弱委托和.xib文件

Question

Why are weak delegates not neeeded when using.xib files?

I made a simple project that doesn't use storyboards. It just uses.xib files associated with UIViewControllers. It presents the user with a button that when pressed, pushes a new viewcontroller and allows them to go back. If the navDelegate below is listed as weak, then navigation to the DetailViewController never happens since navDelegate will always be nil.

SceneDelegate.swift

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?


    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let winScene = scene as? UIWindowScene else {
            return
        }
        window = UIWindow(windowScene: winScene)
        AppCoordinator().start()
    }
}

AppCoordinator.swift

import UIKit

protocol Coordinator: AnyObject {
    func start()
}

class AppCoordinator: Coordinator {
    let router = UINavigationController()

    func start() {
        let vc = RootViewController()

        vc.navDelegate = self
        router.viewControllers = [vc]
        let window = UIApplication.shared.windows.first
        window?.rootViewController = router
        window?.makeKeyAndVisible()
    }
}

extension AppCoordinator: NavDelegate {
    func buttonTapped() {
        let vc = DetailViewController()
        router.pushViewController(vc, animated: true)
    }
}

RootViewController.swift

import UIKit

protocol NavDelegate: AnyObject {
    func buttonTapped()
}

class RootViewController: UIViewController {

    // Does this navDelegate need to be weak when using .xib files?
    var navDelegate: NavDelegate?

    init() {
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    @IBAction func tapDetailButton(_ sender: Any) {
        // Always prints 2 no matter how many times I navigate back and forth
        let refCount = CFGetRetainCount(navDelegate)
        print("Ref Count: \(refCount)")
        navDelegate?.buttonTapped()
    }
}

DetailViewController.swift

import UIKit

class DetailViewController: UIViewController {

    init() {
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
    }

}

So with further insight from @matt, it seems that if you have the AppCoordinator as an instance variable of the SceneDelegate then having the navDelegate as weak works. I suppose the mistake is calling the navDelegate a delegate. It should just be called something like a navHelper so there's no confusion as to whether it should be weak or not.

Answer 1

Nothing to do with xibs. Or delegates either, really.

If you know what weak means, you know that it means "If no one else is holding a strong reference to this thing, let it go out of existence."

So when you say vc.navDelegate = self , where self in AppCoordinator instance, then if no other reference to this AppCoordinator instance retains it, it vanishes in a puff of smoke immediately afterward.

Well, what's the life story of this instance?

 AppCoordinator().start()

What does that line do? It creates an AppCoordinator instance, calls start on it, and then drops the AppCoordinator on the floor, letting it go out of existence if no one else is holding on to it. And no one else is holding on to it, because the only reference that anyone else had to it was weak .