ActiveSupport::Concernとは 〜ソースコードリーディング〜

extend ActiveSupport::Concern

module M
  #モジュールの中でActiveSupport::Concernモジュールをextendする
  extend ActiveSupport::Concern

  def instance_method
    puts "M#instance_method"
  end
  
  included do
    attr_accessor :name
  end

  #ClassMethodsモジュールの中で、ベースクラスに追加するクラスメソッドを定義する
  module ClassMethods
    def class_method
      puts "M.class_method"
    end
  end
end

class C
  #ActiveSupport::Concernをextendしたモジュールをincludeする
  include M
end

c = C.new
c.instance_method 
# => M#instance_method

c.name = "C#name"
c.name
# => C#name

C.class_method
# => M.class_method

module M
  extend ActiveSupport::Concern

  def instance_method
    puts "M#instance_method"
  end
  
  included do
    attr_accessor :name
  end

  module ClassMethods
    def class_method
      puts "M.class_method"
    end
  end
end

class C
  include M
end

# app/models/user.rb

class User < ApplicationRecord
  validates :username, presence: true
end

# app/models/application_record.rb

class ApplicationRecord < ActiveRecord::Base
end

# rails/activerecord/lib/active_record/base.rb

module ActiveRecord # :nodoc:
  class Base
    include ActiveModel::API
    ...
  end
end

# rails/activemodel/lib/active_model/api.rb

module ActiveModel
  module API
    extend ActiveSupport::Concern
    include ActiveModel::Validations
  end
end

# activemodel/lib/active_model/validations/validates.rb

module ActiveModel
  module Validations
    module ClassMethods
      def validates(*attributes)
        defaults = attributes.extract_options!.dup
        validations = defaults.slice!(*_validates_default_keys)

        raise ArgumentError, "You need to supply at least one attribute" if attributes.empty?
        raise ArgumentError, "You need to supply at least one validation" if validations.empty?

        defaults[:attributes] = attributes

        validations.each do |key, options|
          key = "#{key.to_s.camelize}Validator"

          begin
            validator = const_get(key)
          rescue NameError
            raise ArgumentError, "Unknown validator: '#{key}'"
          end

          next unless options

          validates_with(validator, defaults.merge(_parse_validates_options(options)))
        end
      end
      ...
    end
  end
end

# rails/activesupport/lib/active_support/concern.rb

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

    class MultiplePrependBlocks < StandardError # :nodoc:
      def initialize
        super "Cannot define multiple 'prepended' blocks for a Concern"
      end
    end

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

    def append_features(base) # :nodoc:
      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 prepend_features(base) # :nodoc:
      if base.instance_variable_defined?(:@_dependencies)
        base.instance_variable_get(:@_dependencies).unshift self
        false
      else
        return false if base < self
        @_dependencies.each { |dep| base.prepend(dep) }
        super
        base.singleton_class.prepend const_get(:ClassMethods) if const_defined?(:ClassMethods)
        base.class_eval(&@_prepended_block) if instance_variable_defined?(:@_prepended_block)
      end
    end

    def included(base = nil, &block)
      if base.nil?
        if instance_variable_defined?(:@_included_block)
          if @_included_block.source_location != block.source_location
            raise MultipleIncludedBlocks
          end
        else
          @_included_block = block
        end
      else
        super
      end
    end

    def prepended(base = nil, &block)
      if base.nil?
        if instance_variable_defined?(:@_prepended_block)
          if @_prepended_block.source_location != block.source_location
            raise MultiplePrependBlocks
          end
        else
          @_prepended_block = block
        end
      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

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

# このように定義されていた場合...
module M
  extend ActiveSupport::Concern

  class_methods do
    def hoge
      'M#hoge'
    end
  end
end

# 以下のように定義し直されるということ
module M
  module ClassMethods
    def hoge
      'M#hoge'
    end
  end
end

def included(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_included_block)
      if @_included_block.source_location != block.source_location
        raise MultipleIncludedBlocks
      end
    else
      @_included_block = block
    end
  else
    super
  end
end

**module M2
  include M1
end**

**module M1
  extend ActiveSupport::Concern

  included do    
    def hoge
      puts "M#hoge"
    end
  end
end**

def append_features(base) # :nodoc:
  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 prepended(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_prepended_block)
      if @_prepended_block.source_location != block.source_location
        raise MultiplePrependBlocks
      end
    else
      @_prepended_block = block
    end
  else
    super
  end
end

def prepend_features(base) # :nodoc:
  if base.instance_variable_defined?(:@_dependencies)
    base.instance_variable_get(:@_dependencies).unshift self
    false
  else
    return false if base < self
    @_dependencies.each { |dep| base.prepend(dep) }
    super
    base.singleton_class.prepend const_get(:ClassMethods) if const_defined?(:ClassMethods)
    base.class_eval(&@_prepended_block) if instance_variable_defined?(:@_prepended_block)
  end
end