different-decoding.swift

import Foundation

let json = """
{
  "userId": 0,
  "name": "Donny"
}
""".data(using: .utf8)!

let fakeSQLite = """
{
  "user_id": 0,
  "name": "Donny"
}
""".data(using: .utf8)!

struct User: Decodable {
  enum JSONDecodingKeys: CodingKey {
    case userId, name
  }
  
  enum SQLiteDecodingKeys: String, CodingKey {
    case userId = "user_id"
    case name
  }
  
  let userId: Int
  let name: String
  
  init(from decoder: Decoder) throws {
    if let origin = decoder.userInfo[.sqliteOrigin] as? Bool, origin == true {
      let container = try decoder.container(keyedBy: SQLiteDecodingKeys.self)
      self.userId = try container.decode(Int.self, forKey: .userId)
      self.name = try container.decode(String.self, forKey: .name)
    } else {
      let container = try decoder.container(keyedBy: JSONDecodingKeys.self)
      self.userId = try container.decode(Int.self, forKey: .userId)
      self.name = try container.decode(String.self, forKey: .name)
    }
  }
}

extension CodingUserInfoKey {
  static let sqliteOrigin = CodingUserInfoKey(rawValue: "sqliteOrigin")!
  static let jsonOrigin = CodingUserInfoKey(rawValue: "jsonOrigin")!
}

let sqliteDecoder = JSONDecoder()
sqliteDecoder.userInfo[.sqliteOrigin] = true
let user1 = try! sqliteDecoder.decode(User.self, from: fakeSQLite)

let jsonDecoder = JSONDecoder()
sqliteDecoder.userInfo[.jsonOrigin] = true
let user2 = try! jsonDecoder.decode(User.self, from: json)