JSONと異なる形でDecodeしたい場合、`init(from decoder: Decoder) throws`を実装

CustomNestedJSONDecoder.swift

import Foundation

struct Response: Decodable {
    let main: Main

    struct Main: Decodable {
        let sub: Sub

        struct Sub: Decodable {
            let a: UInt
            let b: String

            enum Keys: String, CodingKey {
                case a
                case b
            }
        }

        init(from decoder: Decoder) throws {
            let container = try decoder.container(keyedBy: Sub.Keys.self)
            // 一旦文字列として取得
            let a = try container.decode(UInt.self, forKey: .a)
            let b = try container.decode(String.self, forKey: .b)
            self.sub = Sub.init(a: a, b: b)
        }
    }

    private enum Keys: String, CodingKey {
        case main
    }
}

let json = """
{
    "main": {
        "a": 10,
        "b": "Hoge",
    }
}
""".data(using: .utf8)!

let res = try JSONDecoder().decode(Response.self, from: json)
print(res.main.sub.a) // 10
print(res.main.sub.b) // Hoge