From: Michael Neumann <mneumann@...>
Date: 2004-11-11T04:23:43+09:00
Subject: Re: automatically call function on attribute set

On Thu, Nov 11, 2004 at 03:13:30AM +0900, Joe Laughlin wrote:
> I have a class that represents an airplane.  This class has a bunch of data
> members: x/y/z positions, x/y/z velocities, pitches, rolls, etc.  What I did
> is let the user configure what different attributes the airplane had in a
> YAML config file, then have the airplane class read in that file and setup
> attr_accessors for each attribute in the file.
> 
> I have another object (call it airplane_drawing) that is responsible for
> drawing this airplane on the screen.  It needs to have access to some of the
> attributes of the airplane, and it needs have some of its own attributes
> changed whenever airplane's attributes changes.
> 
> What is the best way to get airplane_drawing to be notified whenever some
> attribute of airplane is changed?  Should I have a thread (or similar) that
> checks the airplane object to see if anything's changed?

You should use an Observer.

  require 'observer'

  class Airplane
    include Observable

    def initialize
      @x = 0
    end

    def fly
      # airplane changes it's coordinate
      @x += 10

      changed
      notify_observers(self)
    end
  end

  class AirplaneDrawing
    def update(obj)
      # ...
    end
  end

  airbus = Airplane.new
  drawing = AirplaneDrawing.new
  airbus.add_observer(drawing)
  airbus.fly

Regards,

  Michael