通过高度变化来切换所有匹配元素的可见性，并在切换完成后可选地触发一个回调函数;
这个动画效果只调整元素的高度，可以使匹配的元素以“滑动”的方式隐藏或显示。在jQuery 1.3中，上下的padding和margin也会有动画，效果更流畅
实例:
<html lang='zh-cn'>
<head>
<title>Insert you title</title>
<meta http-equiv='description' content='this is my page'>
<meta http-equiv='keywords' content='keyword1,keyword2,keyword3'>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type='text/javascript' src='./js/jquery-1.12.1.min.js'></script>
<style type="text/css">
*{margin:0;padding:0;}
html{font:400 15px/1.2em 'Courier New';color:#666;750px;margin:25px auto;}
#show{250px;height:400px;background:red;display:none;color:#FFF;line-height:400px;text-indent:8px;padding:10px 30px;margin:70px;text-align:center;}
button{padding:8px;border:0;outline:none;background:#00E92C;border-radius:2px;}
.wrapAll{color:#FF96EC;}
</style>
<script type='text/javascript'>
$(function(){
$('button').click(function(){
$('#show').slideToggle(1000,'linear',function(){
var num = 0;
console.log(num++);
});
});
});
</script>
</head>
<body>
<div id='demo'>
<button type="button">button</button>
<div id='show'>Test text</div>
</div>
</body>
</html>