From: ara.t.howard@...
Date: 2006-05-23T05:57:15+09:00
Subject: Re: initializing instance variables in a module

On Tue, 23 May 2006, Jeff Rose wrote:

> Well, it took a while but I found the answer.  This is done in ActiveRecord 
> to wrap a possible setup method in the unit tests for auto-instrumentation of 
> fixtures.  Once I knew what to look for, however, I found an old post by 
> Florian Gross that lays out the exact answer nicely:
>
> http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/129834
>
> For educational purposes though, I'll explain a few things I found...
>
> 1) The append_features method is deprecated, and the included method is now 
> the preferred callback.  This method is called when a module is included, and 
> it is passed the module that has done the including.
>
> 2) There is another callback, method_added, which is a meta-class method that 
> is called every time a new method is defined for its class, and the symbol 
> for the method name is passed.
>
> So, the trick is to add a custom method_added method to the meta-class of the 
> class that does the including by extending it in the included callback.  The 
> custom method_added needs to redefine initialize after it is added so that it 
> does the custom setup, then calls the original.
>
> Phew...  Too bad we can't just have another callback that lets us do this 
> easily.  Maybe that feels too much like multiple inheritance or something?
>
> module Foo
>  def module_initialize
>    @myvar = Hash.new {|k,v| k[v] = []}
>  end
>
>  def bar(key)
>    @mybar[key]
>  end
> end
>
> Isn't this a common thing to want?

and precisely one of the reasons i wrote traits - with traits you can simply
ignore the issue altogether:

   harp:~ > gem install traits

   harp:~ > cat a.rb
   require 'rubygems'
   require 'traits'

   module Foo
     trait 'myvar' => Hash.new{|k,v| k[v] = []}
     def bar(key) myvar[key] end
   end

   class Bar
     include Foo
   end

   b = Bar.new

   p b.bar(:k)

   p b.myvar


   harp:~ > ruby a.rb
   []
   {:k=>[]}



;-)

-a
-- 
be kind whenever possible... it is always possible.
- h.h. the 14th dali lama