Help for dynamicReplacement

Using Swift
1

I have some questions when use dynamicReplacement.

1.Why we need dynamicReplacement?

class ViewController: UIViewController {
}

extension UIViewController {
    @_dynamicReplacement(for: viewDidLoad())
    func _viewDidLoad() {
        print("view didLoad")
        viewDidLoad()
    }
}

Nothing happens.

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

extension ViewController {
    @_dynamicReplacement(for: viewDidLoad())
    func _viewDidLoad() {
        print("view didLoad")
        viewDidLoad()
    }
}

This will print "view didLoad".
So why not just

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        print("view didLoad")
    }
}

class ViewController: UIViewController {
}

extension ViewController {
    @_dynamicReplacement(for: viewDidLoad())
    func _viewDidLoad() {
        print("view didLoad")
        viewDidLoad()
    }
}

It will print "view didLoad" endless, why?

2

Seems you can't use @_dynamicReplacement with UIViewController because it's methods would need to be @dynamic which they aren't.

3

You rarely rarely need it. Normally you just subclass and override relevant methods.

See some details about _dynamicReplacement here.