Ruby method interception

Question

I want to intercept method calls on a ruby-class and being able to do something before and after the actual execution of the method. I tried the following code, but get the error:

MethodInterception.rb:16:in before_filter': (eval):2:inalias_method': undefined method say_hello' for classHomeWork' (NameError) from (eval):2:in `before_filter'

Can anybody help me to do it right?

class MethodInterception

  def self.before_filter(method)
    puts "before filter called"
    method = method.to_s
    eval_string = "
      alias_method :old_#{method}, :#{method}

      def #{method}(*args)
        puts 'going to call former method'
        old_#{method}(*args)
        puts 'former method called'
      end
    "
    puts "going to call #{eval_string}"
    eval(eval_string)
    puts "return"
  end
end

class HomeWork < MethodInterception
  before_filter(:say_hello)

  def say_hello
    puts "say hello"
  end

end

Answer 1

Jörg W Mittag's solution is pretty nice. If you want something more robust (read well tested) the best resource would be the rails callbacks module.

Answer 2

Less code was changed from original. I modified only 2 line.

class MethodInterception

  def self.before_filter(method)
    puts "before filter called"
    method = method.to_s
    eval_string = "
      alias_method :old_#{method}, :#{method}

      def #{method}(*args)
        puts 'going to call former method'
        old_#{method}(*args)
        puts 'former method called'
      end
    "
    puts "going to call #{eval_string}"
    class_eval(eval_string) # <= modified
    puts "return"
  end
end

class HomeWork < MethodInterception

  def say_hello
    puts "say hello"
  end

  before_filter(:say_hello) # <= change the called order
end

This works well.

HomeWork.new.say_hello
#=> going to call former method
#=> say hello
#=> former method called