Swift重写协议扩展保持扩展行为

Question

I have this simple class and an hypothetical protocol called 'ShowAlert', it's extension to do the default implementation and a default ViewController and it's ShowAlert protocol implementation.

protocol ShowAlert {
    var titleForAlert: String! { get }
    func messageForAlert() -> String!
    func show()
}

extension ShowAlert where Self: UIViewController {

    func show(){
        let alert = UIAlertController(title: self.titleForAlert, message: self.messageForAlert(), preferredStyle: .Alert)
        alert.addAction(UIAlertAction(title: "Ok", style: .Cancel, handler: nil))
        self.presentViewController(alert, animated: true, completion: nil)
    }
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

    }


    @IBAction func showItNow(sender: AnyObject) {
        self.show()
    }



}

extension ViewController: ShowAlert {
    var titleForAlert: String! {
        get{
            return "Foo"
        }
    }


    func messageForAlert() -> String! {
        return "Bar"
    }

    func show() {
    // here I want to call the default implementation of the protocol to show the alert, then do something else
        print("Good day sir!")
    }
}

It's like on a subclass where I could call a 'super.show()' and then continue implementing whatever I want to do after that.

There's any way to do it? Or my logic go against what protocols are design for and that don't suppose to happen?

Answer 1

There is a simple solution: Just add a defaultShow method to the extension.

extension ShowAlert where Self: UIViewController {

    func defaultShow(){
        let alert = UIAlertController(title: self.titleForAlert, message: self.messageForAlert(), preferredStyle: .Alert)
        alert.addAction(UIAlertAction(title: "Ok", style: .Cancel, handler: nil))
        self.presentViewController(alert, animated: true, completion: nil)
    }

    func show() {
        defaultShow()
    }
}

So in your code you can just call defaultShow :

extension ViewController: ShowAlert {
    // ...

    func show() {
        self.defaultShow()
        print("Good day sir!")
    }
}

There is also another solution where you can call .show() instead of .defaultShow() . However it uses casting and breaks encapsulation. If you want to see it let me know.