_.swift

protocol PushNotificationPayload: Decodable {
}

protocol SubscribeContainer {
    func parse(_ json: Data)
}

extension SubscribeContainer {
    func parse(_ json: Data) {

    }
}

class Tsuchi {
    static let shared = Tsuchi()
    private init() {}

    private struct Container<T: PushNotificationPayload>: SubscribeContainer {
        let handler: (T) -> Void

        func parse(_ data: Data) {
            guard let payload = try? JSONDecoder().decode(T.self, from: data) else {
                return
            }
           handler(payload)
        }
    }

    private struct AnyContainer: SubscribeContainer {
        let base: SubscribeContainer
    }

    private var container: AnyContainer?

    func subscribe<T: PushNotificationPayload>(_ type: T.Type, handler: @escaping (T) -> Void) {
        let container = Container<T>(handler: handler)
        self.container = AnyContainer(base: container)
    }

    func received() {
        guard let container = container else {
            return
        }

        let json = ["name": "Taro"]
        container.base.parse(try! JSONSerialization.data(withJSONObject: json, options: []))
    }
}


struct TestPayload: PushNotificationPayload {
    let name: String
}

Tsuchi.shared.subscribe(TestPayload.self) { (payload) in
    print("received: \(payload)")
}

Tsuchi.shared.received() // received: TestPayload(name: "Taro")