鼠标拖动div，div跟随鼠标移动效果

<div id="boxDiv" style='width:20px;height:20px;position:absolute;background:red;'>
 
 
$('#boxDiv').bind('mousedown',function(){
$(document).bind('mousemove',function(event){
var x = event.clientX + 'px';
var y = event.clientY + 'px';
$('#boxDiv').css({"left":x,"top":y});
});
$(document).bind('mouseup',function(event){
$(document).unbind('mousemove');
  });
});