rails concern

rails ActiveSupport::Concern封装了包含并扩展技术，解决了链式包含的问题一个模块可以通过扩展Concern模块来实现包含并扩展的功能。
concern.rb 代码如下

module ActiveSupport
module Concern
    class MultipleIncludedBlocks < StandardError #:nodoc:
      def initialize
        super "Cannot define multiple 'included' blocks for a Concern"
      end
    end

    def self.extended(base) #:nodoc:
      base.instance_variable_set(:@_dependencies, [])
    end

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

    def included(base = nil, &block)
      if base.nil?
        raise MultipleIncludedBlocks if instance_variable_defined?(:@_included_block)

        @_included_block = block
      else
        super
      end
    end

    def class_methods(&class_methods_module_definition)
      mod = const_defined?(:ClassMethods, false) ?
        const_get(:ClassMethods) :
        const_set(:ClassMethods, Module.new)

      mod.module_eval(&class_methods_module_definition)
    end
  end
end

append_features 是在模块被include时调用，该方法实现把模块加入base的祖先链中，通过@_dependencies记录模块的继承关系，分别并把 class_methods 或 ClassMethods 里的块使用module_eval定义为类方法，进而实现扩展
使用如下

require 'active_support'
module A
  def a_class_method
    puts 'a_class_method'
  end
end

module B
  extend ActiveSupport::Concern
  class_methods do
     def b_class_method
         puts 'b_class_method'
     end
    include A
  end
end

class X
   include B
end
X.a_class_method
X.b_class_method

完美实现了链式包含的问题