From: Sean O'Halpin <sean.ohalpin@...>
Date: 2007-10-20T07:38:55+09:00
Subject: Re: Puzzle...cleaner way to redefine a method?

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Hi,

maybe this is slightly cleaner in interface (in that you pass in pre and
post procs rather than a string) but you still need to use eval somewhere
(in Ruby 1.8 anyway) to be able to use 'def' so you can pass on the block.
Shows the principle anyway.

module Wrapper
  def wrap(meth, pre_block, post_block)
    m = Module.new do
      define_method "pre_#{meth}", &pre_block
      define_method "post_#{meth}", &post_block
      eval %[
      def #{meth}(*a, &b)
        pre_#{meth}
        rv = super(*a, &b)
        post_#{meth}
        rv
      end
      ]
    end
    self.extend m
  end
end

class Thing
  include Wrapper

  def working
    yield
  end

  def main
    working do
      p 'test'
    end
  end
end

a = Thing.new
a.main
a.wrap :working, proc { p 'pre' }, proc { p 'post' }
a.main
__END__
"test"
"pre"
"test"
"post"

If you don't want to include Wrapper in the class, you could use
a.extend(Wrapper)
instead.

Regards,
Sean

On 10/19/07, Blackie <mudsweatandears@gmail.com> 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
>
>
>

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