From: Mark Hubbart <discordantus@...>
Date: 2005-01-01T06:43:12+09:00
Subject: Re: attr / attr_accessor

On Thu, 30 Dec 2004 22:34:40 +0900, Markus Werner
<markus.werner@wobcom.de> wrote:
> Hi Ruby Folk,
> 
> I'm quite a newbie in programming. Last year I came to perl and now I'm
> testing ruby.
> 
> I like the methods attr, attr_*. But there is one drawback. You can't tell the
> methode that there should be a standard code that will be executed on every
> access. IMHO The best would be a pre and a posthook.


Perhaps:

class Module
  def attr_writer_proc(*args, &block)
    names = args.map{|s| s.to_sym}
    names.each do |name|
      define_method "#{name}=" do |val|
        instance_variable_set "@#{name}", block.call(val)
      end
    end
  end
  def attr_reader_proc(*args, &block)
    names = args.map{|s| s.to_sym}
    names.each do |name|
      define_method name do
        block.call instance_variable_get("@#{name}")
      end
    end
  end
end

This would act as a combo pre-hook and data filter:

class Foo
  # make sure the values are strings
  attr_writer_proc :foo, :bar do |val|
    val.to_str
  end

  # we only want to store copies the objects passed
  attr_writer_proc(:baz){|val| val.dup }
  
  # log changes to quux
  attr_writer_proc(:quux){|val| log("setting quux to %p"%[val]); val }
end

this could conceivably be wrapped up into the attr_writer/attr_reader
methods, since they currently don't take blocks... Also, it might be
handy to check the arity of the block and pass the variable name as an
optional second argument.

hth,
Mark