What does class_methods do in concerns?

Question

I am reading some codes which use the concerns in Rails 4.

I read some articles to say, if we would like to include class methods using module ClassMethods, but the code I read using something like:

class_methods do
  def **** 
  end
end

Answer 1

It's just for convenience. module ClassMethods is pure Ruby, but class_methods is defined in ActiveSupport::Concern for convenience. If you look at a source code you'll find that class_methods does exactly the same thing

  # activesupport/lib/concern.rb
  def class_methods(&class_methods_module_definition)
    mod = const_defined?(:ClassMethods, false) ?
      const_get(:ClassMethods) :
      const_set(:ClassMethods, Module.new)

    mod.module_eval(&class_methods_module_definition)
  end