codable-inner-json.swift

public protocol CodableByInnerJSONType : Codable {
    associatedtype JSON: Codable
    
    init(fromInnerJSON json: JSON)
    func encodeToInnerJSON() -> JSON
}

extension CodableByInnerJSONType {
    public init(from decoder: Decoder) throws {
        let json = try JSON(from: decoder)
        self.init(fromInnerJSON: json)
    }
    
    public func encode(to encoder: Encoder) throws {
        let json = encodeToInnerJSON()
        try json.encode(to: encoder)
    }
}

// example

public struct UserCredential : CodableByInnerJSONType {
    public struct JSON : Codable {
        public var id: Int
        public var password: String
    }
    
    public init(fromInnerJSON json: JSON) {
        self.id = json.id
        self.password = json.password
    }
    
    public var id: Int
    public var password: String
    
    public func encodeToInnerJSON() -> JSON {
        return JSON(id: id,
                    password: password)
    }
}