module main;

class SystemShutdownNotifier {

    Subscriber[] subscribers;

    void addSubscriber(Subscriber subscriber) {
        subscribers ~= subscriber;
    }
}

interface Message {
}

class MsgShutdownNotifierStarted: Message {
    public:
        this(SystemShutdownNotifier notifier) {
            this.notifier = notifier;
        }

        SystemShutdownNotifier notifier;
        invariant int type = 1;
}

class Subscriber {
    public:
        void onShutdownNotifierStarted(const (MsgShutdownNotifierStarted) cmd) {
            cmd.notifier.addSubscriber(this);
        }
}

void main() {
    auto subscriber = new Subscriber();

    auto notifier = new SystemShutdownNotifier();

    invariant message = cast(invariant) (new MsgShutdownNotifierStarted(notifier));

    subscriber.onShutdownNotifierStarted(message);
}