ruby-on-rails - 为什么要使用 ActiveSupport::Concern 而不仅仅是一个普通的模块？

Question

我从来不明白为什么必须使用 ActiveSupport::Concern 用于 mixins 而不是普通模块。对于 ActiveSupport::Concern 提供的内容(至少在 Rails 5 中)是否有一个简单的答案，即不使用 ActiveSupport::Concern 的简单模块会做什么？

Answer 1

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上面的模块可以写成:

require 'active_support/concern'

module M
  extend ActiveSupport::Concern

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

  class_methods do
    ...
  end
end

此外，它优雅地处理模块依赖关系。给定一个 Foo模块和 Bar依赖于前者的模块，我们通常会编写以下内容:

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关心 Bar的依赖，即 Foo ?我们可以尝试将这些隐藏在 Host 中。直接包括 Foo在 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，这不起作用，因为 Foo包括在内，它的基础是 Bar模块，而不是 Host类(class)。与 ActiveSupport::Concern ，模块依赖关系得到正确解决:

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