A prototype for a MUD where a Door blocks attempts to pass through a Way.

messages.rb

class ListenerQueue
  attr_reader :listeners
  def initialize
    @listeners = []
  end
  def register(listener)
    @listeners << listener
  end
  def share_message(receiver, method_name, args, kwargs)
    @listeners.each do |listener|
      return false unless listener.allow?(receiver, method_name, args, kwargs)
    end
    true
  end
end

class Door
  attr_accessor :state
  def allow?(receiver, method_name, args, kwargs)
    return true if method_name != :pass
    if state == :closed
      puts "The door blocks your way!"
      return false
    end
    true
  end
end

module MethodBroadcaster
  def broadcast_calls_to(*method_names)
    proxy = Module.new
    method_names.each do |method_name|
      proxy.define_method(method_name) do |*args, **kwargs, &block|
        return false unless listener_queue.share_message(self, method_name, args, kwargs)
        super(*args, **kwargs, &block)
      end
    end
    self.prepend proxy
  end
end

class Thing
  attr_accessor :name
  attr_accessor :current_area
  def initialize(name:, current_area:)
    @name, @current_area = name, current_area
  end
end

class Area
  attr_accessor :description
  attr_accessor :things
  attr_accessor :exits
  def initialize(description: nil, things: [], exits: [])
    @description, @things, @exits = description, things, exits
  end
end

class Way
  extend MethodBroadcaster
  attr_accessor :listener_queue
  attr_accessor :destination
  attr_accessor :direction
  def initialize(destination:, direction:)
    @destination, @direction = destination, direction
  end
  def pass(thing)
    thing.current_area = destination
  end
  broadcast_calls_to :pass
end

hall = Area.new(description: "hall")
room = Area.new(description: "room")
way = Way.new(destination: room, direction: "north")
way.listener_queue = ListenerQueue.new
door = Door.new
door.state = :closed
way.listener_queue.register(door)
player = Thing.new(name: "player", current_area: hall)
puts "Currently in #{player.current_area.description}"
way.pass(player)
puts "Currently in #{player.current_area.description}"
door.state = :open
way.pass(player)
puts "Currently in #{player.current_area.description}"