js iframe富文本 光标移到文本最后解决办法

    //续加<br>ff
   $("iframe").contents().find("body").append("<br>ff");
         var obj=$("iframe").contents().find("body");
         obj.focus();
         var len = obj.html().length;
        if (document.selection) {
            var sel = obj.createTextRange();
            sel.moveStart('character',len);
            sel.collapse(true);
            sel.select();
         } else if (typeof obj.selectionStart == 'number' && typeof obj.selectionEnd == 'number') {
            obj.selectionStart = obj.selectionEnd = len;
         }