Simpler method for decoding JSON from multiple services in Swift

Question

Premise

I have a struct that conforms to Decodable, so it can decode JSON from a variety of responses via init(from:). For each type of JSON response I expect to decode, I have an enum that conforms to CodingKey.

Example

Here's a simplified example, which can be dropped into a Swift playground:

import Foundation

// MARK: - Services -

struct Service1 {}
struct Service2 {}

// MARK: - Person Model -

struct Person {
    let name: String
}

extension Person: Decodable {
    enum CodingKeys: String, CodingKey {
        case name = "name"
    }

    enum Service2CodingKeys: String, CodingKey {
        case name = "person_name"
    }

    // And so on through service n...

    init(from decoder: Decoder) throws {
        switch decoder.userInfo[.service] {
        case is Service1.Type:
            let container = try decoder.container(keyedBy: CodingKeys.self)
            name = try container.decode(String.self, forKey: .name)
        case is Service2.Type:
            let container = try decoder.container(keyedBy: Service2CodingKeys.self)
            name = try container.decode(String.self, forKey: .name)
        // And so on through service n...
        default:
            fatalError("Missing implementation for service.")
        }
    }
}

// MARK: - CodingUserInfoKey -

extension CodingUserInfoKey {
    static let service = CodingUserInfoKey(rawValue: "service")!
}

// MARK: - Responses -

// The JSON response from service 1.
let service1JSONResponse = """
[
    {
        "name": "Peter",
    }
]
""".data(using: .utf8)!

// The JSON response from service 2.
let service2JSONResponse = """
[
    {
        "person_name": "Paul",
    }
]
""".data(using: .utf8)!

// And so on through service n... where other services have JSON responses with keys of varied names ("full_name", "personName").

// MARK: - Decoding -

let decoder = JSONDecoder()

decoder.userInfo[.service] = Service1.self
let service1Persons = try decoder.decode([Person].self, from: service1JSONResponse)

decoder.userInfo[.service] = Service2.self
let service2Persons = try decoder.decode([Person].self, from: service2JSONResponse)

Problem

The problem I'm running into is that I have a lot of different services that I needed to decode responses from, and a model with many more properties than this simplified example. As the number of services increases, so does the number of cases needed to decode those responses.

Question

How can I simplify my init(from:) implementation to reduce all this code duplication?

Attempts

I've tried storing the correct CodingKey.Type for each service and passing that into container(keyedBy:), but I get this error:

Cannot invoke 'container' with an argument list of type '(keyedBy: CodingKey.Type)'.

init(from decoder: Decoder) throws {
    let codingKeyType: CodingKey.Type

    switch decoder.userInfo[.service] {
    case is Service1.Type: codingKeyType = CodingKeys.self
    case is Service2.Type: codingKeyType = Service2CodingKeys.self
    default: fatalError("Missing implementation for service.")
    }

    let container = try decoder.container(keyedBy: codingKeyType) // ← Error
    name = try container.decode(String.self, forKey: .name)
}

Answer 1

Rather than trying to solve this with CodingKeys and an increasingly complicated init, I suggest composing it via a protocol:

protocol PersonLoader: Decodable {
    var name: String { get }
    // additional properties
}

extension Person {
    init(loader: PersonLoader) {
        self.name = loader.name
        // additional properties, but this is one-time
    }
}

Alternately, particularly if Person is a read-only simple data object, you could just make Person a protocol, and then you could avoid this extra copying step.

You can then define the interfaces for each service independently:

struct Service1Person: PersonLoader {
    let name: String
}

struct Service2Person: PersonLoader {
    let person_name: String

    var name: String { person_name }
}

And then map into Persons when you're done:

let service2Persons = try decoder.decode([Service2Person].self,
                                         from: service2JSONResponse)
    .map(Person.init)

If you went with a protocol-only approach, it would look like this instead:

protocol Person: Decodable {
    var name: String { get }
    // additional properties
}

struct Service1Person: Person {
    let name: String
}

struct Service2Person: Person {
    var name: String { person_name }
    let person_name: String
}

let service2Personsx = try decoder.decode([Service2Person].self,
                                         from: service2JSONResponse) as [Person]

Answer 2

Without a bunch of custom per-service (or per-service type) functionality in Person's init(from:), I think it would be difficult to do. You can't pass custom CodingKey-conformant enums into decoder.container(keyedBy:) because it is generic over the type of that enum.

One way you could do it is by using a custom key decoding strategy and perform the mapping from a dictionary or by a function in the custom key decoding method/closure.

In the example below, I've used an enum to represent the services. The mapping dictionary is keyed off the enum case, so reflects the service/service type key mapping. Hopefully, this can serve as a useful roadmap for your more complex real-world use case.

import Foundation

// MARK: - Custom Key Decoding -

struct MyCodingKey: CodingKey {
    var stringValue: String
    var intValue: Int?

    init?(stringValue: String) {
        self.stringValue = stringValue
        self.intValue = nil
    }

    init?(intValue: Int) {
        self.stringValue = String(intValue)
        self.intValue = intValue
    }
}

// MARK: - Services -

enum Services: String {
    case service1
    case service2
}

extension Services {

    var mapping: [String:String] {
        switch self {
        case .service1: return [:]
        case .service2: return ["person_name": "name"]
        }
    }

    func getPersons(jsonData: Data) throws -> [Person] {
        let decoder = JSONDecoder()

        decoder.keyDecodingStrategy = .custom { (keys: [CodingKey]) -> CodingKey in
            let lastKey = keys.last!
            guard lastKey.intValue == nil else {
                return MyCodingKey(intValue: lastKey.intValue!)!
            }
            guard let stringValue = self.mapping[lastKey.stringValue] else {
                return lastKey
            }

            return MyCodingKey(stringValue: stringValue)!
        }

        let persons = try decoder.decode([Person].self, from: jsonData)
        return persons
    }
}

// MARK: - Person Model -

struct Person: Decodable {
    let name: String
}

// MARK: - Responses -

// The JSON response from service 1.
let service1JSONResponse = """
[ {  "name": "Peter", } ]
""".data(using: .utf8)!

// The JSON response from service 2.
let service2JSONResponse = """
[ { "person_name": "Paul", } ]
""".data(using: .utf8)!

// MARK: - Sample Calls -

print((try? Services.service1.getPersons(jsonData: service1JSONResponse))!)
print((try? Services.service2.getPersons(jsonData: service2JSONResponse))!)