method_interception.rb

module MethodInterception
  def method_added(meth)
    return unless (@intercepted_methods ||= []).include?(meth) && !@recursing

    @recursing = true # protect against infinite recursion

    old_meth = instance_method(meth)
    define_method(meth) do |*args, &block|
      puts 'before'
      old_meth.bind(self).call(*args, &block)
      puts 'after'
    end

    @recursing = nil
  end

  def before_filter(meth)
    (@intercepted_methods ||= []) << meth
  end
end


class HomeWork
  extend MethodInterception

  before_filter(:say_hello)

  def say_hello
    puts "say hello"
  end
end


HomeWork.new.say_hello
# before
# say hello
# after