Add button doesn't add text to UITextView in swift - swift

Question

I'm testing the my add button in the view controller file. It should go to validateName function in my model file and which returns true and then enables my addButton. But when I click on "Add" it doesn't change the text of the UITextView.
In viewcontroller.swift:
import UIKit
class ViewController: UIViewController,UITextFieldDelegate{
#IBOutlet weak var nameInputText: UITextField!
#IBOutlet weak var usernameInputText: UITextField!
#IBOutlet weak var passwordInputText: UITextField!
#IBOutlet weak var phoneInputText: UITextField!
#IBOutlet weak var emailInputText: UITextField!
#IBOutlet weak var addButton: UIButton!
#IBOutlet weak var textviewText: UITextView!
let model = Model()
#IBAction func addButton(_ sender: UIButton) {
textviewText.text = "hi"
}
func textField(_ textField: UITextField,
shouldChangeCharactersIn range: NSRange,
replacementString string: String)
-> Bool
{
addButton.isEnabled = false
let text: String = nameInputText.text!
var test_name: Bool
var name_array = [String]()
test_name = model.validateName(nametext: text)
if test_name == true{
addButton.isEnabled = true
name_array.append(text)
return true
}
return (true)
}
override func viewDidLoad() {
super.viewDidLoad()
usernameInputText.delegate = self
nameInputText.delegate = self
passwordInputText.isSecureTextEntry = true
}
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
self.view.endEditing(true)
}
func textFieldShouldReturn(_ textField: UITextField) -> Bool{
usernameInputText.resignFirstResponder()
nameInputText.resignFirstResponder()
return (true)
}
}
//In model.swift:
import Foundation
class Model {
func validateName(nametext: String) -> Bool{
return true
}
}