Wrapper оборачивает вызовы методов вызовами before- и after-методов

wrapper.rb

# wrap_method оборачивает вызовы методов вызовами before- и after-методов
# Возможно множественная обертка, в ходе которой образуется method-chain
module Wrappable
  class WrapperOptions
    attr_reader :before_callback, :after_callback

    def initialize(&block)
      instance_eval(&block)
    end

    def before(method_name)
      @before_callback = method_name    
    end
    def after(method_name)
      @after_callback = method_name
    end
  end

  def wrap_method(method_name, &block)
    @wrap_count = @wrap_count.nil? ? 0 : @wrap_count + 1

    old_method_name = "#{method_name}_wrappable_#{@wrap_count}"   

    wrapper_options_hash = instance_variable_get("@wrapper_options")
    instance_variable_set("@wrapper_options", wrapper_options_hash = {}) if (wrapper_options_hash.nil?)
    wrapper_options_hash[old_method_name] = WrapperOptions.new(&block)    

    alias_method old_method_name, method_name

    define_method(method_name) do
      wrapper_options = self.class.instance_variable_get("@wrapper_options")[old_method_name]
      send(wrapper_options.before_callback) unless wrapper_options.before_callback.nil?
      send(old_method_name)
      send(wrapper_options.after_callback) unless wrapper_options.after_callback.nil?
    end
  end
end


class My
  extend(Wrappable)

  def target_method
    puts "puts target_method"
  end

  def before1
    puts "before1"
  end
  def before2
    puts "before2"
  end


  def after1
    puts "after1"
  end
  def after2
    puts "after2"
  end

  wrap_method :target_method do
    before :before1
    after  :after1
  end

  wrap_method :target_method do
    before :before2
    after  :after2
  end
end

My.new.target_method
# => before2
# => before1
# => puts target_method
# => after1
# => after2

# злоумышленники случайно или нет перезаписали локальную переменную
My.instance_variable_set("@wrapper_options", nil)
My.new.target_method
# NoMethodError: undefined method `[]' for nil:NilClass

Реализовано на без статьи http://blog.crowdint.com/2011/01/14/building-a-basic-dsl-to-create-callbacks-in-ruby.html