Swift 5: How can i get data from JSONDecoder().decode?

Question

I get JSON from Google translator API:

{
    data =     {
        translations =         (
                        {
                detectedSourceLanguage = en;
                translatedText = "\U0434\U0430\U043d\U043d\U044b\U0435";
            }
        );
    };
}

Then i do this:

     if data != nil && error == nil {
        
        let decoder = JSONDecoder()
        
        do {
        
            let translated = try! decoder.decode(Data.self, from: data!)
            dump(translated)
        } catch {
            print(error)
        }
    }

And i fetch this:

MyVocabulary.JSONTranslator.Data
  ▿ data: MyVocabulary.JSONTranslator.Data.Translations
    ▿ translations: 1 element
      ▿ MyVocabulary.JSONTranslator.Data.Translations.TranslatedText
        - detectedSourceLanguage: "en"
        - translatedText: "Работа"

My structs:

 struct Data: Codable {
        
        struct Translations: Codable {
            
            struct  TranslatedText: Codable {
                var detectedSourceLanguage: String
                var translatedText: String
                
                enum CodingKeys: String, CodingKey{
                    case detectedSourceLanguage
                    case translatedText
                }
                
                init(from decoder: Decoder) throws {
                    let container = try decoder.container(keyedBy: CodingKeys.self)
                    self.translatedText = try container.decode(String.self, forKey: .translatedText)
                    self.detectedSourceLanguage = try container.decode(String.self, forKey: .detectedSourceLanguage)
                  }
            }
            var translations: ([TranslatedText])
        }
        let data: Translations
    }

how can I get properties from TranslatedText to var in "do" block after decode?

Answer 1

The major mistake is to name a custom struct Data because it interferes with the Foundation struct Data and decoding Data.self fails.

You don't need a custom initializer nor CodingKeys, these 3 structs are sufficient

struct Root: Decodable {  let data: Translations }
struct Translations: Decodable { let translations: [TranslatedText] }
struct TranslatedText: Decodable { let detectedSourceLanguage, translatedText : String }

Now decode Root.self and get translatedText as described below

let decoder = JSONDecoder()
do {
    let translated = try decoder.decode(Root.self, from: data!)
    let translatedText = translated.data.translations.first?.translatedText
    print(translatedText)
} catch {
    print(error)
}

Answer 2

The JSON you provided is not valid. I will assume you meant the JSON to be something like this:

{
    "data": {
        "translations": [
                {
                    "detectedSourceLanguage": "en",
                    "translatedText": "\U0434\U0430\U043d\U043d\U044b\U0435"
                }
        ]
    }
}

You don't have to put a CodingKeys enum or an init(from:) unless you want custom JSON decoding. You can make it simple by letting the default decoding happen automatically.

Modify your model to this:

struct Response: Codable {
    var data: Data
}

struct Data: Codable {
    var translations: [Translation]
}

struct Translation: Codable {
    var detectedSourceLanguage: String
    var translatedText: String
}

Inside your do block, you should not force unwrap the try. You should change try! to try, this way when the try fails, it will trigger the catch block. Otherwise it will crash the app if you use try!

Inside do you can say:

do {
    let response = try decoder.decode(Response.self, from: data!)
    let data = response.data
    let translations = data.translations
    let firstTranslation = translations[0]
    let detectedSourceLanguage = firstTranslation.detectedSourceLanguage
    let translatedText = firstTranslation.translatedText
}