Handy extension for Codable + transformers

CodableExtensions.swift

import Foundation

protocol TransformerType {
    associatedtype Object
    associatedtype JSON

    func transformFromJSON(_ value: Any) throws -> Object
    func transformToJSON(_ value: Object) throws -> JSON
}

extension KeyedDecodingContainer {
    func decode<T: Decodable>(_ key: K) throws -> T {
        return try decode(T.self, forKey: key)
    }

    func decodeIfPresent<T: Decodable>(_ key: K) throws -> T? {
        return try decodeIfPresent(T.self, forKey: key)
    }

    func decode<T: Decodable, Transform: TransformerType>(_ key: K, transformer: Transform) throws -> T where Transform.Object == T, Transform.JSON: Decodable {
        let decodedValue = try decode(Transform.JSON.self, forKey: key)
        let transformedValue = try transformer.transformFromJSON(decodedValue)
        return transformedValue
    }

    func decodeIfPresent<T: Decodable, Transform: TransformerType>(_ key: K, transformer: Transform) throws -> T? where Transform.Object == T, Transform.JSON: Decodable {
        guard let decodedValue = try decodeIfPresent(Transform.JSON.self, forKey: key) else { return nil }
        let transformedValue = try transformer.transformFromJSON(decodedValue)
        return transformedValue
    }
}

extension KeyedEncodingContainer {
    mutating func encode<Value, Transformer: TransformerType>(_ value: Value, forKey: K, transformer: Transformer) throws where Value == Transformer.Object, Transformer.JSON: Encodable {
        let transformedValue = try transformer.transformToJSON(value)
        try self.encode(transformedValue, forKey: forKey)
    }

    mutating func encode<Value, Transformer: TransformerType>(_ value: Value?, forKey: K, transformer: Transformer) throws where Value == Transformer.Object, Transformer.JSON: Encodable {
        guard let value = value else { return }
        let transformedValue = try transformer.transformToJSON(value)
        try self.encode(transformedValue, forKey: forKey)
    }
}