A way to implement transformer when using Codable

CodableTransformer.swift

import Foundation

// Assume the server return `Int` number in a `String`. If it happen on all models, we can't customise all models with `init(decoder:)` and `encode(encoder:)`, because it will make the code a mess.

// Conform this protocol to implement transformation
protocol Transformer {
    associatedtype Origin: Codable
    associatedtype Target

    static func decode(value: Origin) throws -> Target
    static func encode(value: Target) throws -> Origin
}

// This struct help to connect Transformer and Codable
struct Transform<ATransformer: Transformer>: Codable {
    enum TransformError: Error {
        case codingFailed
    }

    var value: ATransformer.Target

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let originalValue = try container.decode(ATransformer.Origin.self)
        self.value = try ATransformer.decode(value: originalValue)
    }

    func encode(to encoder: Encoder) throws {
        let originalValue = try ATransformer.encode(value: value)
        var container = encoder.singleValueContainer()
        try container.encode(originalValue)
    }
}

Usage.swift

import Foundation

// This is how the model look like when using this way.
struct User: Codable {
    var yearOld: Transform<IntFromString>?
}

// This transformer help to decode String to Int and vice versa
struct IntFromString: Transformer {
    enum SelfError: Error {
        case invalidResponse
    }
    
    static func decode(value: String) throws -> Int {
        guard let intValue = Int(value) else {
            throw SelfError.invalidResponse
        }
        
        return intValue
    }
    
    static func encode(value: Int) throws -> String {
        return "\(value)"
    }
}

let jsonString = """
{
    "yearOld": "10"
}
"""

if let data = jsonString.data(using: .utf8) {
    do {
        let user = try JSONDecoder().decode(User.self, from: data)
        print(user.yearOld?.value ?? -1)
    } catch {
        print(error)
    }
}