How to tell when controller has resumed from background?

README.md

background.swift

import UIKit

class ViewController: UIViewController {
    
    @IBOutlet weak var pasteLabel: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(appWillEnterForeground),
                                               name: UIApplication.willEnterForegroundNotification,
                                               object: nil)
    }
    
    
    @objc func appWillEnterForeground() {
        // run when app enters foreground
        @IBOutlet weak var pasteLabel: UILabel!
        
        let pasteboardString: String? = UIPasteboard.general.string
        if let theString = pasteboardString {
            pasteLabel.text = theString
        }
    }
    
}