def on_event(event_handler = nil, &block)
      event_handler ||= block
      raise ArgumentError, "no event_handler provided" if event_handler.nil?
      @event_handler = event_handler
    end
def on_event(event_handler = nil, &block)
      event_handler ||= block
      raise ArgumentError, "no event_handler provided" if event_handler.nil?
      @event_handler = event_handler
    end
Register a handler for reporting of stack events.
 @param [Proc] event_handler