completion handler for a function that takes a parameter

countAndUpdateUI.swift

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        DispatchQueue.global(qos: .userInteractive).async {
            //Background thread
            
            // NOTE1: remove the `func` keyword and store the result
            let result = countTo(10) //not sure how to wait for this to be finished before updating the button label on main thread
          
            DispatchQueue.main.async {
                //Update Button Label after count is finished
                
                // NOTE2: This captures both the result and `self`, which is ok in this case
                self.label.text = result
            }
        }
        

    }

    func countTo(number: Int) -> Int {
        var counter = 0
        
        while counter < number {
            counter += 1
            print(counter)
        }
        
        return counter
    }

    
}