From: Hassan Schroeder <hassan.schroeder@...>
Date: 2011-05-08T08:27:03+09:00
Subject: Re: Cross access between instance variables

2011/5/7 Stefan Salewski <mail@ssalewski.de>:
>
> Of course this is a way to solve the problem. It may look not very
> natural, that we have to tell the fish the parameters of its tank, when
> we ask the fish about its mood:
>
>>mood =  fish.tell_mood({:temp => t.temp, :food => t.food_type})
>
> But OK, this is the way to do it. Now I know that I have not missed
> something.

A little late to this, but isn't this a good case for the Observer pattern?
So the fish are always "aware" of the tank environment? Very simply:

-----------------------------
require 'observer'

class Tank
  include Observable

  def temperature
    @temperature
  end

  def temperature=(temperature)
    @temperature = temperature
    changed
    notify_observers(temperature)
  end

end

class Fish

  attr_accessor :tank_temperature

  def initialize(tank)
    @tank_temperature = tank.temperature
    tank.add_observer(self)
  end

  def update(temperature)
    puts "temperature changed to #{temperature}"
    @tank_temperature = temperature
  end

end
-----------------------------
And exercise with:
-----------------------------
require 'tank'
require 'fish'

tank = Tank.new
tank.temperature= 100
fish = Fish.new(tank)
puts "fish.tank_temperature: #{fish.tank_temperature}"  # => 100

tank.temperature= 102
puts "fish.tank_temperature: #{fish.tank_temperature}" # => 102
-----------------------------

FWIW,
-- 
Hassan Schroeder ------------------------ hassan.schroeder@gmail.com
twitter: @hassan