From: Hugh Sasse <hgs@...>
Date: 2005-10-21T01:40:48+09:00
Subject: Re: Reflection, observ(er|able) instance vars?

On Fri, 21 Oct 2005, Edward Faulkner wrote:

> On Thu, Oct 20, 2005 at 09:01:45PM +0900, Hugh Sasse wrote:
> > I'd like to hold a hash (number not Hash.new()) for this object so I
> > can compare it quickly with another.  How do I detect that it has
> > changed so that the hash gets updated correctly?
> 
> I think the simplest solution it to define your own attribute writer
> and always use it:

        [...]

If only I could add it to object.  I've tried, but _Why's metastuff
confuses me just sufficiently to stop me making the transition.

Given  metaid.rb thusly:
<quote>
#!/usr/local/bin/ruby -w
#
# Due to Whytheluckystiff.
# http://www.whytheluckystiff.net/articles/seeingMetaclassesClearly.html
# Reformatted to work around bugs in vim indent.
#
#
class Object
  # The hidden singleton lurks behind everyone
  def metaclass
    class << self
      self
    end
  end
  def meta_eval &blk
    metaclass.instance_eval &blk
  end

  # Adds methods to a metaclass
  def meta_def name, &blk
    meta_eval { define_method name, &blk }
  end

  # Defines an instance method within a class
  def class_def name, &blk
    class_eval { define_method name, &blk }
  end
end

</quote>

and  hashing.rb thusly:

<quote>
#!/usr/local/bin/ruby -w 
#
#  Make a class keep its hash up to date by using 
#  accessor methods that call hash on update.
#
#  Hugh Sasse
#  created: 20-OCT-2005  
#
#

require 'metaid'   # See http://www.whytheluckystiff.net/articles/seeingMetaclassesClearly.html

 class Object
   def self.attr_hashing( *arr )
     attr_reader *arr
     arr.each do |var|
       meta_def var do |val|
         @var 
       end
       vareq = (var.to_s + "=")
       meta_def vareq do |val|
         instance_variable_set( "@#{var}", val )
         self.hash
       end
     end
=begin
     class_def :initialize do
       self.class.traits.each do |k,v|
         instance_variable_set( "@#{k}", v )
       end
     end
=end
   end
 end

</quote>

which I've produced from some badly aimed kicks at Dwemthy's Array,
I can't actually get the results I'd like.


<quote>
#!/usr/local/bin/ruby -w
#
# Test out the hashing prog.
#

require 'hashing'

class Wombat
  attr_hashing :wom, :bat
  attr_reader :thehash

  def initialize
    @wom = "wom"
    @bat = "bat"
    @thehash = hash
  end

  def hash
    puts "hash called"
    @thehash = (@wom.to_s + @bat.to_s).hash
  end

  def to_s
    @wom.inspect + @bat.inspect
  end
end

x = Wombat.new  # produce the hash message
puts x
puts x.wom
puts x.bat
puts x.methods.sort.join(', ')
x.wom = "WOM"   # produce the hash message
puts x.wom
x.bat = "BAT"   # produce the hash message
puts x.bat
</quote>

Does anyone feel inclined to help me out?  :-)
> 
> 
> regards,
> Ed
> 
        Thank you,
        Hugh