Decode Arbitrary JSON in Swift

Question

I am trying decode a JSON with below structure and convert to a Swift struct object.

"{\"createdAt\":\"2021-07-20T06:53:05.755Z\",\"extraPayload\":\"32bd7d6691964519\",\"messageID\":\"a5f77d0a27da2e14a78ce4d736590bb3c369d5e5bebd36339553e3b699b82d13\",\"publishedAt\":\"2021-07-20T06:53:05.836655Z\"}"

Both the publishedAt and messageID fields are fixed and any other fields in the JSON can be arbitrary based on a particular use-case. In this case both fields createdAt and extraPayload are completely optional and another case it could be something like appConfig:true etc apart from the 2 fixed fields.

How can I map into a struct with below format which has 2 mandatory fixed fields and arbitrary part captured using a Dictionary of type [String:Any]?

public struct MessagePayload {
    /**
     * Message identifier — opaque non-empty string.
     */
    let messageID: String
    
    /**
     * ISO 8601 date indicating a moment when the message was published to the ACN back-end service.
     */
    let publishedAt: String
    
    /**
     * Application-specific fields.
     */
    let messageObject: [String:Any]?
}

I have tried using Swift Codable but it does not support [String:Any].

@HariJustForFun You need to be more explicit. What are the different possibilities for `messageObject`? If you can share atleast 2 styles (in json examples) then we can help you explore the world of [Custom Decoders](https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types). — staticVoidMan, Jul 20 '21 at 07:25
Thanks staticVoidMan. Updated the question. Please let me know if it's clearer. — HariJustForFun, Jul 20 '21 at 07:29
Anothere duplicate of https://stackoverflow.com/questions/68254792/swift-decode-json-response-and-store-nested-json-as-string-or-json ? — Sulthan, Jul 20 '21 at 07:32
Use a custom Codable decoder for the fixed fields, and SwiftyJSON for the other fields that you want to use as an untyped dictionary. — Damiaan Dufaux, Jul 20 '21 at 09:25

score 2 · Accepted Answer · answered Jul 20 '21 at 11:05

It seems like JSONSerialization from Foundation is probably a better fit for your use case. This will decode the JSON data directly into an Any type, which you can then cast to [String: Any] to access the underlying properties.

let decoded = try? JSONSerialization.jsonObject(with: myJsonData, options: []) as? [String: Any]
decoded?["messageID"]
decoded?["publishedAt"]
// ...access other properties as needed

Decode Arbitrary JSON in Swift

1 Answers1