module_eval,class_eval,singleton

http://onestepback.org/articles/10things/page027.html
http://my.donews.com/idlecat/2006/08/20/IpzenftEYkWGFjJpxPTDTJTCDOGKqTPzCaPZ/llllll
http://blackanger.blog.51cto.com/140924/84057
http://fcamel.twbbs.org/archives/2008/04/07/474/
http://tw.myblog.yahoo.com/weijenlu/article?mid=99&prev=109&next=-1

class Module
  def trace_attr(sym)
  self.module_eval <<-EOS      <= 也可以說是動態產生 method 的方法
     def #{sym}
          printf "Accessing %s with value %s\n",
        "#{sym}", @#{sym}.inspect
        @#{sym}
      end
   EOS
   end
end

class Dog
trace_attr :name
def initialize(string)
   @name = string
end
end
p Dog.new("Fido").name

class Test
end

code =<<-EOF
   def hello_module
     'hello world, use module'
    end
EOF Test.module_eval(code)          <= 動態塞段 code 進去, method 就生了, 太神啦 
p Test.new.hello_module()


Test.class_eval <<-END          <= 有直接塞進class的 fu
def hello_class
'  hello world, use class'
end
END
p Test.new.hello_class()

t = Test.new
class << t                      <= 可以直接塞method 進  t 這個  object
 def hello_singleton
   'hello singleton'
 end
end

class << Test                <= Test class也是 oject 當然也可以塞
 def class_singleton       
   'class singleton'
 end
end


class Announce < ActiveRecord::Base
 require_dependency 'forum/forum_model_mixin'
 include ForumModelMixin
end

module ForumModelMixin
 def self.included(klass)                  <= klass可以知道是哪個 class include了這個lib
     klass.class_eval do               

     has_many :forum_comments, :as => :forum
     alias_method :old_destroy, :destroy
     validates_presence_of :content, :subject, :user_id

     def destroy(r)
       if r
         old_destroy
       else
         update_attribute(:removed_at, Time.now)  if !removed_at
       end
     end
   end
 end
end