meta.rb

module AOP
  def around(fn_name)
    old_method = instance_method(fn_name)
    define_method(fn_name) do |*args|
      yield :before, args if block_given?
      old_method.bind(self).call *args
      yield :after, args if block_given?
    end
  end
end

class Foo
  extend AOP

  def process(msg)
    puts msg
  end
end

Foo.around('process') do |state, args|
  if state == :before
    puts 'Blah 1'
  elsif state == :after
    puts 'Blah 2'
  end
end

Foo.new.process('hai')