ios - 通过扩展向协议(protocol)添加功能的原因是什么，为什么不把它放在协议(protocol)本身的定义中呢？

Question

我一直想知道为什么当我看到协议(protocol)示例时，人们倾向于通过扩展来添加大部分功能。像这样:

protocol Flashable {}//Can be empty becuase function is in extension

extension Flashable where Self: UIView //Makes this protocol work ONLY if object conforms to UIView (ie. uilable, uibutton, etc.)
{
    func flash() {
        UIView.animate(withDuration: 0.3, delay: 0, options: .curveEaseIn, animations: {
            self.alpha = 1.0 //Object fades in
        }) { (animationComplete) in
            if animationComplete == true {
                UIView.animate(withDuration: 0.3, delay: 2.0, options: .curveEaseOut, animations: {
                    self.alpha = 0.0 //Object fades out
                    }, completion: nil)
            }
        }
    }
}

扩展背后的意义是什么？为什么不将其包含在初始协议(protocol)定义中？

Answer 1

why not just include it in the initial protocol definition

因为那是不合法的。协议(protocol)可能包含函数声明，但不包含函数体(实现)。协议(protocol)扩展是关于包含默认实现。这就是协议(protocol)扩展。