DecodableWithArrayOfObjects.swift

import Foundation

struct User: Decodable {
    var firstName: String
    var lastName: String
    var pets: [Pet]
    
    struct Pet: Decodable {
        var species: String
        var breed: String
        var name: String
        var age: Int
    }

    enum CodingKeys: String, CodingKey {
        case firstName, lastName, pets
    }

    public init(from decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: CodingKeys.self)

        firstName = try container.decode(String.self, forKey: .firstName)
        lastName = try container.decode(String.self, forKey: .lastName)
        pets = try container.decode([Pet].self, forKey: .pets)
    }
    
}

let data = """
{
    "firstName": "Beryl",
    "lastName": "Test",
    "pets": [
        {
            "species": "cat",
            "breed": "Persian",
            "name": "Sirius",
            "age": 2
        },
        {
            "species": "dog",
            "breed": "spaniel",
            "name": "Minerva",
            "age": 5
        },
    ]
}
""".data(using: .utf8)!

let user = try? JSONDecoder().decode(User.self, from: data)
print(user)