easyui "form" clear problem

[mars610]

Hi,
   I recently used easyui form of usage, use the clear method, found that after emptying the form data elements which can not be re-set the properties。for example:
<script type="text/javascript">
   $('#ff').form('clear');
  $('#bb').attr('checked',true);
</script>

html:
   <form id="ff">
      <input id="bb" type="checkbox" value="0">
   </form>

I found $('#bb').attr('checked',true);  is not work. The checkbox is not checked and  when I using the "iButton" plug, if  use  "clear" method, iButton effect in Firefox would fail. That is, can not choose