use JSONDecoder on a string

JSONDecoder+Utils.swift

import Foundation

enum ExtendedDecodingError: Error {
    case badString
}

extension JSONDecoder { 
    public func decode<T>(_ type: T.Type, from string: String) throws -> T where T: Decodable {
        if let data = string.data(using: .utf8) {
            let result = try decode(type, from: data)
            return result
        } else {
            throw ExtendedDecodingError.badString
        }
    } 
}