Calling selector from protocol extension

问题:

I'm building simple theme engine and would like have an extension which adds UISwipeGestureRecognizer to UIViewController

Here is my code:

protocol Themeable {
    func themeDidUpdate(currentTheme: Theme) -> Void
}

extension Themeable where Self: UIViewController {
    func switchCurrentTheme() {
        Theme.switchTheme()
        themeDidUpdate(Theme.currentTheme)
    }

    func addSwitchThemeGestureRecognizer() {
        let gestureRecognizer = UISwipeGestureRecognizer(target: self, action:#selector(Self.switchCurrentTheme))
        gestureRecognizer.direction = .Down
        gestureRecognizer.numberOfTouchesRequired = 2
        self.view.addGestureRecognizer(gestureRecognizer)
    }
}

Of course compiler can't find #selector(Self.switchCurrentTheme) as it isn't exposed via @objc directive. Is it possible to add this behaviour to my extension?

UPDATE: Theme is a Swift enum, so I can't add @objc in front of Themeable protocol

回答1:

The cleanest, working solution I could come up with was to define a private extension on UIViewController with the method in question. By limiting the scope to private, access to this method is isolated to within the source file where the protocol is defined in. Here's what it looks like:

protocol Themeable {
    func themeDidUpdate(currentTheme: Theme) -> Void
}

fileprivate extension UIViewController {
    @objc func switchCurrentTheme() {
        guard let themeableSelf = self as? Themeable else {
            return
        }

        Theme.switchTheme()
        themeableSelf.themeDidUpdate(Theme.currentTheme)
    }
}

extension Themeable where Self: UIViewController {
    func addSwitchThemeGestureRecognizer() {
        let gestureRecognizer = UISwipeGestureRecognizer(target: self, action:#selector(switchCurrentTheme))
        gestureRecognizer.direction = .Down
        gestureRecognizer.numberOfTouchesRequired = 2
        self.view.addGestureRecognizer(gestureRecognizer)
    }
}

回答2:

Have you considered creating a wrapper to let you call your non-@objc function from an @objc one?

@objc class Wrapper: NSObject {
    let themeable: Themeable

    init(themeable: Themeable) {
        self.themeable = themeable
    }

    func switchCurrentTheme() {
        Theme.switchTheme()
        themeable.themeDidUpdate(Theme.currentTheme)
    }
}

protocol Themeable {
    func themeDidUpdate(currentTheme: Theme) -> Void
}

extension Themeable where Self: UIViewController {
    func addSwitchThemeGestureRecognizer() {
        let wrapper = Wrapper(themeable: self)
        let gestureRecognizer = UISwipeGestureRecognizer(target: wrapper, action:#selector(Wrapper.switchCurrentTheme))
        gestureRecognizer.direction = .Down
        gestureRecognizer.numberOfTouchesRequired = 2
        self.view.addGestureRecognizer(gestureRecognizer)
    }
}

回答3:

I found a solution. May be not the perfect one, but it works. As I can't define Themeable protocol as @objc because it uses Swift-only enum I decided to move method I want to call to "parent" protocol and define this protocol as @objc. It seems like it works but I don't really like it to be honest...

@objc protocol ThemeSwitcher {
    func switchCurrentTheme()
}

protocol Themeable: ThemeSwitcher {
    func themeDidUpdate(currentTheme: Theme) -> Void
}

extension Themeable where Self: UIViewController {
    func switchCurrentTheme() {
        Theme.switchTheme()
        themeDidUpdate(Theme.currentTheme)
    }

    func addSwitchThemeGestureRecognizer() {
        let gestureRecognizer = UISwipeGestureRecognizer(target: self, action:#selector(switchCurrentTheme))
        gestureRecognizer.direction = .Down
        gestureRecognizer.numberOfTouchesRequired = 2
        self.view.addGestureRecognizer(gestureRecognizer)
    }
}

回答4:

I'm not sure but you can access it like

    #selector(switchCurrentTheme)

or you can refer this link to try Visit How to use #selector(myMethodName) in a protocol extension?