AppDelegate.swift

//
//  AppDelegate.swift
//  Adds observer to detect changes in the *rootViewController* of the main window.
//

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

  var window: UIWindow?

  override init() {
    super.init()
    addObserver(self, forKeyPath: #keyPath(window.rootViewController), options: .new, context: nil)
  }

  func application(
    _ application: UIApplication, 
    didFinishLaunchingWithOptions 
    launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    // Override point for customization after application launch.

    let rootVC = AppNavigator.shared.rootViewController
    window?.rootViewController = rootVC

    return true
  }

  override func observeValue(
    forKeyPath keyPath: String?, 
    of object: Any?, 
    change: [NSKeyValueChangeKey: Any]?, 
    context: UnsafeMutableRawPointer?
  ) {
    // Check for the view controller change desired
    if let firstRoot = window?.rootViewController, keyPath == #keyPath(window.rootViewController) {
      //FIRST ROOT VIEW CONTROLLER
      print(firstRoot)
      //Remove observer if all you need is to know about the first VC
      removeObserver(self, forKeyPath: #keyPath(window.rootViewController))

      // You can also detect other changes like .old
    }
  }
}