code: def get_field_callback(self, field, event):
          # type: (str, str) -> Optional[Tuple[Callable, bool]]
          
          try:
              return self.factory_context.field_callbacks[field][event]
          except KeyError:
              return None
docs: Retrieves the registered method for the given event. Returns None if
          not found
  
          :param field: Name of the dependency field
          :param event: A component life cycle event
          :return: A 2-tuple containing the callback associated to the given
                   event and flag indicating if the callback must be called in
                   valid state only