本文概述
下拉菜单是可切换的上下文菜单，用于以列表格式显示链接。它方便用户从预定义列表中选择一个值。可以通过下拉JavaScript插件进行交互。
你必须在.dropdown类中包装下拉菜单，才能创建Bootstrap Dropdown。
Bootstrap下拉示例
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
</head>
<body>
<div class="container">
<h2>Dropdowns</h2>
<p>The .dropdown class is used to indicate a dropdown menu.</p>
<p>Use the .dropdown-menu class to actually build the dropdown menu.</p>
<p>To open the dropdown menu, use a button or a link with a class of .dropdown-toggle and data-toggle="dropdown".</p>
<div class="dropdown">
<button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Dropdown Example
<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">HTML</a></li>
<li><a href="#">CSS</a></li>
<li><a href="#">JavaScript</a></li>
</ul>
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
</body>
</html>Bootstrap下拉分隔线
.divider类用于分隔下拉菜单中的链接：
请参阅以下示例：
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
</head>
<body>
<div class="container">
<h2>Dropdowns</h2>
<p>The .divider class is used to separate links inside the dropdown menu:</p>
<div class="dropdown">
<button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">Tutorials
<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">HTML</a></li>
<li><a href="#">CSS</a></li>
<li><a href="#">JavaScript</a></li>
<li class="divider"></li>
<li><a href="#">About Us</a></li>
</ul>
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
</body>
</html>Bootstrap下拉标题
.dropdown-header类用于在下拉菜单中添加标题。
请参阅以下示例：
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
</head>
<body>
<div class="container">
<h2>Dropdowns</h2>
<p>The .dropdown-header class is used to add headers inside the dropdown menu:</p>
<div class="dropdown">
<button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">Tutorials
<span class="caret"></span></button>
<ul class="dropdown-menu">
<li class="dropdown-header">Dropdown header 1</li>
<li><a href="#">HTML</a></li>
<li><a href="#">CSS</a></li>
<li><a href="#">JavaScript</a></li>
<li class="divider"></li>
<li class="dropdown-header">Dropdown header 2</li>
<li><a href="#">About Us</a></li>
</ul>
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
</body>
</html>Bootstrap下拉列表：禁用项目
使用类.disabled禁用下拉菜单中的项目。
请参阅以下示例：
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
</head>
<body>
<div class="container">
<h2>Dropdowns</h2>
<p>Here, CSS is disable.</p>
<div class="dropdown">
<button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">Tutorials
<span class="caret"></span></button>
<ul class="dropdown-menu">
<li><a href="#">HTML</a></li>
<li class="disabled"><a href="#">CSS</a></li>
<li><a href="#">JavaScript</a></li>
<li class="divider"></li>
<li><a href="#">About Us</a></li>
</ul>
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
</body>
</html>Bootstrap 4禁用和活动项目
要使某个项目处于活动状态，请使用.active类突出显示特定的下拉菜单项（活动项目以蓝色背景色显示）。
要禁用下拉菜单中的项目，你必须使用.disabled类（它会显示浅灰色的文本颜色和悬停时带有“禁止停车标志”的图标）。
例：
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.6/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<h2>Dropdowns Active and Disable Example</h2>
<div class="dropdown">
<button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">
Dropdown button
</button>
<div class="dropdown-menu">
<a class="dropdown-item" href="#">Normal</a>
<a class="dropdown-item active" href="#">Active</a>
<a class="dropdown-item disabled" href="#">Disabled</a>
</div>
</div>
</div>
</body>
</html>下拉位置
用于放置下拉菜单项。默认情况下，下拉菜单位于左侧。你可以通过将.dropdown-menu-right类添加到具有.dropdown-menu的元素来使下拉列表右对齐。
例：
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.6/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<h2>Right Aligned Dropdowns</h2>
<div class="dropdown">
<button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">
Dropdown button
</button>
<div class="dropdown-menu dropdown-menu-right">
<a class="dropdown-item" href="#">Link 1</a>
<a class="dropdown-item" href="#">Link 2</a>
<a class="dropdown-item" href="#">Link 3</a>
</div>
</div>
</div>
</body>
</html>Bootstrap 4 Dropup
如果你希望下拉菜单向上扩展而不是向下扩展，请更改
