如何使用 Decodable 解码 JSON 数组？

Question

我请求 API 向我发送一些数据，我可以成功检索这些数据，但我仍停留在它的解码过程中。这是我收到的 JSON：

[  
   {  
      "challenge_id":1,
      "challenge_title":"newchallenge1",
      "challenge_pts_earned":1000,
      "challenge_description":"description1",
      "start_date":"2017-09-24T00:00:00.000Z",
      "end_date":"2017-09-24T00:00:00.000Z",
      "challenge_category_id":1,
      "status_id":2,
      "createdAt":"2017-09-24T17:21:47.000Z",
      "updatedAt":"2017-09-24T09:40:34.000Z"
   },
   {  
      "challenge_id":2,
      "challenge_title":"challenge1",
      "challenge_pts_earned":100,
      "challenge_description":"description1",
      "start_date":"2017-09-24T00:00:00.000Z",
      "end_date":"2017-09-24T00:00:00.000Z",
      "challenge_category_id":1,
      "status_id":0,
      "createdAt":"2017-09-24T17:22:12.000Z",
      "updatedAt":"2017-09-24T09:22:12.000Z"
   },
   {  
      "challenge_id":3,
      "challenge_title":"new eat title",
      "challenge_pts_earned":600000,
      "challenge_description":"haha",
      "start_date":"2017-01-09T00:00:00.000Z",
      "end_date":"2017-01-10T00:00:00.000Z",
      "challenge_category_id":2,
      "status_id":0,
      "createdAt":"2017-09-27T17:12:10.000Z",
      "updatedAt":"2017-09-27T09:15:19.000Z"
   }
]

我正在尝试创建以下结构来对其进行解码：

   struct challenge : Codable {
    let  id : String?
    let  title : String?
    let  pointsEarned : String?
    let  description : String?
    let  dayStarted : String?
    let  dayEnded : String?
    let  categoryID : String?
    let  statusID : Int?
    let createdAt : Date?
    let updatedAt : Date?

    enum CodingKeys: String, CodingKey {
        case id = "challenge_id"
        case title = "challenge_title"
        case pointsEarned = "challenge_pts_earned"
        case description = "challenge_description"
        case dayStarted = "start_date"
        case dayEnded = "end_date"
        case categoryID = "challenge_category_id"
        case statusID = "status_id"
        case createdAt, updatedAt
    }

}

这是我的实现代码：

 var All_challenges : [challenge]?
 let url = URL(string: API.all_challenges.rawValue)!
        var request = URLRequest(url: url)
        request.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
        request.httpMethod = "GET"
        let task = URLSession.shared.dataTask(with: request) { data, response, error in
            guard let data = data, error == nil else {
                print("error=\(String(describing: error))")
                return
            }
            if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {
                print("statusCode should be 200, but is \(httpStatus.statusCode)")
                print("\(String(describing: response))")
            }
            let responseString = String(data: data, encoding: .utf8)
            guard let result = responseString else { return }
            print(result)
           if let json = try? JSONDecoder().decode([challenge].self , from : data ) {
                self.All_challenges = json

           }

         }
        task.resume()

但是当我尝试调试它时，我永远无法输入 if 语句

if let json = try? JSONDecoder().decode([challenge].self,from:data ) {
self.All_challenges = json

 }

请给我一些关于我的错误在哪里的描述，我对 JSON 配对非常陌生

Answer 1

Please catch the error and read it

Type 'String' mismatch.
Debug Description: Expected to decode String but found a number instead.

You get that error for challenge_id, challenge_pts_earned, challenge_category_id and status_id because the values are Int (actually you can notice that already when reading the JSON)

Secondly, the Date values cannot be decoded because you didn't provide a date strategy (the default is TimeInterval). You have to provide a custom date formatter to decode ISO8601 with fractional seconds.

最后，如评论中所述，通过指定使用驼峰式变量名，CodingKeys并且由于 JSON 始终包含所有键，因此将属性声明为非可选

let jsonString = """
[
{
"challenge_id":1,
"challenge_title":"newchallenge1",
"challenge_pts_earned":1000,
"challenge_description":"description1",
"start_date":"2017-09-24T00:00:00.000Z",
"end_date":"2017-09-24T00:00:00.000Z",
"challenge_category_id":1,
"status_id":2,
"createdAt":"2017-09-24T17:21:47.000Z",
"updatedAt":"2017-09-24T09:40:34.000Z"
},
{
"challenge_id":2,
"challenge_title":"challenge1",
"challenge_pts_earned":100,
"challenge_description":"description1",
"start_date":"2017-09-24T00:00:00.000Z",
"end_date":"2017-09-24T00:00:00.000Z",
"challenge_category_id":1,
"status_id":0,
"createdAt":"2017-09-24T17:22:12.000Z",
"updatedAt":"2017-09-24T09:22:12.000Z"
},
{
"challenge_id":3,
"challenge_title":"new eat title",
"challenge_pts_earned":600000,
"challenge_description":"haha",
"start_date":"2017-01-09T00:00:00.000Z",
"end_date":"2017-01-10T00:00:00.000Z",
"challenge_category_id":2,
"status_id":0,
"createdAt":"2017-09-27T17:12:10.000Z",
"updatedAt":"2017-09-27T09:15:19.000Z"
}
]
"""

struct Challenge : Decodable {

//    private enum CodingKeys : String, CodingKey {
//        case challengeId = "challenge_id"
//        case challengeTitle = "challenge_title"
//        case challengePtsEarned = "challenge_pts_earned"
//        case challengeDescription = "challenge_description"
//        case startDate = "start_date"
//        case endDate = "end_date"
//        case challengeCategoryId = "challenge_category_id"
//        case statusId = "status_id"
//        case createdAt, updatedAt
//    }

    let challengeId : Int
    let challengeTitle : String
    let challengePtsEarned : Int
    let challengeDescription : String
    let startDate : String // or Date
    let endDate : String // or Date
    let challengeCategoryId : Int
    let statusId : Int
    let createdAt : Date
    let updatedAt : Date
}

let data = Data(jsonString.utf8)
let formatter = DateFormatter()
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SZ"
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .formatted(formatter)
decoder.keyDecodingStrategy = .convertFromSnakeCase 
do {
    let challenges = try decoder.decode([Challenge].self, from: data)
    print(challenges)
} catch { print(error) }

笔记：

在开发 JSON 编码/解码时，强烈建议使用全套错误处理。它使调试变得更加容易

do {
   try JSONDecoder().decode ...

} catch DecodingError.dataCorrupted(let context) {
    print(context)
} catch DecodingError.keyNotFound(let key, let context) {
    print("Key '\(key)' not found:", context.debugDescription)
    print("codingPath:", context.codingPath)
} catch DecodingError.valueNotFound(let value, let context) {
    print("Value '\(value)' not found:", context.debugDescription)
    print("codingPath:", context.codingPath)
} catch DecodingError.typeMismatch(let type, let context)  {
    print("Type '\(type)' mismatch:", context.debugDescription)
    print("codingPath:", context.codingPath)
} catch {
    print("error: ", error)
}

编辑

在 Swift 4.1 及更高版本中，您可以通过添加 keyDecodingStrategy

decoder.keyDecodingStrategy = .convertFromSnakeCase