通过扩展将功能添加到协议的原因是什么，为什么不仅仅将其放在协议本身的定义中呢?

问题描述

我一直想知道为什么当我看到协议示例时，人们倾向于通过扩展来添加大多数功能.像这样:

I've always wondered why when I see examples of protocols people tend to add most of the functions via an extension. Like this:

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)
            }
        }
    }
}

扩展名的含义是什么?为什么不将其包括在初始协议定义中?

What's the point behind the extension? why not just include it in the initial protocol definition?

推荐答案

因为那是不合法的.协议可以包括功能声明，但不包括功能主体(实现).协议扩展即将包括默认实现.这就是协议扩展 的含义.

Because that's not legal. A protocol may include a function declaration but not the function body (the implementation). A protocol extension is about including a default implementation. That's what a protocol extension is.

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