如何确保传入的quickFix消息具有限定的大小？

import quickfix as fix

class MessageSizeValidator(fix.MessageCracker):
    MAX_MESSAGE_SIZE = 10 * 1024 * 1024  # 10 MB

    def onMessage(self, message, sessionID):
        if len(message.toString()) > self.MAX_MESSAGE_SIZE:
            raise ValueError("Message size exceeds the limit")
        super().onMessage(message, sessionID)

# 配置QuickFix引擎
settings = fix.SessionSettings('config.cfg')
application = MyApplication()
storeFactory = fix.FileStoreFactory(settings)
logFactory = fix.FileLogFactory(settings)
initiator = fix.SocketInitiator(application, storeFactory, settings, logFactory)

# 注册自定义的MessageCracker
initiator.setMessageCracker(MessageSizeValidator())

# 启动引擎
initiator.start()