APNS のペイロードデータを struct にマッピングする

struct APNSPayload: Decodable {
    struct APS: Decodable {
        struct Alert: Decodable {
            let title: String
            let body: String
        }

        let alert: Alert
        let badge: Int?
    }

    struct CustomField: Decodable {
        let hogeList: [String]
        let fuga: String?
    }

    let aps: APS
    let customField: CustomField?

    init(decoding userInfo: [AnyHashable: Any]) throws {
        let json = try JSONSerialization.data(withJSONObject: userInfo, options: [])

        // snake_caseをcamelCaseに変換する場合
        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase

        self = try decoder.decode(APNSPayload.self, from: json)
    }
}

// {
//   "aps" : {
//     "alert" : {
//       "title" : "タイトル",
//       "body" : "本文",
//     },
//     "badge": 9,
//   },
//   "custom_field": {
//     "hoge_list": [
//       "piyo"
//     ]
//   }
// }
let payload: APNSPayload? = try? APNSPayload(decoding: userInfo)

var userInfo: [AnyHashable : Any] { get }

guard let aps = userInfo["custom_field"] as? NSDictionary,
      let hogeList = aps["hoge_list"] as? [String] else {
    return
}