Swift4_JSON_06.swift

import Foundation

// framework側のコード

let ExtensionKey = CodingUserInfoKey(rawValue: "extensionKey")!

struct Group: Codable {
    let users: [User]
}

struct User: Codable {
    let name: String
    let additionalInfo: ExtensibleJSON?
    
    enum CodingKeys: String, CodingKey {
        case name
        case additionalInfo = "additional_info"
    }
}

struct ExtensibleJSON: Codable {
    var extensions: [String:Codable] = [:]
    
    init(from decoder: Decoder) throws {
        if let extensions = decoder.userInfo[ExtensionKey] as? [String:Codable.Type] {
            for (key, ExtensionCodable) in extensions {
                self.extensions[key] = try ExtensionCodable.init(from: decoder)
            }
        }
    }
    
    func encode(to encoder: Encoder) throws {
        // do something
    }
}

public class ExtensibleJSONUnarchiver {
    let group: Group
    
    public init(data: Data, extensions: [String:Codable.Type]? = nil) throws {
        let decoder = JSONDecoder()
        decoder.userInfo[ExtensionKey] = extensions
        self.group = try decoder.decode(Group.self, from: data)
    }
}


// ユーザ側のコード

let json = """
{
    "users": [
        {
            "name": "userA",
            "additional_info": {
                "special_info": {
                    "foo": "bar"
                },
                "website": {
                    "name": "DarkHorse",
                    "url": "http://darkhorse2.0spec.jp"
                },
            }
        },
        {
            "name": "userB"
        },
        {
            "name": "userC",
            "additional_info": {}
        },
        {
            "name": "userD",
            "additional_info": {
                "website": {
                    "name": "Gist",
                    "url": "https://gist.github.com/"
                },
                "other_info": {}
            }
        }
    ]
}
""".data(using: .utf8)!

var userWebsites = [String:URL]()

struct WebsiteExtension: Codable {
    struct Website: Codable {
        enum CodingKeys: String, CodingKey {
            case name
            case url
        }
        
        init(from decoder: Decoder) throws {
            let container = try decoder.container(keyedBy: CodingKeys.self)
            let name = try container.decode(String.self, forKey: .name)
            let url = try container.decode(URL.self, forKey: .url)
            userWebsites[name] = url
        }
        
        func encode(to encoder: Encoder) throws {
            // do something
        }
    }
    let website: Website?
}

do {
    let extensions: [String:Codable.Type] = [
        "website": WebsiteExtension.self
    ]
    let unarchiver = try ExtensibleJSONUnarchiver(data: json, extensions: extensions)
    for (name, url) in userWebsites {
        print("\(name): \(url)")
    }
} catch DecodingError.keyNotFound(let key, let context) {
    print("keyNotFound: \(key): \(context)")
} catch {
    print("\(error.localizedDescription)")
}

/*
 Gist: https://gist.github.com/
 DarkHorse: http://darkhorse2.0spec.jp
*/