Swift Codable with nested json as string - json

Question

The services I use for backend calls returns all this json-structure:
{
"status" : "OK",
"payload" : **something**
}
Where something can be a simple string:
{
"status" : "OK",
"payload" : "nothing changed"
}
or a nested json (any json with any properties), for example:
{
"status" : "OK",
"payload" : {
"someInt" : 2,
"someString" : "hi",
...
}
}
This is my struct:
struct GenericResponseModel: Codable {
let status:String?
let payload:String?
}
I want to decode "payload" always as a string. So in the second case I want that the payload property of my "GenericResponseModel" contains the json string of that field, but If I try to decode that response I get the error:
Type 'String' mismatch: Expected to decode String but found a dictionary instead
Is possible to archive what I want?
Many thanks

Answer 1

How about this…
Declare a PayloadType protocol…
protocol PayloadType: Decodable { }
and make String, and struct Payload conform to it…
extension String: PayloadType { }
struct Payload: Decodable, PayloadType {
let someInt: Int
let someString: String
}
Then make GenericResponseModel generic…
struct GenericResponseModel<T: PayloadType>: Decodable {
let status: String
let payload: T
enum CodingKeys: CodingKey {
case status, payload
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
status = try container.decode(String.self, forKey: .status)
payload = try container.decode(T.self, forKey: .payload)
}
}
Then you can decode as follows…
let data = """
{
"status" : "OK",
"payload" : "nothing changed"
}
""".data(using: .utf8)!
print(try JSONDecoder().decode(GenericResponseModel<String>.self, from: data))
// GenericResponseModel<String>(status: "OK", payload: "nothing changed")
and
let data2 = """
{
"status" : "OK",
"payload" : {
"someInt" : 2,
"someString" : "hi"
}
}
""".data(using: .utf8)!
print(try JSONDecoder().decode(GenericResponseModel<Payload>.self, from: data2))
// GenericResponseModel<Payload>(status: "OK", payload: Payload(someInt: 2, someString: "hi"))
Of course, this relies on you knowing the payload type in advance. You could get around this by throwing a specific error if payload is the wrong type…
enum GenericResponseModelError: Error {
case wrongPayloadType
}
and then…
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
status = try container.decode(String.self, forKey: .status)
do {
payload = try container.decode(T.self, forKey: .payload)
} catch {
throw GenericResponseModelError.wrongPayloadType
}
}
Then handle this error when you decode…
let data = """
{
"status" : "OK",
"payload" : {
"someInt" : 2,
"someString" : "hi"
}
}
""".data(using: .utf8)!
do {
let response = try JSONDecoder().decode(GenericResponseModel<String>.self, from: data) // Throws
print(response)
} catch let error as GenericResponseModelError where error == .wrongPayloadType {
let response = try JSONDecoder().decode(GenericResponseModel<Payload>.self, from: data2) // Success!
print(response)
}