Swift Playground: This code demonstrate deepest view controller to a certain view controller conforms to a certain protocol by taking advantage of responder chain.

delegate_chaining_workaround.swift

//	The MIT License (MIT)
//
//	Copyright (c) 2020 Electricwoods LLC, Kaz Yoshikawa.
//
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//	of this software and associated documentation files (the "Software"), to deal
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//	The above copyright notice and this permission notice shall be included in
//	all copies or substantial portions of the Software.
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//	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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//	THE SOFTWARE.



// This code demonstrates how to take advantage of using UIResponder chain to reach a certain responder
// skipping over multiple view controllers.  The deepest QuaternaryViewController trys to
// reach PrimaryViewController's save() method using UIResponder chain.

import UIKit
import PlaygroundSupport

extension UIResponder {
	func findResponder<T>() -> T? {
		var responder = self.next
		repeat {
			if let responder = responder as? T {
				return responder
			}
			responder = responder!.next
		} while responder != nil
		return nil
	}
}

protocol SaveProtocol: AnyObject {
	func save()
}

class PrimaryViewController : UIViewController, SaveProtocol {
	let secondaryViewController = SecondaryViewController()
	override func loadView() {
		let view = UIView()
		view.backgroundColor = .white
		
		let label = UILabel()
		label.frame = CGRect(x: 150, y: 200, width: 200, height: 20)
		label.text = "Hello World!"
		label.textColor = .black
		
		view.addSubview(label)
		self.view = view
	}
	override func viewDidLoad() {
		super.viewDidLoad()
		self.view.addSubview(self.secondaryViewController.view)
		self.addChild(self.secondaryViewController)
		self.secondaryViewController.didMove(toParent: self)
	}
	func save() {
		print("Saved !!")
	}
}
class SecondaryViewController: UIViewController {
	let tertiaryViewController = TertiaryViewController()
	override func viewDidLoad() {
		super.viewDidLoad()
		self.view.addSubview(self.tertiaryViewController.view)
		self.addChild(self.tertiaryViewController)
		self.tertiaryViewController.didMove(toParent: self)
	}
}
class TertiaryViewController: UIViewController {
	let quaternaryViewController = QuaternaryViewController()
	override func viewDidLoad() {
		super.viewDidLoad()
		self.view.addSubview(self.quaternaryViewController.view)
		self.addChild(self.quaternaryViewController)
		self.quaternaryViewController.didMove(toParent: self)
	}
}
class QuaternaryViewController: UIViewController {
	override func viewDidLoad() {
		super.viewDidLoad()
	}
	override func viewDidAppear(_ animated: Bool) {
		super.viewDidAppear(animated)
		self.save()
	}
	func save() {
		if let saver = self.findResponder() as SaveProtocol? {
			saver.save()
		}
	}
}

PlaygroundPage.current.liveView = PrimaryViewController()