What problems did active_support concerns solve?

In the Rails project some of the most commonly used methods are extracted and generally placed in the corresponding concerns directory. generally different concern files are stored in these directories. a file is a module for example:

module UserPlan
  extend ActiveSupport::Concern
  included do 
    belongs_to :plan
  end
  class_method do 
    def cached_all_plan 
      #bala bala
    end
  end
end

Note that the module extend ActiveSupport::Concern is used in this module. we will explore its benefits and why this module exists.

Let's start with the following example:

module Foo
  def self.included(base)
    base.class_eval do
      def self.method1
        ...
      end
    end
  end
end

module Bar
  def self.included(base)
    base.method1
  end
end

class Host
  include Foo # The Foo module was introduced because the Bar module depends on it
  include Bar # Bar reference is to call its method1 method
end

If you want to use the method1 method in the Host class, you must refer to the Bar module but the Bar module depends on the Foo module.

Did you find any problems? the Host class does not care about the Foo module but in order to call the method1 method when the Bar is included. you have to Introduce the Foo module.

There is an improved code along the above logic:

module Bar
  include Foo
  def self.included(base)
    base.method1
  end
end
class Host
  include Bar
end

The Host only cares about the reference of Bar so the Foo module is put in the Bar. It seems that there is no problem but when you implement the running code, you will find that the code cannot run.

In the included hook method of the Bar module the base object points to the Host and the include Foo in Bar will put method1 on the instance object of Bar. when the instance object of the Host calls method1, it will throw a method not found exception

How to do it?

require 'active_support/concern'
module Foo
  extend ActiveSupport::Concern
  included do
    def self.method1
      ...
    end
  end
end

module Bar
  extend ActiveSupport::Concern
  include Foo

  included do
    self.method1
  end
end

class Host
  include Bar 
end

By referring to ActiveSupport::Concern to solve the problem of interdependence between modules and modules.

The comparison found that the code is much simpler and the call is also very convenient. the readability of the code has also become very good and very clear. now ActiveSupport::Concern is quite widely used and many good gems will be used. after all It was put in the ActiveSupport Gem.

Hope it can help you :)