Object access heirarchies

[Leslie Viljoen]

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

[Leslie Viljoen]

Leslie Viljoen wrote:

Ah, one thread up. Perhaps my question has already been answered in the
last day.

[Daniel Schierbeck]

What about a single base class?

class Game
attr_reader :parser, :world, :map, :things, :player

def initialize
@parser = Parser.new(self)
@world = World.new(self)
@map = Map.new(self)
@things = Things.new(self)
@player = Player.new(self)
end

class World
def initialize(game)
@game = game
end

def map
@game.map
end

# ....
end

# ....
end

Cheers,
Daniel

[Joel VanderWerf]

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

[Lyndon Samson]

If it really bothers you you could construct an ObjectManager, which could
allow you to lookup names in a heirachy.

ball = ObjectManager.reference("/gameobjects/world/objects/balls/1")

Sort of JNDI like.

[Edwin van Leeuwen]

leslie wrote:
> 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)

You could do it with class variables:
class Map
private_class_method :new
@@map = nil
def Logger.create
@@map = new unless @@map
@@map
end
end

Now you can call from each object that needs the map object:
map=Map.create and it will create a new one if none exists, or send you
the already created object.

Edwin

--
Posted via http://www.ruby-forum.com/.

[anne001]

I am a newbie on ruby, and I am a bit confused by this code, it seems
like a snake eating its own tail! game contains an object world which
is defined as containing game?

Can someone explain this a little to me? How is that different from
using a @@ class object?

[Daniel Schierbeck]

Say you have three classes, A, B, and C, and you want their instances to
be able to communicate. These three classes will all be instantiated
together, so we know that the other objects are there. You then write a
class, let's just call it Base, that you instantiate instead of the
three classes. It will insatntiate them instead, and pass a reference to
itself to each of them.

class Base
attr_reader :a, :b, :c

def initialize
@a, @b, @c = A.new(self), B.new(self), C.new(self)
end

# We'll keep the other classes in here, so we don't
# pollute the main namespace
class A
def initialize(base)
@base = base
@b, @c = base.b, base.c
end
end

class B
def initialize(base)
@base = base
@a, @c = base.a, base.c
end
end

class C
def initialize(base)
@base = base
@a, @b = base.a, base.b
end
end
end

[Leslie Viljoen]

Daniel Schierbeck wrote:

Ah, that is so clever.

[Leslie Viljoen]

Joel VanderWerf wrote:

Thanks a lot for that reply, it's very interesting indeed.
It's a little over my head, so I am looking into DI.

Les