docs: def subscribe(self, message, handler):
          '''
          Adds hander for specified message.
  
          :param str message:
              Name of message to subscribe to.
  
          :param callable handler:
              Handler for this message type. Handler must receive single parameter
              and that parameter will be instance of sent message.
          '''
          with self._lock:
              ref = WeakCallable(handler, self._on_collect)
              self._subscribers[message].append(ref)