javascript 拖动层

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<title>Drag and Drop example</title>
<style type="text/css">
#a {height:50px;width:50px;left:10px;top:50px;border:1px solid pink;position:absolute;background-color:#EFEFFF;}
#b {height:50px;width:50px;left:70px;top:50px;border:1px solid pink;position:absolute;background-color:#FFEFFE;}
</style>
<script type="text/javascript">
/*
    Copyright Robert Nyman, http://www.robertnyman.com
    Free to use if this text is included
    // modified slightly based on Prototype
*/
function getStyle(oElm, strCssRule){
    var strValue = "";
    if(document.defaultView && document.defaultView.getComputedStyle){
        var css = document.defaultView.getComputedStyle(oElm, null);
        strValue = css ? css.getPropertyValue(strCssRule) : null;
    }
    else if(oElm.currentStyle){
        strCssRule = strCssRule.replace(/\-(\w)/g, function (strMatch, p1){
            return p1.toUpperCase();
        });
        strValue = oElm.currentStyle[strCssRule];
    }
    return strValue;
}
function Draggable(el)
{
var xDelta = 0, yDelta = 0;
var xStart = 0, yStart = 0;
// remove the events
function enddrag()
{
    document.onmouseup = null;
    document.onmousemove = null;
}
// fire each time it's dragged
function drag(e)
{
    e = e || window.event;
    xDelta = xStart - parseInt(e.clientX);
    yDelta = yStart - parseInt(e.clientY);
    xStart = parseInt(e.clientX);
    yStart = parseInt(e.clientY);
    el.style.top = (parseInt(el.style.top) - yDelta) + 'px';
    el.style.left = (parseInt(el.style.left) - xDelta) + 'px';
}
// initiate the drag
function md(e)
{
    e = e || window.event;
    xStart = parseInt(e.clientX);
    yStart = parseInt(e.clientY);
    el.style.top = parseInt(getStyle(el,'top')) + 'px';
    el.style.left = parseInt(getStyle(el,'left')) + 'px';
    document.onmouseup = enddrag;
    document.onmousemove = drag;
    return false;
}
// tie it into the element
el.onmousedown = md;
}
window.onload = function()
{
new Draggable(document.getElementById('a'));
new Draggable(document.getElementById('b'));
}
</script>
</head>
<body>
<div id="a">test</div>
<div id="b">test</div>
</body>
</html>