dynamic_empty_callback.rb

unless File.exists?('./Gemfile')
  File.write('./Gemfile', <<~GEMFILE)
    source 'http://rubygems.org'
    gem 'activesupport'
  GEMFILE
  system 'bundle install --path=vendor/bundle'
end

require 'bundler/setup'
require 'active_support'

class User
  include ActiveSupport::Callbacks
  define_callbacks :find
  set_callback :find, :after, :run_after_find

  def init_with
    run_callbacks(:find) { puts 'init_with!!' }
  end

  def run_after_find
    puts 'after find!!'
  end
end

puts '---'
u1 = User.allocate
u1.init_with

puts '---'
u2 = User.allocate
u2.extend(Module.new { def run_after_find; end })
u2.init_with

puts '---'
u3 = User.allocate
u3.init_with