Transition with completion

Playground.swift

//: Playground - noun: a place where people can play

import UIKit
import XCPlayground
import PlaygroundSupport

extension UINavigationController {
    func popViewController(completion: @escaping () -> ()) {
        CATransaction.begin()
        CATransaction.setCompletionBlock(completion)
        popViewController(animated: true)
        CATransaction.commit()
    }
}

class MyNavController: UINavigationController {

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

}

class MyViewController: UIViewController {

    lazy var button: UIButton = {
        let button = UIButton(frame: CGRect(x: 0, y: 50, width: 100, height: 50))
        button.backgroundColor = .blue
        button.setTitle("Tap Me", for: .normal)
        button.setTitleColor(.white, for: .normal)
        button.addTarget(self, action: #selector(pop), for: .touchUpInside)
        return button
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        view.addSubview(button)
    }

    func pop() {
        navigationController?.popViewController() {
            print("popped")
        }
    }

}

class MyOtherViewController: UIViewController {

    lazy var button: UIButton = {
        let button = UIButton(frame: CGRect(x: 0, y: 50, width: 100, height: 50))
        button.backgroundColor = .blue
        button.setTitle("Tap Me", for: .normal)
        button.setTitleColor(.white, for: .normal)
        button.addTarget(self, action: #selector(push), for: .touchUpInside)
        return button
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        view.addSubview(button)
    }

    func push() {
        let vc = MyViewController()
        navigationController?.pushViewController(vc, animated: true)
        print("pushed")
    }

}

let vc = MyOtherViewController()
let navController = MyNavController(rootViewController: vc)
//let myvc = MyViewController()
//navController.pushViewController(myvc, animated: true)
PlaygroundPage.current.liveView = navController