CKEDITOR添加键盘监听后，当编辑的内空清空时，键盘监听变成无效

粗略的看了下源代码，每次清空的时候好像是会重新构建一个document改成这样可以解决问题，但是不知道有没有新的问题了。。
<!DOCTYPE html>
<html>
  <head>
    <title>测试ctrl+Enter</title>
             
       <script type="text/javascript" src="ckeditor.js"></script>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />      
 
  </head>
  <body>
    <textarea name="ckeditorContent" id="ckeditorContent" cols="80" rows="30"></textarea>
     
    <script type="text/javascript">
        CKEDITOR.replace('ckeditorContent');
         
        function keydown(e){
          if(e.keyCode==13&&e.ctrlKey)
          {
          alert('按下了ctrl+Enter');
           CKEDITOR.instances['ckeditorContent'].setData("",function(){
                if(this.document.$.addEventListener) 
                     this.document.$.addEventListener('keydown',keydown,false);
                 else if(this.document.$.attachEvent)
                     this.document.$.attachEvent('onkeyup',function(e){keydown(e)});  
           });//加了这句后，再次按下ctrl+Enter，认不到了，怎么解决?
           }
        }       
         
              
            CKEDITOR.on('instanceReady', function (e) { 
            if(e.editor.document.$.addEventListener) 
                 e.editor.document.$.addEventListener('keydown',keydown,false);
             else if(e.editor.document.$.attachEvent)
                 e.editor.document.$.attachEvent('onkeyup',function(e){keydown(e)});  
            });
     </script> 
  </body>  
</html>