import Foundation

class ValuesModel {
    var value1 = ""
    var value2 = ""

    init(v1: String, v2: String) {

    self.value1 = v1
    self.value2 = v2
    }

    func getValues() {
        NotificationCenter.default.addObserver(forName: .saveValue1, object: nil, queue: OperationQueue.main) {
            (notification) in
            let val1 = notification.object as! Value1ViewController
            self.value1 = val1.v1
            UserDefaults.standard.set(self.value1, forKey: "value1")
        }

        NotificationCenter.default.addObserver(forName: .saveValue2,  object: nil, queue: OperationQueue.main) {
            (notification) in
            let val2 = notification.object as! Value2ViewController
            self.value2 = val2.v2
            UserDefaults.standard.set(self.value2, forKey: "value1")
        }
    }

}

import UIKit

class MainViewController: UIViewController {

    @IBOutlet weak var calcLbl: UILabel!
    @IBOutlet weak var value1: UILabel!
    @IBOutlet weak var value2: UILabel!

    var data = calculations()

    override func viewDidLoad() {
        super.viewDidLoad()

        NotificationCenter.default.addObserver(forName: .saveValue1, object: nil, queue: OperationQueue.main) {
            (notification) in
            let val1 = notification.object as! Value1ViewController
            self.value1.text = val1.v1
            UserDefaults.standard.set(self.value1.text, forKey: "value1")
        }

        NotificationCenter.default.addObserver(forName: .saveValue2, object: nil, queue: OperationQueue.main) {
            (notification) in
            let val2 = notification.object as! Value2ViewController
            self.value2.text = val2.v2
            UserDefaults.standard.set(self.value2.text, forKey: "value1")
        }
    }

    override func viewWillAppear(_ animated: Bool) {
        if let lastValue1 = UserDefaults.standard.object(forKey: "value1") as? String {
            value1.text = lastValue1
        }
        if let lastValue2 = UserDefaults.standard.object(forKey: "value2") as? String {
            value2.text = lastValue2
        }
    }

    @IBAction func calculateBtn(_ sender: UIButton) {
        calcLbl.text = data.calc1()

    }


}

import UIKit

class Value1ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var value1: UITextField!
    @IBOutlet weak var popUpView: UIView!

    var v1 = ""

    override func viewWillAppear(_ animated: Bool) {
        if let lastValue1Data = UserDefaults.standard.object(forKey: "value1") as? String {
            value1.text = lastValue1Data
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        popUpView.layer.cornerRadius = 10
        popUpView.layer.masksToBounds = true
    }

    @IBAction func done_touchUpInside(_ sender: UIButton) {
        v1 = value1.text!
        NotificationCenter.default.post(name: Notification.Name.saveValue1, object: self)
        dismiss(animated: true, completion: nil)
        UserDefaults.standard.set(self.v1, forKey: "value1")
    }
}

import Foundation

class calculations {
    let v: ValuesModel = ValuesModel(v1: "", v2: "")

    func calc1() -> String {
        let x = Double(self.v.value1)!
        let y = Double(self.v.value2)!
        let z = x + y
        return "(z)"
    }
}