js实现拖拽

1 .box {
2     position: absolute;
3     width: 100px;
4     height: 100px;
5     background-color: #f00;
6     cursor: move;
7 }

<div class="box" id="box" style="left: 0px;top: 0px;">

</div>

 1 var box = document.getElementById("box");
 2 
 3 box.onmousedown = down;  //绑定鼠标按下事件
 4 
 5 function down(e) {
 6 
 7     e = e || window.event;  //处理兼容
 8 
 9     this["startX"] = e.clientX;  //在绑定元素身上添加自定义属性 记录鼠标的起始位置与元素的坐标值
10     this["startY"] = e.clientY;
11     this["startL"] = parseFloat(this.style.left);
12     this["startT"] = parseFloat(this.style.top);
13 
14     //IE和火狐提供了setCapture方法防止鼠标移动过快，焦点丢失问题,但是谷歌不兼容
15     if(this.setCapture) {
16         this.setCapture();
17     } else {
18         var that = this;  //如果不支持，就把事件绑定到document上，不论鼠标移动多快，都不会丢失焦点
19         document.onmousemove = function(e) {
20             move.call(that, e);  //这里用call改变了this指向，因为如果不使用call，此时执行move方法的this是window，移动元素无效果。
21         };
22         document.onmouseup = function() {
23             up.call(that, e);
24         };
25     }
26 }
27 
28 function move(e) {
29     e = e || window.event;
30 
31     var curL = (e.clientX - this["startX"]) + this["startL"];
32     var curT = (e.clientY - this["startY"]) + this["startT"];
33 
34     var minL = 0,
35         minT = 0,
36         maxL = (document.documentElement.clientWidth || document.body.clientWidth) - this.offsetWidth,
37         maxT = (document.documentElement.clientHeight || document.body.clientHeight) - this.offsetHeight;
38     //边界值
39     curL = curL < minL ? minL : (curL > maxL ? maxL : curL);
40     curT = curT < minT ? minT : (curT > maxT ? maxT : curT);
41 
42     this.style.left = curL + "px";
43     this.style.top = curT + "px";
44 }
45 
46 function up(e) {
47     e = e || window.event;
48 
49     if(this.releaseCapture) { //鼠标抬起，解绑事件
50         this.releaseCapture();
51     } else {
52         document.onmousemove = null;
53         document.onmouseup = null;
54     }
55 }