Protocols are not working in Swift (Getting nil) - ios

Question

Refernce image Im new to protocols, I tried the following implementation to achieve the protocols, but im getting nil value for the protocol object
import UIKit
//MARK: step 1 Add Protocol here.
protocol MyDelegate: class {
func changeBackgroundColor(_ color: UIColor?)
}
class ViewController: UIViewController {
//MARK: step 2 Create a delegate property here.
weak var delegate: MyDelegate?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
//MARK: step 3 Add the delegate method call here.
delegate?.changeBackgroundColor(UIColor.red)
}
}
Here I am getting delegate value nil and protocol not getting called.
here is the implementation
import UIKit
class HomeViewController: UIViewController, MyDelegate {
func changeBackgroundColor1(_ color: UIColor?) {
self.view.backgroundColor = color
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
callVC()
}
func callVC() {
let vc = ViewController()
vc.delegate = self
}
func changeBackgroundColor(_ color: UIColor?) {
self.view.backgroundColor = color
}

Answer 1

In short:
You have forgotten to set the delegate to the object that conforms the protocol.
Detailed Answer:
Protocol is an agreement between two objects so that you know a delegate will have some specified functions ready to be called. Let's explain with an example:
Suppose that you have two view controllers called A and B. You have defined your protocol in global (as you did in your code). Then you create a property called delegate in A which will hold a weak reference of any object conforms the protocol. In this case, this held object reference is B.
So you need to have a property in A like below:
weak var delegate: MyDelegate?
Then determine this delegate to be the reference of what you need. In our example, it's B. So you need to set it in B as below.
// Somewhere you have the reference of the object or where you initialize it.
instanceOfA.delegate = self
Finally you conform the protocol in B like:
extension B: MyProtocol {
func changeBackgroundColor(_ color: UIColor?) {
// some implementation goes here
}
}
There you are. Now, you can make sure that you have delegate object and protocol methods are getting called if you have completed steps above correctly.

Answer 2

The problem lies in your callVC method:
func callVC()
{
let vc = ViewController()
vc.delegate = self
}
You create an instance of your ViewController, but you are not doing anything with it. No methods, aside from initializer, will be called on that view controller because it's not a part of active navigation stack. Also, since it's just a local variable (not retained anywhere) it will be deallocated immediately after leaving the method scope.
What you need to do is to present the view controller somehow - either with a navigation controller, or as a child to the current view controller
func callVC()
{
let vc = ViewController()
vc.delegate = self
addChildViewController(vc)
view.addSubview(vc.view)
vc.didMove(toParentViewController: self)
}
or if you're using navigation controller
func callVC()
{
let vc = ViewController()
vc.delegate = self
navigationController?.pushViewController(vc, animated: true)
}

Answer 3

Hi Smart i think the problem is when you set the delegate
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
callVC()
}
func callVC() {
let vc = ViewController()
vc.delegate = self
}
Because you set the delegate for the class ViewController, but you don't present the viewController at all; you should set the delegate before presenting the ViewController, one way to do it is to present the ViewController and in function callVc add this
func callVC() {
let vc = ViewController()
vc.delegate = self
//Present the viewController with this
present(vc, animated: true, completion: nil)
}

Answer 4

Your optional delegate variable is nil. You need to set it first from the UIViewController you are segueing from. For example.
class OtherViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
self.performSegue(withIdentifier: "addHereTheSegueIdFromStoryBoard", sender: nil)
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "addHereTheSegueIdFromStoryBoard" {
guard let vc = segue.destination as? ViewController else {return}
vc.delegate = self
}
}
}

Answer 5

import UIKit
class ViewController: UIViewController {
weak var delegate: MyDelegate?
override func viewDidLoad() {
super.viewDidLoad()
if delegate != nil {
delegate?.changeBackgroundColor(UIColor.red)
}
}
}
Call like
class SecondViewController: UIViewController(), MyDelegate {
func callVC() {
let vc = ViewController()
vc.delegate = self
}
func changeBackgroundColor(_ color: UIColor?) {
//Code here
}
}