From: David Alan Black <dblack@...>
Date: 2002-08-05T08:29:06+09:00
Subject: Re: Detecting when an instance variable is created/set

Hi --

On Mon, 5 Aug 2002, Harry Ohlsen wrote:

> > > Unfortunately, for it to be useful to me, I'd specifically need it to
> > > work in initialize() as well.
> >
> > I don't know why it does not work in initialize (I assumed it would).
> > A dirty fix:
> >
> > module Initializer
> >   def initialize(*args)
> >     puts "Here I'll handle these variables: #{instance_variables.inspect}"
> >   end
> > end
> >
> > module MonitorVariables
> >   ...
> > end
> >
> > class Test
> >   def initialize(*args)
> >     @a = 1
> >     @b = 'world'
> >     super(*args)
> >   end
>
> What is that super() calling?  It seems to be the initialize() in Initializer,
> but I don't understand why.  I would have thought that it would be calling
> the one from perhaps Object (since Test doesn't specifically subclass
> anything else).
>
> > I also don't know why, if put in MonitorVariables, initialize won't be
> > called, and needs to be put in a different module.
>
> I added the code in your Initializer module into MonitorVariables and it
> worked OK, so while it seems initialize() seems to need to be handled
> separately, it doesn't appear to need to be in a different module.

My experience was the same as Massimiliano's: the #initialize didn't
seem to get found by 'super' if it was in the same module as
#append_features.  I don't know why not.

(On #initialize being handled separately -- see earlier post about
private methods.)

> I still have the issue, though, that with this module included, all of the
> instance variables end up being set to "true", rather than to the values they
> were actually assigned.
>
> Any idea why that is and how to fix it?

Yes -- the dynamically created replacement methods in M.'s
append_features method aren't returning the value of the call to the
original method.  Here's what I think is a drop-in replacement (a few
variable names tweaked just to conform to my habits while hacking :-)

  module MonitorVariables
    def self.append_features(klass)
      (klass.instance_methods + ['initialize']).each do |method|
        oldmeth = "___#{method}"
        klass.class_eval <<-EOF
        alias #{oldmeth} #{method}
        def #{method}(*params,&block)
          old_iv = instance_variables
          res = #{oldmeth}(*params,&block)
          (diff = instance_variables - old_iv).empty? or
            puts "variable(s) " + diff.inspect +
             " added in method #{method}"
          return res
        end
        EOF
      end
    end
  end


I'm wondering, though, whether there's a simpler way to do what you're
trying to do (the self.a = x approach maybe?).


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav