Retrieving JSON String in Decodable decode function

Question

During decoding of ClassA that conforms to Decodable, I would like to retrieve the value of one of the properties for custom decoding. How do I achieve that?

class ClassA: Decodable {

    let title: String?
    let data: MyCustomNotDecodableNSObject?

    private enum CodingKeys: String, CodingKey {
        case title
        case data
    }

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        title = try? container.decode(String.self, forKey: .title)

        let dataRawValue = ? // somehow retrieve the raw value that coresponds to the key "data" (JSON string)
        let theData = MyCustomNotDecodableNSObject(rawData: dataRawValue)
        data = theData
    }
}

Example of parsed JSON:

{
    "classA" : {
         "title" = "a title"
         "data" : {
             "key1" : "value 1",
             "key2" : "value 2",
             "key3" : "value 3"
          }
}

The thing I am after is:

"key1" : "value 1",
"key2" : "value 2",
"key3" : "value 3"

Please, do not suggest making MyCustomNotDecodableNSObject conform to Decodable protocol. This class cannot be modified.

Hi, but why MyCustomNotDecodableNSObject should not be decodable? You can easily decode values like that if your type is conformed to decodable, `let dataRawValue = try? container.decode(MyCustomNotDecodableNSObject.self, forKey: .data)` — Andrew, Jun 30 '19 at 09:40
@Andrew Please, do not suggest making MyCustomNotDecodableNSObject conform to Decodable protocol. This class cannot be modified. — Luda, Jun 30 '19 at 10:55
Your update does not answer my question of "do you expect to get a `Data`? Or do you want to get it as a string?". — Sweeper, Jun 30 '19 at 10:56
Possible duplicate of [How to decode a property with type of JSON dictionary in Swift 4 decodable protocol](https://stackoverflow.com/questions/44603248/how-to-decode-a-property-with-type-of-json-dictionary-in-swift-4-decodable-proto) — Luda, Jun 30 '19 at 12:25

score 1 · Accepted Answer · edited Jun 30 '19 at 12:25

It is somewhat difficult to do this. One way I found is to first decode the part you want as a [String: Any], using the method described in this post. Then, convert that dictionary to Data using JSONSerialization, which is quite a long-winded way of doing things, but I couldn't find a better way.

if let dict = try container.decodeIfPresent([String: Any].self, forKey: .data) {
    let dataRawValue = try JSONSerialization.data(withJSONObject: dict, options: .prettyPrinted) // prettyPrinted is optional here
    data = MyCustomNotDecodableNSObject(rawData: dataRawValue)
} else {
    data = nil
}

If you actually want a String to pass into MyCustomNotDecodableNSObject.init, just call String.init(data:encoding:).

Extension from the linked post is needed!

**Extension from the linked post is needed!** – Luda Jun 30 '19 at 12:25 — Luda, Jun 30 '19 at 12:25

Retrieving JSON String in Decodable decode function

1 Answers1