Swift conditional conformance with two cases - json

Question

The short question is how can I make a type conditionally conform to a protocol with two conditions when either one of them is met?
I have a generic type NetworkResponse<Data>. It represents server response. Here is how it is defined:
enum NetworkResponse<Data> {
case success(Data)
case error(ServerError)
}
I want to make NetworkResponse to conform Decodable. Here is my server response format:
{
"data": {
"someKey": "someValue",
"anotherKey": 15
},
"meta": {
"returnCode": 0,
"returnMessage": "operation is successful"
}
}
The data part depends on what request is made. The meta part represents some meta data about response. Like whether it is success or if not what is the error.
So here is how I implemented Decodable:
extension NetworkResponse: Decodable where Data: Decodable {
enum CodingKeys: CodingKey {
case meta
case data
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let metaValue = try container.decode(ServerError.self, forKey: .meta)
if metaValue.code != 0 {
self = .error(metaValue)
} else {
self = .success(try container.decode(Data.self, forKey: .data))
}
}
}
So far so good. But here is my problem. For some apis which don't need to return any data the data part in response is omitted. In this case my response would look like this:
{
"meta": {
"returnCode": 0,
"returnMessage": "operation is successful"
}
}
In this case I want to decode the response json as NetworkResponse<Void>. But since Void can not conform to Decodable (since it is non nominal type) compiler gives error.
To overcome this I tried to create more specialized extension of Decodable where Data is Void like this:
extension NetworkResponse: Decodable where Data == Void {
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let metaValue = try container.decode(AppErrors.Server.self, forKey: CodingKeys.meta)
if metaValue.code != 0 {
self = .error(metaValue)
} else {
self = .success(())
}
}
}
But still compiler complines like: Conflicting conformance of 'NetworkResponse<Data>' to protocol 'Decodable'; there cannot be more than one conformance, even with different conditional bounds.
So how can I create seperate init(from:) function that is used for when Data is Void?

Answer 1

I strongly urge you to change the generic parameter's type name away from Data since it's too easily confused with Foundation.Data which is widely used throughout Swift.
On to the problem itself, you can make an empty struct to represent "void" and add a new case to your NetworkResponse:
struct EmptyData: Decodable {}
enum NetworkResponse<T> {
case success(T)
case successWithEmptyData
case error(ServerError)
}
extension NetworkResponse: Decodable where T: Decodable {
private enum CodingKeys: CodingKey {
case meta
case data
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let metaValue = try container.decode(ServerError.self, forKey: .meta)
if metaValue.code != 0 {
self = .error(metaValue)
} else if T.self == EmptyData.self {
self = .successWithEmptyData
} else {
self = .success(try container.decode(T.self, forKey: .data))
}
}
}
let response = try! JSONDecoder().decode(NetworkResponse<EmptyData>.self, from: jsonData)
Alternatively, you can make your .success case contains a T? when you expect no data in return and decode accordingly.