Passing Data Between viewControllers Programmatically

import UIKit

@UIApplicationMain class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

window = UIWindow(frame: UIScreen.main.bounds)
  window?.makeKeyAndVisible()

let navigationController = UINavigationController(rootViewController: MainViewController())

window?.rootViewController = navigationController

window?.backgroundColor = UIColor.yellow

application.statusBarStyle = .lightContent

  return true
}

}

import UIKit

class MainViewController: UIViewController {

let label = UILabel()

let textField = UITextField()

override func viewDidLoad() {

super.viewDidLoad()

  view.backgroundColor = UIColor.gray
  setupLabel()
  setupTextField()
  setupButton()
}

func setupLabel() {

  label.frame = CGRect(x: 40, y: 80, width: 300, height: 60)
  label.text = "welcome to my world"
  label.textColor = UIColor.yellow
  label.font = UIFont.boldSystemFont(ofSize: 25)
  label.textAlignment = .center
  label.layer.borderWidth = 2
  label.layer.borderColor = UIColor.yellow.cgColor
  label.layer.cornerRadius = 5
  view.addSubview(label)
}

func setupTextField() {

  textField.frame = CGRect(x: 10, y: 200, width: self.view.frame.size.width - 20, height: 60)
  textField.placeholder = "text here"
  textField.textAlignment = .center
  textField.font = UIFont.systemFont(ofSize: 25)
  textField.layer.borderWidth = 2
  textField.layer.borderColor = UIColor.yellow.cgColor
  textField.layer.cornerRadius = 5
  view.addSubview(textField)
}

func setupButton() {

  let button = UIButton()
  button.frame = CGRect(x: 50, y: 300, width: self.view.frame.size.width - 100, height: 60)
  button.setTitle("Enter", for: .normal)
  button.setTitleColor(UIColor.yellow, for: .normal)
  button.layer.borderWidth = 2
  button.layer.borderColor = UIColor.yellow.cgColor
  button.layer.cornerRadius = 5
  button.addTarget(self, action: #selector(buttonTarget), for: .touchUpInside)
  view.addSubview(button)
}

func buttonTarget() {

let vcPass = SecondViewController()
vcPass.stringPassed = textField.text!
self.navigationController?.pushViewController(vcPass, animated: true)
}

}

import UIKit

class SecondViewController: UIViewController {

let secondLabel = UILabel()

var stringPassed: String?

override func viewDidLoad() {
  super.viewDidLoad()

setupLabelSecond()

  print(stringPassed)
}

func setupLabelSecond() {

secondLabel.frame = CGRect(x: 40, y: 80, width: 150, height: 60)

secondLabel.text = stringPassed

  secondLabel.textColor = UIColor.green
  secondLabel.font = UIFont.boldSystemFont(ofSize: 25)
  secondLabel.textAlignment = .center
  secondLabel.layer.borderWidth = 2
  secondLabel.layer.borderColor = UIColor.green.cgColor
  secondLabel.layer.cornerRadius = 5
  view.addSubview(secondLabel)
}

}