From: "Stephan Kämper" <Stephan.Kaemper@...>
Date: 2004-01-02T07:11:46+09:00
Subject: Drb and observer pattern

Hi all and have a happy new year everybody!

I'm trying to built a set of programs that
- may run on different machines, or as different processes on one machine
- I'd like to have one program to notify other(s) of certain events

Now I thought that combining DRb and the Observer pattern might be the 
way to go, but as I'm posting a question I apparently ran into an error.

BTW, I'm running a SuSE Linux 9.0 with ruby 1.8.1
[884] stk@tao ~/devel/util: ruby -v
ruby 1.8.1 (2003-12-25) [i686-linux]



I boiled down the server part to this:

==> File: Notifier.rb

require "drb/drb"
require "observer"

class Notifier
   include Observable
   def change( i )
     puts "Changing: #{'%2d' % i}"
     changed( true )
     notify_observers( Time.now, i )
   end
end

DRb.start_service( 'druby://localhost:4711', Notifier.new )
puts DRb.uri
DRb.thread.join



The program using that service got shortened to this:

require 'drb'

class Dummy
   def initialize( observed )
     observed.add_observer( self )
   end

   # define update as required by the Observer module
   def update( *args )
     puts "!!!"
     p *args
   end
end

notifier = DRbObject.new( nil, 'druby://localhost:4711' )
d = Dummy.new( notifier )

# Do something with the notifier to get d's 'update' called
notifier.change( rand( 10 ) )


Now, running the notifier works fine, but starting the client leaves me 
with this:

[884] stk@tao ~/devel/util: ruby dummy.rb
(druby://localhost:4711) /usr/local/lib/ruby/1.8/observer.rb:126:in 
`add_observer': Observer <Dummy> needs to respond to 'update'. 
(NoMethodError)
         from dummy.rb:5:in `initialize'
         from dummy.rb:15:in `new'
         from dummy.rb:15


{ Note that I changed the part of 'observer.rb' to give some information 
about the observer that leads to the exception - just to be sure that 
it's an Object of class Dummy that rose the exception. }

Now, why do I get the exception? I defined 'update' of objects of class 
Dummy, didn't I?

Where's my mistake? Is it that way a good way anyway?

Happy rubying!

Stephan