title: Swift Codable - decode nested dictionary
body: <p>Lets say I have a dictionary like this: </p>
  
  <pre><code>{"user_data":{"id":3,"name":"Damian D","email":"aaa@aaa.pl"},"status":true}
  </code></pre>
  
  <p>How can I use <code>Codable</code> protocol to decode just <code>user_data</code> into such <code>struct</code>:</p>
  
  <pre><code>struct User: Codable {
      private enum CodingKeys: String, CodingKey {
          case id
          case username = "name"
          case email
      }
  
      let id:       Int
      let username: String
      let email:    String
  }
  </code></pre>
  
  <p>Do I need to convert this sub dictionary into Data, or is there a easier way?</p>