Callbacks module for Ruby classes

callbacks.rb

# encoding: utf-8

# Small module to add the ability to register callbacks at class level to use it at instace level.
# With this module you can create small DSLs on your classes.
#
# Example usage:
#
# class Person
#   include ::Callbacks
#
#   def self.setup(&block)
#     _register_callback(:setup, block)
#   end
#
#   def hello(a)
#     _execute_callback(:setup, a)
#   end
# end
#
# Person.setup { |a| puts "Hello #{a}!!" }
# p = Person.new
# p.hello('world')
# # => "Hello world!!"
#
module Callbacks
  def self.included(base)
    base.class_eval do
      class << self
        def _callbacks
          @_callbacks ||= Hash.new { |h, k| h[k] = [Proc.new { |*| }, []] }
        end
        protected :_callbacks

        def _register_callback(name, block, *arguments)
          _callbacks[name] = [block, arguments]
        end
        protected :_register_callback
      end

      def _execute_callback(name, *arguments)
        block, extra = _registered_callbacks[name]
        arguments = arguments + extra

        instance_exec *arguments, &block
      end
      protected :_execute_callback

      def _registered_callbacks
        @_registered_callbacks ||= self.class.send(:_callbacks).dup
      end
      protected :_registered_callbacks

      # Modify a callback at instance level, without mess with the
      # callbacks registered in the class
      def _modify_callback(name, block, *arguments)
        _registered_callbacks[name] = [block, arguments]
      end
      protected :_modify_callback
    end
  end
end

person.rb

# encoding: utf-8

class Person
  include ::Callbacks

  def self.setup(&block)
    _register_callback(:setup, block)
  end

  def hello(a)
    _execute_callback(:setup, a)
  end
end

Person.setup { |a| puts "Hello #{a}!!" }

test.rb

# encoding: utf-8

require './callbacks'
require './person'

someone = Person.new
$stdout.puts someone.hello "there" # => Hello there!!