rails/activesupport/lib/active_support/concern.rb

# A typical module looks like this:
#
#   module M
#     def self.included(base)
#       base.send(:extend, ClassMethods)
#       base.send(:include, InstanceMethods)
#       scope :foo, :conditions => { :created_at => nil }
#     end
#
#     module ClassMethods
#       def cm; puts 'I am a class method'; end
#     end
#
#     module InstanceMethods
#       def im; puts 'I am an instance method'; end
#     end
#   end
#
# By using <tt>ActiveSupport::Concern</tt> the above module could instead be written as:
#   
#   require 'active_support/concern'
#
#   module M
#     extend ActiveSupport::Concern
#
#     included do
#       scope :foo, :conditions => { :created_at => nil }
#     end
#
#     module ClassMethods
#       def cm; puts 'I am a class method'; end
#     end
#
#     module InstanceMethods
#       def im; puts 'I am an instance method'; end
#     end
#   end
#
# Moreover, it gracefully handles module dependencies. Given a Foo module and a Bar module which depends on the former, we would typically write the following:
#
#   module Foo
#     def self.included(base)
#       # Define some :enhanced_method for Host class
#       base.class_eval do
#         def self.enhanced_method
#           # Do enhanced stuff
#         end
#       end
#     end
#   end
#
#   module Bar
#     def self.included(base)
#       base.send(:enhanced_method)
#     end
#   end
#
#   class Host
#     include Foo # We need to include this dependency for Bar
#     include Bar # Bar is the module that Host really needs
#   end
#
# But why should Host care about Bar's dependencies, namely Foo? We could try to hide these from Host directly including Foo in Bar:
#
#   module Foo
#     def self.included(base)
#       # Define some :enhanced_method for Host class
#       base.class_eval do
#         def self.enhanced_method
#           # Do enhanced stuff
#         end
#       end
#     end
#   end
#
#   module Bar
#     include Foo 
#     def self.included(base)
#       base.send(:enhanced_method)
#     end
#   end
#
#   class Host
#     include Bar
#   end
#
# Unfortunately this won't work, since when Foo is included, its <tt>base</tt> is Bar module, not Host class.
# With <tt>ActiveSupport::Concern</tt>, module dependencies are properly resolved:
#
#   require 'active_support/concern'
#
#   module Foo
#     extend ActiveSupport::Concern
#     included do
#       class_eval do
#         def self.enhanced_method
#           # Do enhanced stuff
#         end
#       end
#     end
#   end
#
#   module Bar
#     extend ActiveSupport::Concern
#     include Foo
#
#     included do
#       self.send(:enhanced_method)
#     end
#   end
#
#   class Host
#     include Bar # Host only needs to care about Bar without needing to know about its dependencies
#   end
#
module ActiveSupport
  module Concern
    def self.extended(base)
      base.instance_variable_set("@_dependencies", [])
    end

    def append_features(base)
      if base.instance_variable_defined?("@_dependencies")
        base.instance_variable_get("@_dependencies") << self
        return false
      else
        return false if base < self
        @_dependencies.each { |dep| base.send(:include, dep) }
        super
        base.extend const_get("ClassMethods") if const_defined?("ClassMethods")
        base.send :include, const_get("InstanceMethods") if const_defined?("InstanceMethods")
        base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
      end
    end

    def included(base = nil, &block)
      if base.nil?
        @_included_block = block
      else
        super
      end
    end
  end
end