class EventHandler:
    _instances = set()

    def __init__(self, server_connector):
        type(self)._instances.add(self)
        self.server_connector = server_connector

    def start(self):
        self.server_connector.register_callback(self._event_callback)

    def _event_callback(self, message):
        self.handle_event(message, self.server_connector)

    def handle_event(self, message, server_connector):
        raise NotImplementedError()

    @classmethod
    def stop_all_instances(cls):
        for instance in cls._instances:
            instance.stop()

    def stop(self):
        self.server_connector.close()
        self.cleanup()

    def cleanup(self):
        pass