Calculator app does not get multiple-digit numbers and operations return zero at all times?

I'm using Swift 4 and the numbers are all in Int.

So I made a calculation class file:

import Foundation
import UIKit

class Calculation {

    var result = Int()
    var operationButton = String()
    var currentNumber = Int()

    func calculator() {

        switch operationButton {
        case "+":
            result = result + currentNumber
        case "-":
            result = result - currentNumber
        case "*":
            result = result * currentNumber
        case "/":
            result = result / currentNumber
        case "=":
            result = currentNumber
        default:
            break
        }

    }

        func textClear() {
            result = 0
            currentNumber = 0
            operationButton = "="
    }

And then I made 3 functions in UIControllerView for AC button, numbers button, and operations button to get the "0-9" and "-,+,*,/,=" directly from the button title.

    class ViewController: UIViewController {


    @IBOutlet weak var resultTextField: UITextField!

    var calculation = Calculation()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        calculation.operationButton = "="
        resultTextField.text = "\(calculation.result)"
    }


    @IBAction func allClearButtonPressed(_ sender: UIButton) {
        calculation.textClear()
        resultTextField.text = "\(calculation.result)"
    }

    @IBAction func operationButtonPressed(_ sender: UIButton) {

        calculation.operationButton = sender.titleLabel!.text as String!
            calculation.operationButton = sender.titleLabel!.text!
        calculation.calculator()
        resultTextField.text = "\(calculation.result)"
    }

    @IBAction func numberButtonPressed(_ sender: UIButton) {

        calculation.currentNumber = Int((sender.titleLabel?.text)!)!
        let numberInput = String(calculation.currentNumber)
        resultTextField.text = numberInput
    }
}

First problem is that whenever I try to put a number e.g 21, it just returns 2, and then replaced by 1. The second problem would be the operators which always return zero to the "resultTextField".

Your numberButtonPressed function reads the text of the button, replaces the currentNumber and then assign it to the resultTextField.text. What you want is a function that adds the new digit to your current number. Inside your Calculator class.

func pressedDigit(_ digit: Int) {
    currentNumber = currentNumber * 10 + digit
}

Then you can call this function and assign the currentNumber to your resultTextField.

@IBAction func numberButtonPressed(_ sender: UIButton) {
    guard let digit = sender.titleLabel?.text?.flatMap({ Int(String($0)) }) else { return }
    calculation.pressedDigit(value: digit)
    resultTextField.text = "\(calculation.currentNumber)"
}

You also have to reset currentNumber after each calculation.