js拖弹出的登录框

<script type="text/javascript">

function _move(){
var objthis=this;
this.x=null;
this.y=null;
this.xx=null;
this.yy=null;
this.xxx=null;
this.yyy=null;
this.obja=null;
this.co=null;
this.t=null;
document.onmousemove=function(){
try{objthis.x=arguments[0].screenX;objthis.y=arguments[0].screenY;}
catch(err){objthis.x=window.event.x;objthis.y=window.event.y;}
}
}
_move.prototype.create=function(a,b){
if(this.obja==null){
this.co=b;
var objthis=this;
this.obja=a;
this.obja.onmouseup=function(){objthis.up()}
this.xx=this.x;
this.yy=this.y;
this.xxx=this.obja.offsetLeft;
this.yyy=this.obja.offsetTop;
this.obja.style.position="absolute";
this.move();
}
}
_move.prototype.move=function(){
var objthis=this;
var x=this.xxx + this.x - this.xx;
var y=this.yyy + this.y - this.yy;
this.obja.style.left = x + "px";
this.obja.style.top = y + "px";
this.t=setTimeout(function(){objthis.move()},1)
}
_move.prototype.up=function(){
clearTimeout(this.t)
if(this.co==1){
this.obja.style.left = this.xxx + "px";
this.obja.style.top = this.yyy + "px";
}
this.xxx=null;
this.yyy=null;
this.obja=null;
}
var moveLoginForm =new _move();

</script>

formDiv要拖动的弹出框ID，div为可以拖动的点，一般为弹出框顶部

<div class="title" onmousedown="moveLoginForm.create($('formDiv'))" onmouseover="this.title='按住左鼠移动窗口位置'"><h3>登  录</h3><span><a href="javascript:$('usernameInfo').innerHTML='';formClose();"></a></span></div>