为什么要使用ActiveSupport :: Concern而不是仅使用普通模块？

来自https://api.rubyonrails.org/classes/ActiveSupport/Concern.html：

一个典型的模块如下所示：

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

  module ClassMethods
    ...
  end
end

通过使用ActiveSupport::Concern上述模块，可以改为：

require 'active_support/concern'

module M
  extend ActiveSupport::Concern

  included do
    scope :disabled, -> { where(disabled: true) }
  end

  class_methods do
    ...
  end
end

而且，它可以优雅地处理模块依赖性。给定一个Foo模块和一个Bar依赖于前者的模块，我们通常将编写以下内容：

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 # We need to include this dependency for Bar
  include Bar # Bar is the module that Host really needs
end

但是为什么要Host在乎Bar的依赖Foo呢？我们可以尝试将这些隐藏起来，使其不Host直接包含Foo在中Bar：

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

class Host
  include Bar
end

不幸的是，这是行不通的，因为当Foo包含时，其基础是Bar模块，而不是Host类。使用ActiveSupport::Concern，可以正确解决模块依赖性：

require 'active_support/concern'

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 # It works, now Bar takes care of its dependencies
end