Active Support Concern

典型的模組如下所示

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

  module ClassMethods
    ...
  end
end

使用 ActiveSupport::Concern，上述模組可以改寫成

require "active_support/concern"

module M
  extend ActiveSupport::Concern

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

  class_methods do
    ...
  end
end

此外，它優雅地處理模組相依性。假設有一個 Foo 模組和一個依賴於前者的 Bar 模組，我們通常會寫以下內容

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，為什麼 Host 要關心 Bar 的相依性，也就是 Foo 呢？我們可以試著直接在 Bar 中包含 Foo 來向 Host 隱藏這些相依性

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

class Host
  include Bar
end

很不幸地，這無法運作，因為當包含 Foo 時，它的 base 是 Bar 模組，而不是 Host 類別。使用 ActiveSupport::Concern，模組相依性會正確地解析

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

前置 concerns

就像 include 一樣，concerns 也支援 prepend，並有一個對應的 prepended do 回呼。module ClassMethods 或 class_methods do 也會前置。

prepend 也用於任何相依性。