用于Value（Structures）类型的Swift协议扩展

Question

public struct KZErrorInfo: Unboxable {

var statusCode = -1
var status: String?
var errorMessage: String?

public init() {

}

public init(unboxer: Unboxer) {
    self.statusCode = unboxer.unbox("StatusCode")
    self.status = unboxer.unbox("Status")
    self.errorMessage = unboxer.unbox("Message")
}

}

protocol KZClientResponse: ETClientResponse {

var errorInfo: KZErrorInfo? { get set }

}

var errorInfo: KZErrorInfo? {
    get {
        if let value = objc_getAssociatedObject(self, &xoAssociationKeyErrorInfo) as? KZErrorInfo {
            return value
        }
        return nil
    }
    set(newValue) {
        if let error = newValue {
           objc_setAssociatedObject(self, &xoAssociationKeyErrorInfo, error, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
    }
}

My objective is to have a default implantation for the protocol KZClientResponse and Xcode is giving me a compile error as below. In the case of value types, how to overcome this issue? Appreciate you suggestions.

Answer 1

As the error message is indicating, objc_getAssociatedObject(_:_:) and objc_setAssociatedObject(_:_:_:_:) require AnyClass as the first argument. You cannot use Swift structs as AnyClass .

Think another way to store errorInfo which works with structs. Why don't you have it as the struct's property?

Answer 2

... giving me a compile error as below. In the case of value types, how to overcome this issue?

You can't overcome the compiler error. You're trying to mix apples with oranges. objc_getAssociatedObject is, by definition, Objective-C. But Objective-C knows nothing of Swift structs; it cannot possibly see them. The only thing it knows about are what it calls objects — that is, classes and their instances. To work with a Swift struct, you cannot use the Objective-C runtime at all: you must operate entirely within Swift itself.