codable-dynamic-key.swift

import UIKit

let userJson = """
  {
    "id": "4yq6txdpfadhbaqnwp3",
    "age": 5,
    "name":"John Doe",
    "properties": {
        "dynamicKeyA": "1",
        "dynamicKeyB": "2",
        "dynamicKeyC": "3"
    }
  }
""".data(using: .utf8)!

typealias Properties = [String: String]

struct User: Decodable {
    let id: String
    let age: Int
    let name: String
    let properties: Properties

    enum CodingKeys: String, CodingKey {
        case id
        case age
        case name
        case properties
    }
}

extension Properties {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        self = try container.decode(Properties.self)
    }
}

let test = try JSONDecoder().decode(User.self, from: userJson)
print(test)