JS拖动技术--- 关于setCapture

<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 >drag me <div>