Decoding empty JSON string value into nil with Decodable

Question

Suppose I have a struct like this:

struct Result: Decodable {
   let animal: Animal?
}

And an enum like this:

enum Animal: String, Decodable {
   case cat = "cat"
   case dog = "dog"
}

but the JSON returned is like this:

{
  "animal": ""
}

If I try to use a JSONDecoder to decode this into a Result struct, I get Cannot initialize Animal from invalid String value as an error message. How can I properly decode this JSON result into a Result where the animal property is nil?

In your case, the animal in JSON is not nil. Neither it is of type Animal. It is of string type. It is just an empty String. — Nilanshu Jaiswal, Dec 02 '18 at 09:45

score 4 · Accepted Answer · answered Dec 02 '18 at 09:56

If you want to treat an empty string as nil, you need to implement your own decoding logic. For example:

struct Result: Decodable {
    let animal: Animal?

    enum CodingKeys : CodingKey {
        case animal
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let string = try container.decode(String.self, forKey: .animal)

        // here I made use of the fact that an invalid raw value will cause the init to return nil
        animal = Animal.init(rawValue: string)
    }
}

Decoding empty JSON string value into nil with Decodable

1 Answers1