javascript实现最简单的拖拽效果

// by zhangxinxu welcome to visit my personal website http://www.zhangxinxu.com/
// zxx.drag v1.0 2010-03-23 元素的拖拽实现

var params = {
    left: 0,
    top: 0,
    currentX: 0,
    currentY: 0,
    flag: false
};
//获取相关CSS属性
var getCss = function(o,key){
    return o.currentStyle? o.currentStyle[key] : document.defaultView.getComputedStyle(o,false)[key];     
};

//拖拽的实现
var startDrag = function(bar, target, callback){
    if(getCss(target, "left") !== "auto"){
        params.left = getCss(target, "left");
    }
    if(getCss(target, "top") !== "auto"){
        params.top = getCss(target, "top");
    }
    //o是移动对象
    bar.onmousedown = function(event){
        params.flag = true;
        if(!event){
            event = window.event;
            //防止IE文字选中
            bar.onselectstart = function(){
                return false;
            }  
        }
        var e = event;
        params.currentX = e.clientX;
        params.currentY = e.clientY;
    };
    document.onmouseup = function(){
        params.flag = false;    
        if(getCss(target, "left") !== "auto"){
            params.left = getCss(target, "left");
        }
        if(getCss(target, "top") !== "auto"){
            params.top = getCss(target, "top");
        }
    };
    document.onmousemove = function(event){
        var e = event ? event: window.event;
        if(params.flag){
            var nowX = e.clientX, nowY = e.clientY;
            var disX = nowX - params.currentX, disY = nowY - params.currentY;
            target.style.left = parseInt(params.left) + disX + "px";
            target.style.top = parseInt(params.top) + disY + "px";
            if (event.preventDefault) {
                event.preventDefault();
            }
            return false;
        }
        
        if (typeof callback == "function") {
            callback(parseInt(params.left) + disX, parseInt(params.top) + disY);
        }
    }    
};

// by zhangxinxu welcome to visit my personal website http://www.zhangxinxu.com/
// zxx.drag v1.0 2010-03-23
var params={left:0,top:0,currentX:0,currentY:0,flag:false};var getCss=function(b,a){return b.currentStyle?b.currentStyle[a]:document.defaultView.getComputedStyle(b,false)[a]};var startDrag=function(a,b,c){if(getCss(b,"left")!=="auto"){params.left=getCss(b,"left")}if(getCss(b,"top")!=="auto"){params.top=getCss(b,"top")}a.onmousedown=function(d){params.flag=true;if(!d){d=window.event;a.onselectstart=function(){return false}}var f=d;params.currentX=f.clientX;params.currentY=f.clientY};document.onmouseup=function(){params.flag=false;if(getCss(b,"left")!=="auto"){params.left=getCss(b,"left")}if(getCss(b,"top")!=="auto"){params.top=getCss(b,"top")}};document.onmousemove=function(i){var j=i?i:window.event;if(params.flag){var f=j.clientX,d=j.clientY;var h=f-params.currentX,g=d-params.currentY;b.style.left=parseInt(params.left)+h+"px";b.style.top=parseInt(params.top)+g+"px";if(i.preventDefault){i.preventDefault()}return false}if(typeof c=="function"){c(parseInt(params.left)+h,parseInt(params.top)+g)}}};

<style type="text/css">
#box{position:absolute; left:100px; top:100px; padding:5px; background:#f0f3f9; font-size:12px; -moz-box-shadow:2px 2px 4px #666666; -webkit-box-shadow:2px 2px 4px #666666;}
#main{border:1px solid #a0b3d6; background:white;}
#bar{line-height:24px; background:#beceeb; border-bottom:1px solid #a0b3d6; padding-left:5px; cursor:move;}
#content{width:420px; height:250px; padding:10px 5px;}
</style>

<div id="box">
    <div id="main">
        <div id="bar">拖拽</div>
        <div id="content">
            内容……
        </div>
    </div>
</div>

<script src="http://www.zhangxinxu.com/study/js/zxx.drag.1.0.js" type="text/javascript"></script>
<script type="text/javascript">
    var oBox = document.getElementById("box");
    var oBar = document.getElementById("bar");
    startDrag(oBar, oBox);
</script>