• 下面用到了 module 的 hook方法 included
• 下面也可以看出来一个module是怎么被引入到一个类里面的 （include extend require）

来看一个 typical 的 module

module M
  def self.included(base)
    base.extend ClassMethods
    base.class_eval do 
      scope :disabled, -> { where(disabled: true) }
    end
  end
  
  module ClassMethods
    
  end
end

然后是一个 active_support/concern

require 'active_support/concern'

module M
  extend ActiveSupport::Concern
  
  included do 
    scope :disabled, -> { where(disabled: true) }
  end
  
  class_methods do 
    
  end
end

然后就是 module 的依赖了 Bar依赖于Foo

module Foo
  def self.included(base)
    base.class_eval do 
      def self.method_injected_by_foo
      
      end
    end
  end
end

module Bar
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Foo
  include Bar
end

使用 ActiveSupport::Concern 之后 依赖就是可以是单独的

module Foo
  extend ActiveSupport::Concern
  included do 
    def self.method_injected_by_foo
    end
  end
end

module Bar
  extend ActiveSupport::Concern
  include Foo
  
  included do
    self.method_injected_by_foo
  end
end

class Host
  include Bar
end

有了上面的这些背景知识 。很多时候一些工具类可以如是扩展

#源码 其实挺精彩的 append_features 也很好的说明了 extend 和 include的区别

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
        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.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