class EventBus:
    _subscribers = {}

    @classmethod
    def subscribe(cls, event_type, callback):
        if event_type not in cls._subscribers:
            cls._subscribers[event_type] = []
        cls._subscribers[event_type].append(callback)

    @classmethod
    def emit(cls, event_type, data=None):
        for callback in cls._subscribers.get(event_type, []):
            callback(data)

    @classmethod
    def unsubscribe(cls, event_type, callback):
        if event_type in cls._subscribers:
            cls._subscribers[event_type].remove(callback)