A quick example of codeable based off of a Hackingwithswift tutorial

codeabe.swift

// https://www.hackingwithswift.com/articles/119/codable-cheat-sheet
import Foundation 

struct Packet: Decodable {
    var value: Int
    var array: [Packet]?    
}

enum Lett {
    indirect case a(m: M) 
    case b
}

enum M {
    case b(m: Lett)
}
let json = """
[
    {
        "name": {
            "first_name": "Taylor",
            "last_name": "Swift"
        },
        "age": 26
    }
]
"""


struct User: Codable {
    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)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(age, forKey: .age)
        
        var name = container.nestedContainer(keyedBy: NameCodingKeys.self, forKey: .name)
        try name.encode(firstName, forKey: .firstName)
        try name.encode(lastName, forKey: .lastName)
    }
}

let decoder = JSONDecoder()
// 1.
decoder.keyDecodingStrategy = .convertFromSnakeCase
// 2.
// let user1 = try! decoder.decode(User.self, from: json.data(using: .utf8)!)
let user2 = try! decoder.decode([User].self, from: json.data(using: .utf8)!)
// print(user1)
print(user2)


/* EDUCATIONAL NOTES:  
1. Things break if I remove `convertFromSnakeCase`.
2. Things break if I decode it to `User.self` instead of `[User].self`.

Attempting to break it, helps you better understand how things work. 

The way I think of `let container = try decoder.container(keyedBy: CodingKeys.self)` is: 
container is just a _way_ to see what was passed to the decoder. In this context it means it's a window into the passed json. 
Your container has different items you can open. Each item is opened with a specific coding key. 

Your json has two fields (name, age). Think of it as two locks (key). Each lock can be opened with a unique key (or matching key). 
Then you have a new container. You can either open more keys (firstName, lastName) in this container or just map it to a field of the type your decoding
*/