Overriding at-objc methods defined in extensions without the dynamic keyword

objc-extension-override-without-dynamic.swift

// Paste this into an iOS playground in Xcode and run
// Expected output:
// ParentVC: handled in ParentCoordinator
// ChildVC: handled in ChildCoordinator
// UIViewController: accountLogin() not handled

import UIKit

NSSetUncaughtExceptionHandler { exception in
    print("Exception: \(exception.name) — \(exception)")
    print("Call stack:\n\(exception.callStackSymbols.joined(separator: "\n"))")
}

extension UIResponder {
    @objc var coordinatingResponder: UIResponder? {
        return next
    }

    @objc /* no dynamic */ func accountLogin() -> String {
        return coordinatingResponder?.accountLogin() ?? "\(#function) not handled"
    }
}

class ParentCoordinator: UIResponder {
    override func accountLogin() -> String {
        return "handled in ParentCoordinator"
    }
}

class ChildCoordinator: UIResponder {
    override func accountLogin() -> String {
        return "handled in ChildCoordinator"
    }
}

class ParentVC: UIViewController {
    let coordinator = ParentCoordinator()
    let child = ChildVC(nibName: nil, bundle: nil)

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .yellow
        view.addSubview(child.view)
        child.view.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            child.view.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
            child.view.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 100),
            child.view.widthAnchor.constraint(equalToConstant: 200),
            child.view.heightAnchor.constraint(equalToConstant: 200)])
        addChild(child)
        child.didMove(toParent: self)
    }

    override var coordinatingResponder: UIResponder? {
        return coordinator
    }

    func didTapLogin() {
        print("ParentVC:", accountLogin())
    }
}

class ChildVC: UIViewController {
    let coordinator = ChildCoordinator()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .green
    }

    override var coordinatingResponder: UIResponder? {
        return coordinator
    }

    func didTapLogin() {
        print("ChildVC:", accountLogin())
    }
}

let parentVC = ParentVC(nibName: nil, bundle: nil)

import PlaygroundSupport
PlaygroundPage.current.liveView = parentVC

parentVC.didTapLogin()
parentVC.child.didTapLogin()

let uivc = UIViewController(nibName: nil, bundle: nil)
print("UIViewController:", uivc.accountLogin())