Active Support Concern¶ ↑

一个典型的模块看起来是这样的

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

  module ClassMethods
    ...
  end
end

使用 ActiveSupport::Concern，上面的模块可以这样写

require "active_support/concern"

module M
  extend ActiveSupport::Concern

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

  class_methods do
    ...
  end
end

此外，它还能很好地处理模块依赖。给定一个 Foo 模块和一个依赖于前者的 Bar 模块，我们通常会这样写

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 Host 为什么要关心 Bar 的依赖，即 Foo 呢？我们可以尝试直接将 Foo 包含在 Bar 中来隐藏这些依赖

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

class Host
  include Bar
end

不幸的是，这行不通，因为当包含 Foo 时，它的 base 是 Bar 模块，而不是 Host 类。有了 ActiveSupport::Concern，模块依赖就会被正确解析

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

Prepending concerns¶ ↑

就像 include 一样，concerns 也支持 prepend，并有相应的 prepended do 回调。module ClassMethods 或 class_methods do 也会被 prepended。

prepend 也用于任何依赖项。