CustomDecoder.swift

import Foundation


let json = """
[
{
"name": {
"firstName": "Taylor",
"lastName": "Swift"
},
"age": 26
}
]
"""

struct User: Decodable {
    var firstName: String
    var lastName: String
    var age: Int
    
    enum CodingKeys: String, CodingKey {
        case name, age
    }
    
    enum NameCodingKeys: String, CodingKey {
        case firstName, lastName
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        age = try container.decode(Int.self, forKey: .age)
        let name = try container.nestedContainer(keyedBy: NameCodingKeys.self, forKey: .name)
        firstName = try name.decode(String.self, forKey: .firstName)
        lastName = try name.decode(String.self, forKey: .lastName)
    }
}


let data = json.data(using: .utf8)
let decoder = JSONDecoder()

let user = try decoder.decode([User].self, from: data!)
print(user)