Swift Decodable - How to decode nested JSON that has been base64 encoded - swift

Question

I am attempting to decode a JSON response from a third-party API which contains nested/child JSON that has been base64 encoded.
Contrived Example JSON
{
"id": 1234,
"attributes": "eyAibmFtZSI6ICJzb21lLXZhbHVlIiB9",
}
PS "eyAibmFtZSI6ICJzb21lLXZhbHVlIiB9" is { 'name': 'some-value' } base64 encoded.
I have some code that is able to decode this at present but unfortunately I have to reinstanciate an additional JSONDecoder() inside of the init in order to do so, and this is not cool...
Contrived Example Code
struct Attributes: Decodable {
let name: String
}
struct Model: Decodable {
let id: Int64
let attributes: Attributes
private enum CodingKeys: String, CodingKey {
case id
case attributes
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.id = try container.decode(Int64.self, forKey: .id)
let encodedAttributesString = try container.decode(String.self, forKey: .attributes)
guard let attributesData = Data(base64Encoded: encodedAttributesString) else {
fatalError()
}
// HERE IS WHERE I NEED HELP
self.attributes = try JSONDecoder().decode(Attributes.self, from: attributesData)
}
}
Is there anyway to achieve the decoding without instanciating the additional JSONDecoder?
PS: I have no control over the response format and it cannot be changed.

Answer 1

If attributes contains only one key value pair this is the simple solution.
It decodes the base64 encoded string directly as Data – this is possible with the .base64 data decoding strategy – and deserializes it with traditional JSONSerialization. The value is assigned to a member name in the Model struct.
If the base64 encoded string cannot be decoded a DecodingError will be thrown
let jsonString = """
{
"id": 1234,
"attributes": "eyAibmFtZSI6ICJzb21lLXZhbHVlIiB9",
}
"""
struct Model: Decodable {
let id: Int64
let name: String
private enum CodingKeys: String, CodingKey {
case id, attributes
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.id = try container.decode(Int64.self, forKey: .id)
let attributeData = try container.decode(Data.self, forKey: .attributes)
guard let attributes = try JSONSerialization.jsonObject(with: attributeData) as? [String:String],
let attributeName = attributes["name"] else { throw DecodingError.dataCorruptedError(forKey: .attributes, in: container, debugDescription: "Attributes isn't eiter a dicionary or has no key name") }
self.name = attributeName
}
}
let data = Data(jsonString.utf8)
do {
let decoder = JSONDecoder()
decoder.dataDecodingStrategy = .base64
let result = try decoder.decode(Model.self, from: data)
print(result)
} catch {
print(error)
}

Answer 2

I find the question interesting, so here is a possible solution which would be to give the main decoder an additional one in its userInfo:
extension CodingUserInfoKey {
static let additionalDecoder = CodingUserInfoKey(rawValue: "AdditionalDecoder")!
}
var decoder = JSONDecoder()
let additionalDecoder = JSONDecoder() //here you can put the same one, you can add different options, same ones, etc.
decoder.userInfo = [CodingUserInfoKey.additionalDecoder: additionalDecoder]
Because the main method we use from JSONDecoder() is func decode<T>(_ type: T.Type, from data: Data) throws -> T where T : Decodable and I wanted to keep it as such, I created a protocol:
protocol BasicDecoder {
func decode<T>(_ type: T.Type, from data: Data) throws -> T where T : Decodable
}
extension JSONDecoder: BasicDecoder {}
And I made JSONDecoder respects it (and since it already does...)
Now, to play a little and check what could be done, I created a custom one, in the idea of having like you said a XML Decoder, it's basic, and it's just for the fun (ie: do no replicate this at home ^^):
struct CustomWithJSONSerialization: BasicDecoder {
func decode<T>(_ type: T.Type, from data: Data) throws -> T where T : Decodable {
guard let dict = try JSONSerialization.jsonObject(with: data) as? [String: Any] else { fatalError() }
return Attributes(name: dict["name"] as! String) as! T
}
}
So, init(from:):
guard let attributesData = Data(base64Encoded: encodedAttributesString) else { fatalError() }
guard let additionalDecoder = decoder.userInfo[.additionalDecoder] as? BasicDecoder else { fatalError() }
self.attributes = try additionalDecoder.decode(Attributes.self, from: attributesData)
Let's try it now!
var decoder = JSONDecoder()
let additionalDecoder = JSONDecoder()
decoder.userInfo = [CodingUserInfoKey.additionalDecoder: additionalDecoder]
var decoder2 = JSONDecoder()
let additionalDecoder2 = CustomWithJSONSerialization()
decoder2.userInfo = [CodingUserInfoKey.additionalDecoder: additionalDecoder]
let jsonStr = """
{
"id": 1234,
"attributes": "eyAibmFtZSI6ICJzb21lLXZhbHVlIiB9",
}
"""
let jsonData = jsonStr.data(using: .utf8)!
do {
let value = try decoder.decode(Model.self, from: jsonData)
print("1: \(value)")
let value2 = try decoder2.decode(Model.self, from: jsonData)
print("2: \(value2)")
}
catch {
print("Error: \(error)")
}
Output:
$> 1: Model(id: 1234, attributes: Quick.Attributes(name: "some-value"))
$> 2: Model(id: 1234, attributes: Quick.Attributes(name: "some-value"))

Answer 3

After reading this interesting post, I came up with a reusable solution.
You can create a new NestedJSONDecodable protocol which gets also the JSONDecoder in it's initializer:
protocol NestedJSONDecodable: Decodable {
init(from decoder: Decoder, using nestedDecoder: JSONDecoder) throws
}
Implement the decoder extraction technique (from the aforementioned post) together with a new decode(_:from:) function for decoding NestedJSONDecodable types:
protocol DecoderExtractable {
func decoder(for data: Data) throws -> Decoder
}
extension JSONDecoder: DecoderExtractable {
struct DecoderExtractor: Decodable {
let decoder: Decoder
init(from decoder: Decoder) throws {
self.decoder = decoder
}
}
func decoder(for data: Data) throws -> Decoder {
return try decode(DecoderExtractor.self, from: data).decoder
}
func decode<T: NestedJSONDecodable>(_ type: T.Type, from data: Data) throws -> T {
return try T(from: try decoder(for: data), using: self)
}
}
And change your Model struct to conform to NestedJSONDecodable protocol instead of Decodable:
struct Model: NestedJSONDecodable {
let id: Int64
let attributes: Attributes
private enum CodingKeys: String, CodingKey {
case id
case attributes
}
init(from decoder: Decoder, using nestedDecoder: JSONDecoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.id = try container.decode(Int64.self, forKey: .id)
let attributesData = try container.decode(Data.self, forKey: .attributes)
self.attributes = try nestedDecoder.decode(Attributes.self, from: attributesData)
}
}
The rest of your code will remain the same.

Answer 4

You could create a single decoder as a static property of Model, configure it once, and use it for all your Model decoding needs, both externally and internally.
Unsolicited thought:
Honestly, I would only recommend doing that if you're seeing a measurable loss of CPU time or crazy heap growth from the allocation of additional JSONDecoders… they're not heavyweight objects, less than 128 bytes unless there's some trickery I don't understand (which is pretty common though tbh):
let decoder = JSONDecoder()
malloc_size(Unmanaged.passRetained(decoder).toOpaque()) // 128