从可编码结构获取错误消息

Question

我正在解码一个 JSON 结构，如果它无法解码，此时在我的错误检查中，这意味着服务器响应中缺少一个字段，我想向用户显示该字段。

解码此结构时：

struct UserResponseObject: Decodable {
let message: String
let data: User
}

这里

do {
            let responseObject = try createDecoder().decode(UserResponseObject.self, from: jsonData)
            //print("RESPONSE MESSAGE: ", responseObject.message)
            //print("GET USER DATA: ",responseObject.data)
            completion!(.success(responseObject.data))
        } catch let error as NSError {
            print("failure to decode user from JSON")
            completion!(.failure(error))
        }

如果没有字段 .data，我想在 catch 块中的 responseObject.message 中返回消息。 但是我不允许将响应重新解码为该结构。

struct ErrorObject: Decodable {
let message: String
}

当第一次解码失败时，我应该如何尝试获取消息。 谢谢

Answer 1

您可以通过添加更多 catch 块来了解错误的确切性质：

do {
    let messages = try JSONDecoder().decode(Results.self, from: data)
} 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)
}

Answer 2

如果您的结构实现可编码，那么您最好使用 JSONEncoder & JSONDecoder

struct Language: Codable {
  var name: String
  var version: Int
}

let swift = Language(name: "Swift", version: 4)
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(swift) {
  // save `encoded` somewhere
 }

if let encoded = try? encoder.encode(swift) {
if let json = String(data: encoded, encoding: .utf8) {
 print(json)
}

let decoder = JSONDecoder()
 if let decoded = try? decoder.decode(Language.self, from: encoded) {
   print(decoded.name)
}

Answer 3

如果 json 中缺少任何字段，您应该首先将其设为可选。 在你的情况下应该是，

struct UserResponseObject: Decodable {
   let message: String? // You should decide, should it be optional or not
   let data: User? // You should decide, should it be optional or not
}

此外，您应该处理 do 块中的无数据情况...因此，在您的 try-catch 块中尝试创建UserResponseObject对象的新实例；

do {
            let responseObject = try createDecoder().decode(UserResponseObject.self, from: jsonData)
            //print("RESPONSE MESSAGE: ", responseObject.message)
            //print("GET USER DATA: ",responseObject.data)
            if(responseObject.data) {
                completion!(.success(responseObject.data))
            }
            else {
                completion!(.failure(//no data error handler))
            }
        } catch let error as NSError {
            let responseObject = UserResponseObject(message: error.localizedDescription, data: nil)
            print("failure to decode user from JSON")
            completion!(.failure(error))
        }