コールバック関数（クロージャ）を使用した値渡しのサンプルを作成してみた

import UIKit

class FirstViewController: UIViewController {


    @IBOutlet weak var label: UILabel!{
        didSet{
            label.text = "0"

        }
    }


    @IBAction func nextButton(_ sender: UIButton) {

            let secondStoryboard = UIStoryboard(name: "Second", bundle: nil)
            let secondVC = secondStoryboard.instantiateInitialViewController() as!SecondViewController


        secondVC.completion = { CountModel in
            print("ーーーー渡されたよーーーーーーー")
            self.label.text = CountModel.count.description
    }

        let nav = self.navigationController!
        nav.pushViewController(secondVC, animated: true)

    }

    override func viewDidLoad() {
        super.viewDidLoad()

 }

}

import UIKit

class SecondViewController: UIViewController{


   private var countmodel = CountModel.init(count: 0)
    var completion:((CountModel)->Void)?

    @IBOutlet weak var countLabel: UILabel!{
        didSet{
            countLabel.text = countmodel.count.description
        }
    }

    @IBAction func countButtun(_ sender: UIButton) {
        countmodel.count = countmodel.count + 1
        countLabel.text = countmodel.count.description
        print(countmodel)
    }


    @IBAction func addButton(_ sender: Any) {
        tap()
        self.navigationController?.popViewController(animated: true)

    }

    //クロージャー関数
    func tap(){
        print("tapしたよ")
        print(countmodel)
       completion?(countmodel)
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        }


    }

import Foundation

struct CountModel {
    var count:Int
}