型クラスライクなDecode.swift

TypeClassLikeDecode.swift

typealias JSON = [String:String]

protocol Decoder {
    associatedtype T
    func decode(json: JSON) -> T
}

protocol Decodable {
    static func decode<D: Decoder>(json: JSON, decoder: D) -> D.T where D.T == Self
}

extension Decodable {
    static func decode<D: Decoder>(json: JSON, decoder: D) -> D.T where D.T == Self {
        return decoder.decode(json: json)
    }
}


class A {
    let a: String

    init(a: String) {
        self.a = a
    }
}
class B: A {
    let b: String
    init(a: String, b: String) {
        self.b = b
        super.init(a: a)
    }
}

extension A: Decodable { }

let json: JSON = ["a":"AAAAA", "b": "BBBBB"]

struct ADecoder: Decoder {
    func decode(json: JSON) -> A {
        return A(a: json["a"]!)
    }
}

struct BDecoder: Decoder {
    func decode(json: JSON) -> B {
        return B(a: json["a"]!, b: json["b"]!)
    }
}

let a = A.decode(json: json, decoder: ADecoder())
//NG
//let b_with_a_decoder = B.decode(json: json, decoder: ADecoder())
let b = B.decode(json: json, decoder: BDecoder())