code: def set_ev_cls(ev_cls, dispatchers=None):
      
      def _set_ev_cls_dec(handler):
          if 'callers' not in dir(handler):
              handler.callers = {}
          for e in _listify(ev_cls):
              handler.callers[e] = _Caller(_listify(dispatchers), e.__module__)
          return handler
      return _set_ev_cls_dec
docs: A decorator for Ryu application to declare an event handler.
  
      Decorated method will become an event handler.
      ev_cls is an event class whose instances this RyuApp wants to receive.
      dispatchers argument specifies one of the following negotiation phases
      (or a list of them) for which events should be generated for this handler.
      Note that, in case an event changes the phase, the phase before the change
      is used to check the interest.
  
      .. tabularcolumns:: |l|L|
  
      =========================================== ===============================
      Negotiation phase                           Description
      =========================================== ===============================
      ryu.controller.handler.HANDSHAKE_DISPATCHER Sending and waiting for hello
                                                  message
      ryu.controller.handler.CONFIG_DISPATCHER    Version negotiated and sent
                                                  features-request message
      ryu.controller.handler.MAIN_DISPATCHER      Switch-features message
                                                  received and sent set-config
                                                  message
      ryu.controller.handler.DEAD_DISPATCHER      Disconnect from the peer.  Or
                                                  disconnecting due to some
                                                  unrecoverable errors.
      =========================================== ===============================