From: John Feminella <johnf@...>
Date: 2011-05-08T08:36:40+09:00
Subject: Re: Cross access between instance variables

You add fish to a tank, not tanks to a fish. An alternative proposal,
that doesn't involve Fish holding a reference to Tanks at all:

class Fish
  # ...

  def comfortable?(temp_in_degrees_celsius)
    (@min..@max).include? temp_in_degrees_celsius
  end
end

class Tank
  # ...

  def add_inhabitant(i)
    self.inhabitants << i
  end

  def update
    # ...
    # possible temperature change!

    self.inhabitants.each do |inhabitant|
      logger.warn("#{inhabitant} is experiencing environmental
stress") unless inhabitant.comfortable?(t)
    end
  end
end
--
John Feminella
Principal Consultant, BitsBuilder
LI: http://www.linkedin.com/in/johnxf
SO: http://stackoverflow.com/users/75170/



On Sat, May 7, 2011 at 19:27, Hassan Schroeder
<hassan.schroeder@gmail.com> wrote:
> 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
>
>