JSONDecoder无法解析映射列表

Question

我已经准备好一个简单的吉特布测试游乐场来演示我的问题：

​

​

我的Swift代码：

struct TopResponse: Codable {
    let results: [Top]
}
struct Top: Codable {
    let uid: Int
    let elo: Int
    let given: String
    let photo: String?
    let motto: String?
    let avg_score: String?
    let avg_time: String?
}

let url = URL(string: "https://slova.de/ws/top")!
let task = URLSession.shared.dataTask(with: url) {
    data, response, error in
    
    let decoder = JSONDecoder()
    guard let data2 = data,
          let tops = try? decoder.decode(TopResponse.self, from:
                                            data2) else { return }
    print(tops.results[4].given)
}
task.resume()

无法解析获取的JSON字符串，并且不打印任何内容。

请问这里有什么问题？

Answer 1

你的密码怎么了？

try?

这是罪魁祸首。为什么？您忽略了decode(_:from:)引发的错误。--您忽略了错误，该错误可能会给您提供确切的原因，或者至少给出失败原因的提示。相反，编写一个适当的do { try ... } catch { ... }。

所以：

guard let data2 = data,
      let tops = try? decoder.decode(TopResponse.self, from:
                                        data2) else { return }
print(tops.results[4].given)

=>

guard let data2 = data else { return }
do {
let tops = try decoder.decode(TopResponse.self, from: data2)
print(tops.results[4].given)
} catch {
    print("Got error while parsing: \(error)")
    print("With response: \(String(data: data2, encoding: .utf8))") //Just in case because I've seen plenty of code where expected JSON wasn't the one received: it was an error, doc changed, etc...
}

第一次打印的输出：

$>Got error while parsing: keyNotFound(CodingKeys(stringValue: "results", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"results\", intValue: nil) (\"results\").", underlyingError: nil))

修正：

struct TopResponse: Codable {
    let results: [Top]

    enum CodingKeys: String, CodingKey {
        case results = "data"
    }
}

或者用data重命名data。

然后，下一个错误：

$>Got error while parsing: typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "data", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0), CodingKeys(stringValue: "avg_score", intValue: nil)], debugDescription: "Expected to decode String but found a number instead.", underlyingError: nil))

从JSON中提取：

    "avg_score": 20.4

它不是一个String (它不是双引号之间的值)，而是一个Double。

修正：

let avg_score: String?

=>

let avg_score: Double?