<!doctype html>
<html>
<head>
<title>image's transition</title>
<script type="text/javascript">
window.onload = function(){
var pic_wrapper = document.getElementById("pic_wrapper");
var ul = document.getElementsByTagName("ul")[0];
var lis= document.getElementsByTagName("li");
var imgs = pic_wrapper.getElementsByTagName("img");
var img1 = imgs[0];
var img2 = imgs[1];
//使用for循环绑定事件处理函数，通过测试
// for (var i = 0; i < lis.length; i++) {
// var img = imgs[i];
// var li = lis[i];
// setOpacity(li,img);
// function setOpacity(li1,img1){
// li1.onmouseover = function(){
// setAllOpacity();
// img1.style.opacity = "1";
// };
// }
// };
function setAllOpacity(){
for(var i = 0; i < imgs.length; i++){
imgs[i].style.opacity = "0";
}
}
//使用事件委托方式，但存在问题
ul.onmouseover = function(event){
event = event || window.event;
var target = event.target || event.srcElement;
switch(target.id){//问题在于target.id，通过正常交互无法获取target.id
case "li1":
setAllOpacity();
imgs[0].style.opacity = "1";alert("li1");
break;
case "li2":
setAllOpacity();
imgs[1].style.opacity = "1";alert("li2");
break;
case "li3":
setAllOpacity();
imgs[2].style.opacity = "1";
break;
case "li4":
setAllOpacity();
imgs[3].style.opacity = "1";
break;
}
};
function fire(elem, type){
var evt;
if(document.createEventObject){
elem.fireEvent('on'+type);
}else{
evt = document.createEvent('MouseEvents');
evt.initEvent(type, true, true);
elem.dispatchEvent(evt);
}
}
fire(lis[0],"mouseover");//手动触发事件可以获取得到target
};
</script>
<style type="text/css">
*{
padding: 0;
border: 0;
margin: 0;
}
#wrapper{
width: 800px;
height: 600px;
margin: 20px auto;
position: relative;
overflow: hidden;
}
#pic_wrapper{
position: absolute;
}
#pic_wrapper img{
width: 800px;
height: 600px;
position: absolute;
transition: opacity 0.5s ease-in;
opacity: 0;
}
.num{
position: absolute;
bottom: 50px;
left: 390px;
}
li{
display: inline;
cursor: pointer;
}
</style>
</head>
<body>
<div id="wrapper">
<div id="pic_wrapper">
<img src="transition1.jpg">
<img src="transition2.jpg">
<img src="transition3.jpg">
<img src="transition4.jpg">
</div>
<ul class="num">
<li id="li1"><img src="num.jpg"></li>
<li id="li2"><img src="num.jpg"></li>
<li id="li3"><img src="num.jpg"></li>
<li id="li4"><img src="num.jpg"></li>
</ul>
</div>
</body>
</html>
switch处修改为：
switch(event.target.id || event.target.parentNode.id)
因为 <li> 和 <img> 的尺寸太接近，有时候 mouseover 是在 <img> 上触发，最后冒泡到 <ul> 上，事件目标是 <img>，它没有 id，所以就出错了。在 handler 中要先判断一下目标的类型：
if(event.target.nodeName.toLowerCase() === 'li') {
...
}