快速覆盖方法

Question

Basically I am currently adding functionX to everywhere i present an UIAlertController as seen below:

let alert = UIAlertController(title: "", message: "", preferredStyle: .alert)
let okAction = UIAlertAction(title: "ok", style: .default)
alert.addAction(okAction)
functionX(actionSheet: alert, controller: self)
self.present(alert, animated: true, completion: nil)
// or it can be
// tableviewController.present(alert, animated: true, completion: nil)

Instead of calling functionX every time, I want to override a present method and call functionX there. I attempted the following:

extension UIViewController {
    override func present(_ viewControllerToPresent: UIViewController, animated flag: Bool, completion: (() -> Void)? = nil) {
        if (viewControllerToPresent is UIAlertController) {
            functionX(actionSheet: viewControllerToPresent, controller: /* what should this be? */ )
        }
        super.present() //error here
    }
}

Is this an appropriate approach? Can you help me fill the missing parameters?

ie:

• What should be the controller? What would self or tableviewController from the first code stub be in the overriding present function?

• How should I call the present method in the overriding present function?

Answer 1

According to the Swift guide ,

Extensions can add new functionality to a type, but they cannot override existing functionality.

So you shouldn't really be overriding an existing method in a UIViewController in an extension.

What you could do is, to add your own present , called functionXAndPresent :

extension UIViewController {
    func functionXAndPresent(_ viewControllerToPresent: UIViewController, animated flag: Bool, completion: (() -> Void)? = nil) {
        if (viewControllerToPresent is UIAlertController) {
            // to answer your second question, you should use "self" here
            functionX(actionSheet: viewControllerToPresent, controller: self)
        }
        present(viewControllerToPresent, animated: flag, completion: completion)
    }
}

You can't do this by overriding because as you have found out, you can't really refer to the "non-overridden" method at the end. super.present doesn't work because you are in an extension, not a subclass.

Answer 2

You can simply create a common method showAlert(with:and:) in a UIViewController extension and call functionX when the alert is presented, ie

extension UIViewController {
    func showAlert(with title: String?, message: String?) {
        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        let okAction = UIAlertAction(title: "OK", style: .default)
        alert.addAction(okAction)
        self.present(alert, animated: true, completion: {
            self.functionX(actionSheet: alert, controller: self)
        })
    }

    func functionX(actionSheet: UIAlertController, controller: UIViewController) {
        //your code here...
    }
}

Usage:

Call the showAlert(with:and:) method from whatever controller you want to, be it a UIViewController or a UITableViewController or any other, ie

self.showAlert(with: "Alery..!!!", message: "This is a sample alert.")

Answer 3

Overrides of NSObject's derivatives in Swift's static extensions is only for Objective-C compatibility. You cannot override in extensions of pure Swift declarations. Think of it in such manner that if the class itself adds an override, and then a static extension adds an override. Which implementation should the linker link? And which implementation does super call refer to? In Objective-C this behavior is undefined, in Swift the extension override is ignored all together.

What you could do instead is move the overriding function from the extension directly to the class.