How do you change an outside variable inside a function passing a parameter Noobie in swift

Question

How do you change an outside variable inside a function with passing a parameter?; Noobie in swift

var varIsTrue:Bool = true

import UIKit
extension UIDevice {

class ViewController: UIViewController {
 override func viewDidLoad() {

  changeVar(varIsTrue , false)
     
 }

 func changeVar(_ varName:? , _ arg:Bool){
         varName = arg // Xcode cannot assign to value: 'varName' is a 'let' constant

             }
// varName:? Don't know if the question mark should be a :String some kind of argument or what?
           }
      }

Answer 1

Assuming your example is just an abstraction and you're trying to understand the concept of changing a variable from within a function this would be a solution.

Like already mentioned in the comments you need to use inout and pass the reference (indicated by the leading &) of your "outside variable".

var varIsTrue: Bool = true

changeVar(&varIsTrue , false)

func changeVar(_ property: inout Bool , _ arg: Bool){
    property = arg
}