code: def _on_event(self, conv_event):
          
          conv = self._conv_list.get(conv_event.conversation_id)
          user = conv.get_user(conv_event.user_id)
          show_notification = all((
              isinstance(conv_event, hangups.ChatMessageEvent),
              not user.is_self,
              not conv.is_quiet,
          ))
          if show_notification:
              self.add_conversation_tab(conv_event.conversation_id)
              if self._discreet_notifications:
                  notification = DISCREET_NOTIFICATION
              else:
                  notification = notifier.Notification(
                      user.full_name, get_conv_name(conv), conv_event.text
                  )
              self._notifier.send(notification)
docs: Open conversation tab for new messages & pass events to notifier.