How can I make sure that after declaring a delegate of a protocol in Swift 5.2, that delegate is not nil when it is called? - swift5

Question

I am trying to clear a textfield in MainViewController from the DetailViewController. I have the following code in a Swift Playground.
import UIKit
import PlaygroundSupport
protocol DetailViewControllerDelegate: class {
func bar()
}
class DetailViewController: UIViewController {
var detailViewControllerDelegate: DetailViewControllerDelegate!
override func loadView() {
let view = UIView()
view.backgroundColor = .white
let button = UIButton()
button.frame = CGRect(x: 100, y: 200, width: 180, height: 20)
button.setTitle("Hello World!", for: .normal)
button.backgroundColor = .blue
button.addTarget(self, action: #selector(handlePress), for: .touchUpInside)
view.addSubview(button)
self.view = view
}
#objc func handlePress() {
print("\(#function)")
if let vrvc = detailViewControllerDelegate {
vrvc.bar()
} else {
print("detailViewControllerDelegate is NIL")
}
}
}
class MainViewController : UIViewController, DetailViewControllerDelegate {
func bar() {
print("Inside Bar")
}
override func loadView() {
let detailViewController = DetailViewController()
detailViewController.detailViewControllerDelegate = self
}
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = DetailViewController()
When the button is pressed, detailViewController is nil. How can I make sure that detailViewController is NOT nil when the button is pressed?

Answer 1

You have two distinct DetailViewControllers:
override func loadView() {
let detailViewController = DetailViewController()
detailViewController now references a new instance of DetailViewController
detailViewController.detailViewControllerDelegate = self
}
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = DetailViewController()
liveView now references a new instance of DetailViewController which is different from the one above and in which you have not set detailViewControllerDelegate.
If you need to create a DetailViewController in one place and reference it in another you need to store a reference to it in a property that is accessible in both places.
That said, the second instance of DetailViewController is being created in a statement which looks like an attempt to test code in the Playground so maybe you just need to think about how you are testing.