InjectedViewController.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    
    class InjectedViewController: UIViewController {
        override func viewWillAppear(_ animated: Bool) {
            super.viewWillAppear(animated)
            print("INJECTED VIEW WILL APPEAR")
        }
    }

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        let injected = InjectedViewController()
        let simple = UIViewController()
        simple.view.setValue(injected, forKeyPath: "_viewDelegate")
        window?.addSubview(simple.view)
        return true
    }
}