Simple callback implementation in plain Ruby

before_callbackable.rb

module BeforeCallbackable
  module ClassMethods
    def before(**arg)
      @before_callbacks = {} unless defined?(@before_callbacks)

      @before_callbacks.merge!(arg)
    end

    def method_added(method_name)
      if before_callbacks.key? method_name
        renamed_method = "before_#{method_name}".to_sym
        fail ArgumentError, "method #{renamed_method} is already defined" if respond_to?(renamed_method)
        
        define_method renamed_method, instance_method(method_name)
        private renamed_method

        before_method = before_callbacks.delete(method_name)
        define_method method_name do |*args|
          send(before_method)
          send(renamed_method, *args)
        end
      end
    end

    private
      attr_reader :before_callbacks
  end

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

class TestBeforeCallback
  include BeforeCallbackable

  before qqq: :www

  def qqq
    'qqq'
  end

  private

    def www
      puts 'www'
    end
end

TestBeforeCallback.new.qqq