知识点
如需向按钮添加下拉菜单，只需要简单地在在一个 .btn-group 中放置按钮和下拉菜单即可。您也可以使用 <span class="caret"></span> 来指示按钮作为下拉菜单。
单按钮下拉菜单
data-toggle="dropdown"，一定要有，否则将不会展示下拉的功能
<div class="btn-group">
<button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">下拉菜单<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
分裂式按钮下拉菜单
在练习的时候遇到了一个问题，根据W3C以及Bootstrap官方实例，页面呈现出来的效果始终如下：
问题按钮
问题所在DOCTYPE声明，我使用的是如下声明：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
必须改为：
<!DOCTYPE html>
尺寸
在.btn上添加.btn-lg类，或在.btn-group 上添加.btn-group-lg类都可以达到效果。
尺寸
向上弹出式菜单
给父元素添加.dropup类就可以。
实践
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gbk"/>
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="Resource-type" content="Document"/>
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>组件-按钮下拉菜单</title>
<link href="css/bootstrap.min.css" rel="stylesheet" type="text/css" />
<script src="js/jquery-2.1.4.min.js" type="text/javascript"></script>
<script src="js/bootstrap.min.js" type="text/javascript"></script>
<style>
<!--
.line{border-top:1px solid #afd9ee;margin:20px 0;}
-->
</style>
</head>
<body>
<div class="container">
<!--单按钮菜单-->
<div class="btn-group">
<button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">下拉菜单<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">下拉菜单 <span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-success dropdown-toggle" data-toggle="dropdown">下拉菜单 <span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-info dropdown-toggle" data-toggle="dropdown">下拉菜单<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-warning dropdown-toggle" data-toggle="dropdown">下拉菜单<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-danger dropdown-toggle" data-toggle="dropdown">下拉菜单<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="line"></div>
<!--分裂式按钮下拉菜单-->
<div class="btn-group">
<button type="button" class="btn btn-default">下拉菜单</button>
<button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">
<span class="caret"></span>
<span class="sr-only">下拉按钮</span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-danger">Action</button>
<button type="button" class="btn btn-danger dropdown-toggle" data-toggle="dropdown">
<span class="caret"></span>
<span class="sr-only">Toggle Dropdown</span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
<li><a href="#">Something else here</a></li>
<li role="separator" class="divider"></li>
<li><a href="#">Separated link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-default">默认</button>
<button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">
<span class="sr-only">切换下拉菜单</span>
<span class="caret"></span>
</button>
<ul class="dropdown-menu" role="menu">
<li><a href="#">功能</a></li>
<li><a href="#">另一个功能</a></li>
<li><a href="#">其他</a></li>
<li class="divider"></li>
<li><a href="#">分离的链接</a></li>
</ul>
</div>
<div class="line"></div>
<!--尺寸-->
<div class="btn-group">
<button type="button" class="btn btn-danger btn-lg dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-danger btn-sm dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group">
<button type="button" class="btn btn-danger btn-xs dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group btn-group-lg">
<button type="button" class="btn btn-danger btn-lg dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group btn-group-sm">
<button type="button" class="btn btn-danger btn-lg dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="btn-group btn-group-xs">
<button type="button" class="btn btn-danger btn-lg dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
<div class="line"></div>
<!--向上弹出-->
<div class="btn-group dropup">
<button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">
下拉菜单
<span class="caret"></span>
</button>
<ul class="dropdown-menu">
<li><a href="#">Dropdown link</a></li>
<li><a href="#">Dropdown link</a></li>
</ul>
</div>
</div>
</body>
</html>
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