Active Support Concern

一个典型的模块看起来像这样

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，上面的模块可以改为以下写法

require "active_support/concern"

module M
  extend ActiveSupport::Concern

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

  class_methods do
    ...
  end
end

此外，它优雅地处理模块依赖。给定一个Foo 模块和一个依赖于前者的Bar 模块，我们通常会写以下代码

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

但为什么Host 要关心Bar 的依赖关系，即Foo？我们可以尝试通过直接在Bar 中包含Foo 来隐藏这些依赖关系，使其对Host 不可见

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

class Host
  include Bar
end

不幸的是，这将不起作用，因为当包含Foo 时，它的base 是Bar 模块，而不是Host 类。使用ActiveSupport::Concern，模块依赖关系将得到正确解决

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

预置关注点

就像include 一样，关注点也支持prepend，并带有一个相应的prepended do 回调。module ClassMethods 或class_methods do 也将被预置。

prepend 也用于任何依赖关系。