App running in background or when the screen is locked

Hi,

My app start to count the seconds when oppened. Follow the code:

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var label1: UILabel! //Display showing the seconds

var i = 0 //Counter

override func viewDidLoad() {

super.viewDidLoad()

// After loading the view.

Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(Estouro), userInfo: nil, repeats: true)

}

@objc func Estouro(){

i = i + 1

label1.text = String(i) //Display seconds

}

}

What can I do to keep counting the secounds in background or after lock the screen?

Thank you,