From: Alex Young <alex@...>
Date: 2007-10-20T04:19:03+09:00
Subject: Re: Puzzle...cleaner way to redefine a method?

Blackie wrote:
> Here's a puzzle. There must be a cleaner way to do this.
> 
> I would like to have users of this class be able to call "set_wrapper"
> and define pre and post behavior on the "working" method. The only
> simple way I've found is evaling a string the redefines "working".
> Searching threads on instance and class_eval tends to turn up an oil
> slick of arguments and misinformation. Help is apprecaited!
> 
> ~~~~~~~~~~
> class Thing
>   def set_wrapper(string)
>     eval(string)
>   end
> 
>   def working
>     yield
>   end
> 
>   def main
>     working do
>       p 'test'
>     end
>   end
> end
> 
> a = Thing.new
> 
> a.main
> 
> a.set_wrapper("def working; p 'pre'; yield; p 'post'; end")
> 
> a.main

Try this (untested):

class Thing
   def add_pre_call(&prc)
     (@pre_procs ||= []) << prc
   end
   def add_post_call(&prc)
     (@post_procs ||= []) << prc
   end
   def working
     @pre_procs.each {|p| p.call} if @pre_procs
     yield
     @post_procs.each{|p| p.call} if @post_procs
   end
end

a = Thing.new
a.main
a.add_pre_call { p 'pre' }
a.add_post_call { p 'post' }
a.main

-- 
Alex