text: from abc import ABCMeta, abstractmethod
  from .event_base import EventBase
  
  
  class UpgradeWorkerBase(EventBase):
      '''
          Upgrade worker base
      '''
      __metaclass__ = ABCMeta
  
      def __init__(self):
          super(UpgradeWorkerBase, self).__init__()
          self._key = None
          self._is_stopped = False
  
      @property
      def key(self):
          return self._key
  
      @key.setter
      def key(self, value):
          self._key = value
  
      @abstractmethod
      def get_upgrade_content_size(self):
          '''get the size of upgrade content'''
          return 0
  
      @abstractmethod
      def work(self):
          '''do the work'''
  
      @abstractmethod
      def stop(self):
          '''stop work'''
  