Question

In Ruby, I can do this:

module Foo
end

class Bar
  include Foo
end

module Foo
  def do_something_instancey
    puts "I'm an instance!"
  end
end

Then, if I instantiate a Bar object, I can call do_something_instancey on it:

b = Bar.new
b.do_something_instancey

However, if I do this…

module Foo
  def self.included(base)
    def base.do_something_classy do
      puts "I'm a class!"
    end
  end
end

My understanding is that because I included Foo in Bar before defining that class method, I cannot call Bar.do_something_classy because it never got “attached” to Bar.

I realize that might be slightly inaccurate/not really the right terminology. Regardless, is there a way, in the above example, to attach a class method to Bar from Foo after the module has already been included?

Answer 1

Here’s an example for both, class and instance methods:

module Foo
  def self.included(base)
    base.extend(ClassMethods)
  end

   module ClassMethods
   end
end

class Bar
  include Foo
end

module Foo
  def do_something_instancey
    puts "I'm an instance!"
  end

  module ClassMethods
    def do_something_classy
      puts "I'm a class!"
    end
  end
end

b = Bar.new
b.do_something_instancey
# => I'm an instance!
Bar.do_something_classy
# => I'm a class!

---

To add class methods to each class that has (already) included a specific module, you could traverse Ruby’s ObjectSpace:

ObjectSpace.each_object(Class) do |klass|
  if klass.include? Foo
    klass.define_singleton_method(:do_something_classy) do
      puts "I'm a class!"
    end
  end
end