Notification not triggering when view controller updated on Navigation stack changed - ios

Question

I'm trying to send a notification on changing the navigation stack update. But it's not triggered. Here is my code. I have a requirement to change the root view controller on button action. I'm trying the below code, but it's not working for me.
import UIKit
class SecondViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
}
#IBAction func replaceThirdViewControllerAsNavigationRoot() {
let nc = NotificationCenter.default
nc.post(name: Notification.Name("Notify"), object: nil)
self.navigationController?.viewControllers = [ThirdViewController.instance()]
}
}
class ThirdViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let nc = NotificationCenter.default
nc.addObserver(self, selector: #selector(userLoggedIn), name: Notification.Name("Notify"), object: nil)
}
#objc func userLoggedIn() {
print("-----")
}
}
extension UIViewController {
static func instance<T: UIViewController>() -> T {
let name = String(describing: self)
guard let controller = UIStoryboard.main.instantiateViewController(withIdentifier: name) as? T else {
fatalError("ViewController '\(name)' is not of the expected class \(T.self).")
}
return controller
}
}
extension UIStoryboard {
static var main: UIStoryboard {
return UIStoryboard.init(name: "Main", bundle: nil)
}
}
Thanks in advance.

Answer 1

The sequence of event is wrong. When you post a notification, you need to make sure that an observer already exists, otherwise the notification will be discarded.
In other words: make sure that
nc.addObserver(self, selector: #selector(userLoggedIn), name: Notification.Name("Notify"), object: nil)
runs before
nc.post(name: Notification.Name("Notify"), object: nil)