gistfile1.ru

module Callbacks
  @@callbacks = {}

  def has_callback_hook(name)
    @@callbacks[name] = []
 
    class_eval <<-CLASS
        def #{name}_add block
          @callbacks[#{name}] << block
        end

        def #{name}
          @callbacks[#{name}].each do |block|
            block.call self
          end
        end
    CLASS
  end
end

gistfile2.ru

require 'callbacks'

class TestingCallbacks
  extend Callbacks

  has_callback_hook :before_foo
  has_callback_hook :after_foo

  attr_accessor :bar

  def initialize
    self.bar = "baz"
  end 

  def foo 
    self.before_foo

    puts self.bar

    self.after_foo
  end 
end

gistfile3.ru

require 'testing_callbacks'

test = TestingCallbacks.new
test.before_foo_add lambda { |t| t.bar = "Hello!" }
test.foo