Codable+NSCoding.swift

extension KeyedDecodingContainer {
    
    func decode<T: NSCoding>(_ type: T.Type, forKey key: KeyedDecodingContainer<K>.Key) throws -> T? {
        let data = try decode(Data.self, forKey: key)
        return try NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(data) as? T ?? nil
    }
}

extension KeyedEncodingContainer {
    
    mutating func encode(_ value: NSCoding, forKey key: KeyedEncodingContainer<K>.Key) throws {
        let data = NSKeyedArchiver.archivedData(withRootObject: value)
        try encode(data, forKey: key)
    }
}