gentle_overwrite.rb

# Blog post at 
# http://pivotallabs.com/users/rolson/blog/articles/1162-redefine-a-method-from-a-module-like-a-gentleman

module Teacher
  
  def self.included(base)
    base.extend ClassMethods
    base.overwrite_initialize
    base.instance_eval do
      def method_added(name)
        return if name != :initialize
        overwrite_initialize
      end
    end
  end
  
  module ClassMethods
    def overwrite_initialize
      class_eval do
        unless method_defined?(:custom_initialize)
          define_method(:custom_initialize) do
            puts "teacher initialized"
            original_initialize
          end
        end

        if instance_method(:initialize) != instance_method(:custom_initialize)
          alias_method :original_initialize, :initialize
          alias_method :initialize, :custom_initialize
        end
      end
    end
  end

end

class Person
  include Teacher

  def initialize
    puts "person initialized"
  end
end


if __FILE__ == $PROGRAM_NAME
  require 'test/unit'

  class TestTeacher < Test::Unit::TestCase
    def test_output_of_initialize
      out = capture_stdout { Person.new }
      lines = out.string.split(/\n/)
      assert_equal "teacher initialized", lines[0]
      assert_equal "person initialized", lines[1]
    end
  end
  
  
  # Test helper for capturing input to standard out
  # http://thinkingdigitally.com/archive/capturing-output-from-puts-in-ruby/
  require 'stringio'
  module Kernel
    def capture_stdout
      out = StringIO.new
      $stdout = out
      yield
      return out
    ensure
      $stdout = STDOUT
    end
  end
end