简易拖拽

onmousedown:存储距离

onmousemove:根据距离，计算div最新位置

问题1：当鼠标移动过快脱离div，此时div不会跟随鼠标移动

解决：改为document.onmouseover

问题2：当鼠标移动div超出浏览器可视区范围，此时找不到div

解决：应判断div是否移动超出可视区范围，若超出，则将div的左边距置为0，右边和顶部底部同理

 1 <!DOCTYPE html>
 2 <html>
 3 <head>
 4 <meta charset="utf-8">
 5 <script type="text/javascript">
 6 window.onload=function(){
 7     var oDiv1=document.getElementById("div1");
 8     var disX=0;
 9     var disY=0;
10     oDiv1.onmousedown=function(ev){
11         oEvent=ev||event;
12         disX=oEvent.clientX-oDiv1.offsetLeft;
13         disY=oEvent.clientY-oDiv1.offsetTop;
14         document.onmousemove=function(ev){
15             oEvent=ev||event;
16             var l=oEvent.clientX-disX;
17             var t=oEvent.clientY-disY;
18             //判断用户拖拽是否拖出浏览器可视区
19             if(l<0)
20                 l=0;
21             else if(l>document.documentElement.clientWidth-oDiv1.offsetWidth)
22                 l=document.documentElement.clientWidth-oDiv1.offsetWidth;
23             if(t<0)
24                 t=0;
25             else if(t>document.documentElement.clientHeight-oDiv1.offsetHeight)
26                 t=document.documentElement.clientHeight-oDiv1.offsetHeight;
27 
28             oDiv1.style.left=l+'px';
29             oDiv1.style.top=t+'px';
30         };
31         document.onmouseup=function(){
32             document.onmousemove='null';
33             document.onmouseup='null';
34         }
35         return false;
36     };
37 };
38 </script>
39 <style>
40 
41 #div1{
42     width:100px;
43     height:80px;
44     background:#ccc;
45     position:absolute;
46 }
47 
48 </style>
49 </head>
50 <body>
51 <div id="div1"></div>
52 
53 </body>
54 </html>