Swift：按下按钮时编辑Label的值

Question

I'm trying to learn Swift and I'm playing around with the basics of variables. I made an app where when you press a button, the value of a label changes, and while the button that adds 1 works, the one that subtracts one doesn't. If I change the first button (plusone) to subtract one, then it works, so I assume it is something to do with how the code is parsed? Would you be kind enough to explain how this works? Thanks in advance.

The code is here:

import UIKit

class ViewController: UIViewController {

    var value = 0
    @IBOutlet var number: UILabel!

    @IBAction func plusone(_ sender: Any) {
        value = value+1
        number.text = "\(value)"
    }

    @IBAction func plustwo(_ sender: Any) {
        value = value-1
        number.text = "\(value)"
    }

}

And the app looks like this if that matters: Screenshot

The second button is named 'plustwo' because I haven't figured out where else I need to rename it, if I just rename it here the app displays a blank screen and nothing else.

Answer 1

From the comments:

I changed plustwo to -2 and it works as intended. Pressing Add One adds one and pressing Substract One substracts one. I am not sure why.

The behavior you describe tells me that you have your Subtract One button wired to both the plusone and plustwo functions. You can check this by control -clicking on your button in Interface Builder and checking the connections. If you see TouchUpInside wired to two functions, remove one by clicking on the x to delete the connection.

Another way you can verify the behavior is to add logging print statements to your functions such as print("in plusone") and print("in plustwo") . When you press Subtract One , if you see both messages in the console then you know you have your button connected to both @IBAction s.

How did this happen? This frequently happens when you copy a button in Interface Builder that already has a connection to an @IBAction . Connecting it to a new @IBAction doesn't replace the first one, it just connects to both of them and calls both when the button is pressed.

The second button is named 'plustwo' because I haven't figured out where else I need to rename it, if I just rename it here the app displays a blank screen and nothing else.

To rename the function, you need to change the name of the function and remove and reconnect the connection in Interface Builder . As described above, control -click on the button in Interface Builder and remove the connection to the old function name, then connect the button to the new function.