From: Joel VanderWerf <vjoel@...>
Date: 2005-11-28T07:39:17+09:00
Subject: Re: Object access heirarchies

Leslie Viljoen wrote:
> I often have this problem in my projects.
> 
> I have a bunch of classes to do various things in my program, but
> once instantiated they each need access to each other. For example,
> in the adventure game I am working on, I have a Parser object that
> needs to manipulate a Player, Map and Things in the map.
> 
> I normally do such things by passing objects around when initializing,
> like so:
> 
> parser = Parser(Player.new(World.new(Map.new, Things.new)))
> 
> That works, although it seems a little long winded - for the
> parser to modify a thing, it needs to set something like:
> player.world.thing["ball"].name = "bat"
> 
> Later, the Things needed access to the Map so I could set thing.location
> to a map.room object. So I changed thing.location to be a string --
> thing.location_name and made a function that could look up a location
> based on it's name: map["garden"].
> 
> What I want to know is: what's the best way? I had globals $player,
> $world, $map
> earlier and that makes things a bit easier but it also makes me nervous
> (perhaps just because I have been anti-global-indoctrinated for 20 years
> now)
> I'm still a bit vague on Modules. Any ideas?
> 
> Les
> 

Modules are typically used not for organizing instances, but for
grouping methods and constants, so that chuncks of code can be reused in
different situations.

Why not simply replace the globals with a Game class and an instance
that has attributes for player, world, map, etc.?

If your program is complex enough, you may find a technique called
"dependency injection" useful. It's kind of like the Game class
suggestion, with a little extra syntactic sugar. DI is a way of
extracting the connection logic (which you are now doing by "passing
objets around when initializing") from the rest of the object creation
code. With DI, you define containers for your instances, and you can
decide whether a particular kind of instance should be unique in the
container, or there should be one instance for each key (such as a
location name).

There are several DI frameworks for ruby (see RAA). Here's an example
using my own MinDI library:

require 'mindi'

Parser = Struct.new :game # add parser state vars here
Player = Struct.new :game, :location, :contents
World = Struct.new :game # add state vars (:time, maybe)
Map = Struct.new :game
  # The :game attr is useful if you want to write methods of
  # these classes (Player for example) that refer to the other
  # game objects. But it's not strictly necessary.

Room = Struct.new :name, :contents

# Sometimes, a struct is not enough...
class Thing
  attr_accessor :name
  attr_reader :location

  def initialize name, location = nil
    @name = name
    @location = location
  end

  # Moving a Thing updates the Room it's in or Player who has it
  def location=(loc)
    @location.contents.delete self if @location
    @location = loc
    @location.contents << self if @location
  end
end

class GameContainer
  extend MinDI::Container

  # These are some singletons--one instance each.
  parser  { Parser.new self }
  player  { Player.new self, start_room, [] }
  world   { World.new self }
  map     { Map.new self }

  # A multiton.
  # The |name| means there can be many things--one for each name.
  # internally, there is a hash  stored in @thing that maps
  # each name strign to a Thing.
  thing { |name| Thing.new name }

  # The shovel (unique with that name).
  shovel { thing "Shovel" }

  room { |name| Room.new name, [] }

  start_room { room "garden" }

  # Set up some initial conditions
  def initialize
    # create and locate the shovel
    shovel.location = start_room
  end
end

game = GameContainer.new

# The shovel is already defined:
p game.shovel
puts

# Create a new thing:
ball = game.thing("ball")
ball.location = game.room "basement"
p ball
puts

player = game.player

# pick up the shovel and anything else in the start room
# This could be made into a #pick_up method of Game or of Player
player.location.contents.each { |thing| thing.location = player }
p player.contents.map {|thing| thing.name}
puts

# move the player
p player.location.name
player.location = game.room "basement"
p player.location.name
puts

# get the ball
player.location.contents.each { |thing| thing.location = player }
p player.contents.map {|thing| thing.name}
puts

# You can define new methods on the GameContainer that access the internal
# storage used by the "service" methods.
class GameContainer
  def rooms
    @room.values
    # because internally the "room" service uses a hash stored in
    # the @room instance var
  end

  def things
    @thing.values
  end
end

# Show all the rooms
p game.rooms

# Show all the things
p game.things.map {|thing| thing.name}

__END__

Output:

#<Thing:0xb7d862fc @name="Shovel", @location=#<struct Room
name="garden", contents=[#<Thing:0xb7d862fc ...>]>>

#<Thing:0xb7d85078 @name="ball", @location=#<struct Room
name="basement", contents=[#<Thing:0xb7d85078 ...>]>>

["Shovel"]

"garden"
"basement"

["Shovel", "ball"]

[#<struct Room name="garden", contents=[]>, #<struct Room
name="basement", contents=[]>]
["ball", "Shovel"]


-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407