From: Michael Sullivan <mps@...>
Date: 2001-11-15T11:48:49+09:00
Subject: [ruby-talk:25241] Re: attr_accessor question

Avi,

Thanks for getting back to me.  We must be thinking along the same lines
because this is what I've come up with:

class Module
   def attr_callback(*accessors)
      accessors.each { |a|
         eval "
            def #{a}=(p)
               @#{a}=p
               self.respond_to?(attr_changed, TRUE) && attr_changed( :#{a} )
               @#{a}
            end
      
            def #{a}
               @#{a}
            end"
         }
   end
end
   
   
class TestClass
   attr_callback :e 
end

TestClass.instance_methods


However, it seems to completely ignore the "eval".  I've replaced the
eval with a puts for testing and the methods appear to be formed
correctly.  When I go to see if the instance methods are defined, I get
an empty array.

BTW, I tried your variant making attr_notify a class method for Module,
but it complains when defining Foo it cannot find attr_notify.

Seems like it should work, but it doesn't.

Any clues?

Mike




On Thu, Nov 15, 2001 at 10:38:39AM +0900, Avi Bryant wrote:
> Michael Sullivan <mps@discomsys.com> wrote:
> > I was wondering if when one has defined "attr_accessor" for their
> > instance variables if there was any way to determine when one of the
> > variables changes.
> 
> We really need a more generic attr framework, so that I can write something like
> 
> class Foo
>   attr [:read, :write, :initialize, :notify] :x, :y
> end
> 
> (and while we're at it, we need to be able to do the same thing for methods
> and classes...)
> 
> But for now, how about:
> 
> def Module.attr_notify(*vars)
>   vars.each do |var|
>     module_eval %{
>       def #{var}() @#{var} end
>       def #{var}=(_#{var})
>          @#{var} = _#{var}
>          changed(:#{var})
>       end
>     }
>   end
> end
> 
> 
> This is untested, but it should allow:
> 
> class Foo
>    attr_notify :x, :y
> 
>    def changed(var)
>      puts "#{var} was changed!"
>    end
> end

-- 
Michael P. Sullivan                                  
Distributed Computing Systems, LLC                         Cell: 516-429-2080 
E-Mail: mps@discomsys.com                           http://www.discomsys.com/ 
    * UNIX Systems and Database Consulting, Architecture and Management *
"Failing to plan, is planning to fail... plan for the worst, hope for the best"