Sample app that demoes events flowing through the responder chain

ResponderChain.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
  var window: UIWindow?
  
  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    window = Window()
    window?.rootViewController = ViewController()
    window?.makeKeyAndVisible()
    return true
  }
  
  @objc func `do`(_ sender: Any, _ event: UIEvent?) {
    print(Self.self, #function, sender, event as Any)
  }
}

@objc class Window: UIWindow {
  @objc func `do`(_ sender: Any, _ event: UIEvent?) {
    print(Self.self, #function, sender, event as Any)
  }
  
  //    override func canPerformAction(_ action: Selector, withSender sender: Any?) -> Bool {
  //        print(action, sender as Any)
  //        return false
  //    }
}

@objc class ViewController: UIViewController {
  override func loadView() {
    view = Control(frame: .zero)
  }
  
  //    @objc func `do`(_ sender: Any, _ event: UIEvent?) {
  //        print(Self.self, #function, sender, event as Any)
  //    }
}

@objc class Control: UIControl {
  override init(frame: CGRect) {
    super.init(frame: frame)
    
    backgroundColor = .systemBackground
    
    let tapAction = #selector(tapped)
    let tapRecognizer = UITapGestureRecognizer(target: self, action: tapAction)
    addGestureRecognizer(tapRecognizer)
  }
  
  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
  
  @objc func tapped(_ event: UIEvent?) {
    sendAction(Selector("do::"), to: nil, for: UIEvent())
  }
  
  //    @objc func `do`(_ sender: Any, _ event: UIEvent?) {
  //        print(Self.self, #function, sender, event as Any)
  //    }
}