19-下拉菜单

将下拉菜单触发器和下拉菜单都包裹在 .dropdown 里，或者另一个声明了 position: relative; 的元素。然后加入组成菜单的 HTML 代码。

引入js

除了引入css的支持外：

<link rel="stylesheet" href="bootstrap-3.3.7/css/bootstrap.min.css" type="text/css"></link>Code language: HTML, XML (xml)

还需要引入下面的两个js：

<script src="https://cdn.bootcss.com/jquery/1.12.4/jquery.min.js"></script>
<script src="https://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>Code language: HTML, XML (xml)

往下拉

<div class="dropdown">
  <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true">
	Dropdown
	<span class="caret"></span>
  </button>
  <ul class="dropdown-menu" aria-labelledby="dropdownMenu1">
	<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>Code language: HTML, XML (xml)

往上拉

<div class="dropup">
  <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu2" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
	Dropup
	<span class="caret"></span>
  </button>
  <ul class="dropdown-menu" aria-labelledby="dropdownMenu2">
	<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>Code language: HTML, XML (xml)