getting error message from codable struct - ios

Question

Im am decoding a JSON struct and if it fails to decode, at this point in my error checking it means that one of the fields is missing from the server response, which I want to display to the user.
When decoding this struct:
struct UserResponseObject: Decodable {
let message: String
let data: User
}
here
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))
}
if there is no field .data, I want to return the message in responseObject.message in the catch block. But I am not allowed to redecode the response into this struct.
struct ErrorObject: Decodable {
let message: String
}
How should I go about trying to get the message when the first decode fails. Thanks

Answer 1

You can understand the exact nature of error by adding more catch blocks:
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

If your struct implemnts codable then you better use 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

If any field is missing from the json, you should make it optional first of all. In your case it should be,
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
}
Also you should handle the no data case in your do block...
As a result in your try-catch block try to create a new instance of UserResponseObject object;
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))
}