method_interceptor.rb

module MethodInterceptor

  def self.included(base)
    base.extend(ClassMethods)
    base.send(:include, InstanceMethods)
    base.class_eval do 
      # we declare the method_list on the class env
      @_instance_method_list = base.instance_methods.inject(Hash.new) do |methods, method_name|
        # we undef all methods
        if !%w(__send__ __id__ method_missing class).include?(method_name)
          methods[method_name.to_sym] = base.instance_method(method_name)
          base.send(:undef_method, method_name)
        end
        methods
      end
    end
  end

  module ClassMethods

    def _instance_method_list
      @_instance_method_list
    end

    def method_added(name)
      return if [:before_method, :method_missing].include?(name)
      _instance_method_list[name] = self.instance_method(name)
      self.send(:undef_method,  name)
      nil
    end

  end

  module InstanceMethods

    def before_method(method_name, *args)
      # by defaults it always will be called
      true
    end

    def method_missing(name, *args)
      if self.class._instance_method_list.key?(name)
        if before_method(name, *args) 
          self.class._instance_method_list[name].bind(self).call(*args)
        else
          super
        end
      else
        super
      end
    end
  end

end

class Say
  include MethodInterceptor

  def before_method(method_name, *args)
    # you cannot say hello world!
    return !(method_name == :say && args[0] == 'hello world')
  end

  def say(msg)
    puts msg
  end

end