鼠标捕获(setCapture，releaseCapture)的学习

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>完美拖拽</title>
<style type="text/css">
html,body{
     
     overflow:hidden;}
body,div,h2,p{
     
     margin:0;padding:0;}
body{
     
     color:#fff;background:#000;font:12px/2 Arial;}
p{
     
     padding:0 10px;margin-top:10px;}
span{
     
     color:#ff0;padding-left:5px;}
#box{
     
     position:absolute;width:300px;height:150px;background:#333;border:2px solid #ccc;top:50%;left:50%;margin:-75px 0 0 -150px;}
#box h2{
     
     height:25px;cursor:move;background:#222;border-bottom:2px solid #ccc;text-align:right;padding:0 10px;}
#box h2 a{
     
     color:#fff;font:12px/25px Arial;text-decoration:none;outline:none;}
</style>
<script type="text/javascript">
window.onload=function (){
    var oBox=document.getElementById("box");
    var oH2 = oBox.getElementsByTagName("h2")[0];
    var oA = oBox.getElementsByTagName("a")[0];
    var aSpan = oBox.getElementsByTagName("span");    
    var disX = disY = 0;
    var bDrag = false;
    var aPos = [{x:oBox.offsetLeft, y:oBox.offsetTop}];
    
    //鼠标按下, 激活拖拽
    oH2.onmousedown = function (event){        
        var event = event || window.event;
        bDrag = true;
        disX = event.clientX - oBox.offsetLeft;
        disY = event.clientY - oBox.offsetTop;        
        aPos.push({x:oBox.offsetLeft, y:oBox.offsetTop})
        
        this.setCapture && this.setCapture();        
        return false;
    };
    
    //拖拽开始
    document.onmousemove = function (event){
        if (!bDrag) return;
        var event = event || window.event;
        var iL = event.clientX - disX;
        var iT = event.clientY - disY;
        var maxL = document.documentElement.clientWidth - oBox.offsetWidth;
        var maxT = document.documentElement.clientHeight - oBox.offsetHeight;
        
        iL = iL < 0 ? 0 : iL;
        iL = iL > maxL ? maxL : iL; 
        
        iT = iT < 0 ? 0 : iT;
        iT = iT > maxT ? maxT : iT;
        
        oBox.style.marginTop = oBox.style.marginLeft = 0;
        oBox.style.left = iL + "px";
        oBox.style.top = iT + "px";    
        aPos.push({x:iL, y:iT})
        
        status();
        
        return false;
    };

    //鼠标释放, 结束拖拽
    document.onmouseup = window.onblur = oH2.onlosecapture = function (){
        bDrag = false;                
        oH2.releaseCapture && oH2.releaseCapture();
        status();
    };
    
    //回放拖动轨迹
    oA.onclick = function (){
        if (aPos.length == 1) return;
        var timer = setInterval(function ()    {
            var oPos = aPos.pop();
            oPos ? (oBox.style.left = oPos.x + "px", oBox.style.top = oPos.y + "px", status()) : clearInterval(timer)
        }, 30);        
        this.focus = false;//去除链接虚线        
        return false;
    };
    
    //阻止冒泡
    oA.onmousedown = function (event){
        (event || window.event).cancelBubble = true
    };
    
    //监听状态函数
    function status (){
        aSpan[0].innerHTML = bDrag;
        aSpan[1].innerHTML = oBox.offsetTop;
        aSpan[2].innerHTML = oBox.offsetLeft;
    }
    
    //初始调用
    status();
};
</script>
</head>
<body>
<div id="box">
    <h2><a href="javascript:;">点击回放拖动轨迹</a></h2>
    <p><strong>Drag:</strong><span></span></p>
    <p><strong>offsetTop:</strong><span></span></p>
    <p><strong>offsetLeft:</strong><span></span></p>
</div>
</body>
</html>