How to guarantee valid JSON in Swift 4? - json

Question

I'm trying to work with JSON data returned from a service. The JSON is, according to JSON validators, valid and is very simple:
[{"ID":"SDS-T589863","TotalRisk":0.2458,"TotalScore":641.032}]
However trying to parse it in my Swift 4 code it is mysteriously (to me at least) invalid. Here's my attempt to parse it:
// make the request
let task = session.dataTask(with: urlRequest) {
(data, response, error) in
// check for any errors
guard error == nil else {
print(error!)
return
}
// make sure we got data
guard let responseData = data else {
print("Error: did not receive data")
return
}
// this is fine:
guard let ddd = String(bytes: responseData, encoding: String.Encoding.utf8) else {
print("can't")
return
}
print(ddd) // prints [{"ID":"SDS-T589863","TotalRisk":0.2458,"TotalScore":641.032}] happily
do {
// cannot serialize
guard let risk = try JSONSerialization.jsonObject(with: responseData, options: [JSONSerialization.ReadingOptions.allowFragments])
as? [String: Any]
else {
print("error trying to convert data to JSON")
return
}
print(risk)
} catch {
print("error trying to convert data to JSON")
return
}
}
task.resume()
}
Assuming that I have no control over the JSON object or the format in which it is returned to me, is there a way to tell what is wrong with the JSON and perhaps format the response so that it can be serialized correctly?

Answer 1

You should cast your data to the [[String: Any]] type because you have array in response.
You are trying to cast to [String: Any], but you have an array of [String: Any] because your response enclosed in [] brackets.
Example:
let risk = try JSONSerialization.jsonObject(with: responseData, options: [JSONSerialization.ReadingOptions.allowFragments]) as? [[String: Any]]
Or if you want to get just only one [String: Any] object from response you can write:
let risk = (try JSONSerialization.jsonObject(with: responseData, options: [JSONSerialization.ReadingOptions.allowFragments]) as? [[String: Any]])?.first
Or if your object can be an array or not an array (but it sounds a little bit strange) you could try to cast to several possible types.

Answer 2

The response type is array of json objects so you have to cast it to [[String: Any]]. Since you are using Swift 4, you can use Decodable type which maps the model to the response.
let task = URLSession().dataTask(with: urlRequest) { (data, response, error) in
// check for any errors
guard error == nil else {
print(error!)
return
}
// make sure we got data
guard let responseData = data else {
print("Error: did not receive data")
return
}
do {
let decoder = JSONDecoder()
let riskArray = try decoder.decode([Risk].self, from: responseData)
print(riskArray)
} catch {
print("error trying to convert data to Model")
print(error.localizedDescription)
}
}
task.resume()
You can define your Model struct like
struct Risk: Decodable {
let id: String
let totalRisk: Double
let totalScore: Double
enum CodingKeys: String, CodingKey {
case id = "ID"
case totalRisk = "TotalRisk"
case totalScore = "TotalScore"
}
}
You can read more about Decodable protocol here