User.login {
    // register remote notification (show dialog if user not determined about notification permission.)
    Tsuchi.shared.register { authorized in
        //...
    }
}

User.logout {
    Tsuchi.shared.unregister { authorized in
        //...
    }
}

struct FCMNotificationPayload: PushNotificationPayload {
    let eventName: String?
    let eventType: EventType?
    var aps: APS?
}

Tsuchi.shared.subscribe(FCMNotificationPayload.self) { result in
    switch result {
    case .success(let (notification, mode)):
        print(notification.name, mode)
    case .failure(let error):
        print(error)
    }
}

public enum AppTopic: String, TopicType {
    case owner
    case member
}

// Subscribe topic
Tsuchi.shared.subscribe(toTopic: AppTopic.owner)

// Unsubscribe topic
Tsuchi.shared.unsubscribe(fromTopic: AppTopic.owner)