001.Playground.swift

import Foundation

struct AnyCodingKey: CodingKey {
    var stringValue: String
    var intValue: Int?
    
    init?(intValue: Int) {
        self.intValue = intValue
        self.stringValue = "\(intValue)"
    }
    init?(stringValue: String) {
        self.intValue = nil
        self.stringValue = stringValue
    }
}


// MARK: - Identifier<T>
struct Identifier<T> {
    let value: String
}
extension Identifier: Codable {
    init(from decoder: Decoder) throws {
        self.value = try String(from: decoder)
    }
    func encode(to encoder: Encoder) throws {
        try value.encode(to: encoder)
    }
}

// MARK: - Identified<T>
struct Identified<T> {
    let identifier: Identifier<T>
    let value: T
}
extension Identified: Codable where T: Codable {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: AnyCodingKey.self)
        
        self.identifier = try container.decode(Identifier<T>.self, forKey: AnyCodingKey(stringValue: "identifier")!)
        self.value = try T.init(from: decoder)
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: AnyCodingKey.self)
        
        try container.encode(identifier, forKey: AnyCodingKey(stringValue: "identifier")!)
        try value.encode(to: encoder)
    }
}


// MARK: - Models
struct Author: Codable {
    let name: String
}

struct Book: Codable {
    let title: String
    let authorIdentifier: Identifier<Author>
}


// MARK: - Usage
let authorJson = """
{
    "identifier": "A13424B6",
    "name": "Robert C. Martin"
}
""".data(using: .utf8)!

let bookJson = """
{
    "identifier": "A161F15C",
    "title": "Clean Code",
    "authorIdentifier": "A13424B6"
}
""".data(using: .utf8)!

let author = try! JSONDecoder().decode(Identified<Author>.self, from: authorJson)
print("Author ID:", author.identifier.value)
print("Author Name:", author.value.name)
print("")
let book = try! JSONDecoder().decode(Identified<Book>.self, from: bookJson)
print("Book ID:", book.identifier.value)
print("Book Title:", book.value.title)
print("Book Author ID:", book.value.authorIdentifier.value)