如何使用解码解码 JSON 数组?

Question

我请求API向我发送一些我可以成功检索的数据，但我被困在它的解码过程中。 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解码它：

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()

然而当我尝试调试它时，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 配对很陌生

Answer 1

请捕获错误并阅读它

键入"字符串"不匹配。
调试说明：预期解码字符串，但找到了一个数字。

对于challenge_id、challenge_pts_earned、challenge_category_id和status_id，您会收到该错误，因为这些值是Int的(实际上您在读取 JSON 时已经注意到了这一点)

其次，无法解码Date值，因为您没有提供日期策略(默认值为TimeInterval)。您必须提供自定义日期格式化程序才能解码具有秒数小数部分的ISO8601。

最后，如注释中所述，通过指定CodingKeys来使用camelCased变量名称，并且由于 JSON 始终包含所有键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) }

---

注意：

在开发 JSON en-/解码时，强烈建议使用完整的错误处理集。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来省略编码键

decoder.keyDecodingStrategy = .convertFromSnakeCase 

Answer 2

你的代码中有几个错误。首先，某些类型的变量是错误的。所有id和challenge_pts_earned都需要是Int型。您可以轻松发现这一点，因为在 JSON 中字符串需要用引号分隔。解码Dates 时，还需要指定要使用的日期格式。

startDate和endDate也是日期，所以即使它们可以解码为String，我也建议您将它们存储为实际的Date对象。

还请遵守 Swift 命名约定，即类型为大 CamelCase，变量和函数名称为 lowerCamelCase。您可以使用符合CodingKey的自定义类型将自定义变量名称映射到其 JSON 表示形式。

struct Challenge : Decodable {
let id: Int?
let title: String?
let pointsEarned: Int?
let description: String?
let startDate: String?
let endDate: String?
let categoryId: Int?
let statusId: Int?
let createdAt: Date?
let updatedAt: Date?
private enum CodingKeys: String, CodingKey {
case id = "challenge_id"
case title = "challenge_title"
case pointsEarned = "challenge_pts_earned"
case description = "challenge_description"
case startDate = "start_date"
case endDate = "end_date"
case categoryId = "challenge_category_id"
case statusId = "status_id"
case createdAt
case updatedAt
}
static var dateFormatter: DateFormatter {
let df = DateFormatter()
df.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
df.locale = Locale(identifier: "en_US_POSIX")
return df
}
}
do {
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .formatted(Challenge.dateFormatter)
let decoded = try decoder.decode([Challenge].self, from: jsonArrayResponse.data(using: .utf8)!)
} catch {
print(error)
}