我看了"Protocol-Oriented Programming in Swift"并阅读了相关文档，但我仍然认为以下示例代码存在冲突(在 Playground 中尝试)。

protocol X {

    // The important part is "static" keyword
    static var x: String { get }
}

extension X {
    // Here "static" again
    static var x: String {
        get {
            return "xxx"
        }
    }
}

// Now I'm going to use the protocol in a class, BUT
// in classes "static" is like "final class",
// i.e. CAN'T BE OVERRIDDEN, right?

// But I'd prefer to have the ability to override that property,
// so I'll try to use "class" keyword.
// Will it break the program? (spoiler: no!)

class Y: X {
    // Here we are allowed to use "class" keyword (but why?).
    class var x: String {
        get {
            return "yyy"
        }
    }
}

class Z: Y {
    override class var x: String {
        get {
            return "zzz"
        }
    }
}

class Test<T: X> {
    func test() -> String {
        return T.x
    }
}

// And finally the property is successfully overridden (but why?).
print(Test<Z>().test()) // "zzz\n"

这是否真的意味着当协议(protocol)在类中使用时，来自协议(protocol)(和可能的默认实现)的 static 关键字可以合法地替换为 class 关键字？您是否知道任何引用资料可以证实这一点？

最佳答案

来自 Language Reference / Declarations我们知道以下内容。

Function Declaration

...

Special Kinds of Methods

...

Methods associated with a type rather than an instance of a type must be marked with the static declaration modifier for enumerations and structures or the class declaration modifier for classes.

即static 关键字(主要)用于枚举和结构，class 关键字用于类。

还有这样的注释:

Type Variable Properties

...

NOTE

In a class declaration, the keyword static has the same effect as marking the declaration with both the class and final declaration modifiers.

即static 关键字实际上可以在类声明中使用，并且表示final class。

那么协议(protocol)呢？

Protocol Method Declaration

...

To declare a class or static method requirement in a protocol declaration, mark the method declaration with the static declaration modifier. Classes that implement this method declare the method with the class modifier. Structures that implement it must declare the method with the static declaration modifier instead. If you’re implementing the method in an extension, use the class modifier if you’re extending a class and the static modifier if you’re extending a structure.

这里的文档指出，当在类或类扩展中实现协议(protocol)时，我们应该将协议(protocol)声明中的 static 关键字替换为 class 关键字(这是原始问题的确切答案)。

奖金

在两种情况下，协议(protocol)的采用将仅限于类。第一个(也是最不明确的)是当协议(protocol)包含可选成员时:

Protocol Declaration

...

By default, types that conform to a protocol must implement all properties, methods, and subscripts declared in the protocol. That said, you can mark these protocol member declarations with the optional declaration modifier to specify that their implementation by a conforming type is optional. The optional modifier can be applied only to protocols that are marked with the objc attribute. As a result, only class types can adopt and conform to a protocol that contains optional member requirements. ...

第二个(明确；下一段):

To restrict the adoption of a protocol to class types only, mark the protocol with the class requirement by writing the class keyword as the first item in the inherited protocols list after the colon. ...

但考虑到 static 和 class 关键字的适用性，他们都没有改变规则。

关于ios - Swift:协议(protocol)扩展中的静态属性可以被覆盖，但为什么呢？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33673900/