to update labelStatus , why don't you do it in the same code that increments numbers ?

@IBAction func buttonAddOne(sender: AnyObject) {
    numbers = numbers + 1
    labelDisplay.text = "\(numbers)"
    if numbers == 5 {
        labelStatus.text = "Numbers variable is equal to 5! Hurray!"
    }
}

Answer 2

The code you put in viewDidLoad is executed once, when the view is loaded - it isn't triggered every time numbers is updated.

To implement what you need, you should make number an instance property, and add a didSet observer:

class ViewController: UIViewController {
    var numbers: Int = 0 {
        didSet {
            if numbers == 5 {
                labelStatus.text = "Numbers variable is equal to 5! Hurray!"
            }
        }
    }
    ...
}

The property observer is automatically executed when the property value is changed.

For more info read Property Observers