JS拖动技术 关于setCapture使用

<script type="text/javascript">
<!--
window.onload=function(){
objDiv = document.getElementById('drag');
drag(objDiv);
};
function drag(dv){
dv.onmousedown=function(e){
var d=document;
e = e || window.event;
var x= e.layerX || e.offsetX;
var y= e.layerY || e.offsetY;
//设置捕获范围
if(dv.setCapture){
dv.setCapture();
}else if(window.captureEvents){
window.captureEvents(Event.MOUSEMOVE | Event.MOUSEUP);
}

d.onmousemove=function(e){
e= e || window.event;
if(!e.pageX)e.pageX=e.clientX;
if(!e.pageY)e.pageY=e.clientY;
var tx=e.pageX-x;
var ty=e.pageY-y;
dv.style.left=tx;
dv.style.top=ty;
};
d.onmouseup=function(){
//取消捕获范围
if(dv.releaseCapture){
dv.releaseCapture();
}else if(window.captureEvents){
window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
}
//清除事件
d.onmousemove=null;
d.onmouseup=null;
};
};
}
//-->
</script>
<div id="drag" style="position:absolute;left:12px;top:24px;width:100;height:150;border:1px solid #000000;z-index:1;background:#eeeeee">drag me</div>