From: Joel VanderWerf <vjoel@...>
Date: 2006-08-25T04:04:58+09:00
Subject: Re: Observing changes in object state

Max Muermann wrote:
> Hi all,
> 
> I was playing around with the observer library and thought I'd
> reimplement somthing I had done in Java a while back. It uses the
> observer pattern but automatically adds interceptors to every
> attribute setter method - once on the first include and then
> dynamically as more setter methods are added. If anybody has some
> comments or hints on how to implement this in a better way, I'd be
> delighted to hear them.

Here's a somewhat different approach, but it might be interesting to 
compare:

require 'observable' # see http://raa.ruby-lang.org/project/observable/

class Model
   extend Observable
   attr_accessor :id # avoid confusion with Object#id
   observable :name, :id
end

# Observer
class Watcher
   def initialize m
     m.when_name Object do
       puts "name set to #{m.name.inspect}"
     end

     m.when_id Object do
       puts "id set to #{m.id.inspect}"
     end
     # note that each method is treated separately
   end
end

# change some stuff
m = Model.new
Watcher.new m
m.id='000'
m.name='test'
m.id='000'
   # note this does not trigger the observer since value
   # doesn't change. Use the "signal" mechanism from the
   # observable lib for that behavior

__END__

Output:

name set to nil
id set to nil
id set to "000"
name set to "test"

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