如何为子类覆盖快速协议功能（例如，UIView中的UILabel）

Question

我正在尝试实现一个扩展功能，该功能应根据使用它的类的类型而有所不同。这些对象必须是UIView（或子类）。它应该始终使用在指定类型上扩展的函数，但如果不符合任何扩展函数，则应改用UIView方法（作为后备）。

这是我要执行的操作的一个示例：

protocol aProtocol {
    typealias completionBlock = (_ finished:Bool)->()
    func doSomething(completion: completionBlock)
}


extension UIView: aProtocol {
    func doSomething(completion: (Bool) -> ()) {
        print("Im an UIView")
    }
}

extension aProtocol where Self: UILabel {
    func doSomething(completion: (Bool) -> ()) {
        print("im an UILabel")
    }
}

extension aProtocol where Self: UIImageView {
    func doSomething(completion: (Bool) -> ()) {
        print("im an UIImageView")
    }
}

执行：

UIView().doSomething { (foo) in } // Should print "Im an UIView"
UIButton().doSomething { (foo) in } // Should print "Im an UIView" (UIButton doesent have specified extended function so should fall back on the UIView function)
UILabel().doSomething { (foo) in } // Should print "im an UILabel"
UIImageView().doSomething { (foo) in } // Should print "im an UIImageView"

现在打印：

Im an UIView
Im an UIView
Im an UIView
Im an UIView

这意味着它始终使用UIView方法，即使我希望它使用自己的方法也是如此。我的目标是这样打印：

Im an UIView
Im an UIView
im an UILabel
im an UIImageView

Answer 1

您可以通过以下方式实现，您只需在中将其方法公开aProtocol给Objective-c运行时即可。overridingextension

@objc protocol aProtocol {
    typealias completionBlock = (_ finished:Bool)->()
    func doSomething(completion: completionBlock)
}

extension UIView: aProtocol {
    func doSomething(completion: (Bool) -> ()) {
        print("Im an UIView")
    }
}

extension UILabel {
    override func doSomething(completion: (Bool) -> ()) {
        // you can call super.doSomething(completion: completion)
        print("im an UILabel")
    }
}

extension UIImageView {
    override func doSomething(completion: (Bool) -> ()) {
        // you can call super.doSomething(completion: completion)
        print("im an UIImageView")
    }
}

输出：

Im an UIView
Im an UIView
im an UILabel
im an UIImageView