Top Level Key Decoding

AnyCodingKey.swift

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

extension AnyCodingKey: ExpressibleByStringLiteral {
  init(stringLiteral value: StringLiteralType) {
    self.init(stringValue: value)
  }
}

TopLevelKeyWrapper.swift

private let topLevelKey = CodingUserInfoKey(rawValue: "topLevelKey")!

private struct Wrapper<T: Decodable>: Decodable {
  let value: T
  
  init(from decoder: Decoder) throws {
    let key = decoder.userInfo[topLevelKey] as! AnyCodingKey
    let container = try decoder.container(keyedBy: AnyCodingKey.self)
    value = try container.decode(T.self, forKey: key)
  }
}

extension JSONDecoder {
  func decode<T: Decodable>(_ type: T.Type, key: AnyCodingKey, from data: Data) throws -> T {
    userInfo[topLevelKey] = key
    return try decode(Wrapper.self, from: data).value
  }
}

Usage.swift

struct User: Codable {
  let age: Int
  let name: String
}

let data = """
  {
    "users": [
      {
        "name": "Ian",
        "age": 31
      },
      {
        "name": "Michael",
        "age": 42
      }
    ]
  }
  """.data(using: .utf8)!

let users = try JSONDecoder().decode([User].self, key: "users", from: data)