swift - 对象不符合协议(protocol)

Question

struct IsProtocol<Pr> {
    static func implementedInObject<T>(object: T) -> Bool {
        return object is Pr
    }
}


protocol A : class {}
class B : A {}

let b = B()
println(IsProtocol<A>.implementedInObject(b))

返回错误。我应该怎么做才能返回 true？

Answer 1

检查协议(protocol)一致性仅适用于与 Objective-C 兼容的协议(protocol)，即用 @objc 指令标记。这是 Swift 语言的局限性，尽管我很希望看到它消失，但我们现在必须使用它。

这里是文档摘录:

You can check for protocol conformance only if your protocol is marked with the @objc attribute, as seen for the HasArea protocol above. This attribute indicates that the protocol should be exposed to Objective-C code and is described in Using Swift with Cocoa and Objective-C. Even if you are not interoperating with Objective-C, you need to mark your protocols with the @objc attribute if you want to be able to check for protocol conformance.

Note also that @objc protocols can be adopted only by classes, and not by structures or enumerations. If you mark your protocol as @objc in order to check for conformance, you will be able to apply that protocol only to class types.

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html

添加@objc后，它可以正常工作:

struct IsProtocol<Pr> {
    static func implementedInObject<T>(object: T) -> Bool {
        return object is Pr
    }
}

@objc protocol A {}
class B : A {}

let b = B()
println(IsProtocol<A>.implementedInObject(b))