Bootstrap 下拉菜单学习笔记
Bootstrap 是一个流行的前端开发框架，其下拉菜单功能在页面导航和菜单设计中十分常见。下面是一些关于 Bootstrap 下拉菜单的学习笔记及实例。
导航条中的下拉菜单
单个下拉菜单
下面是一个简单的导航条中的单个下拉菜单实例：
Copy Code<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">Logo</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNav">
<ul class="navbar-nav">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Contact</a>
</li>
</ul>
</div>
</nav>
其中，.dropdown指定了下拉菜单，.dropdown-toggle指定了菜单的触发按钮，.dropdown-menu指定了菜单的内容，aria-haspopup="true"表示该按钮有下拉菜单并可以展开。
多个下拉菜单
如果导航条中需要多个下拉菜单，可以通过在 .dropdown 中添加 .dropdown-submenu 创建子菜单。下面是一个多个下拉菜单实例：
Copy Code<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">Logo</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNav">
<ul class="navbar-nav">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown1" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown 1
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown1">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item dropdown-toggle" href="#" id="navbarDropdown2" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Submenu
</a>
<ul class="dropdown-menu" aria-labelledby="navbarDropdown2">
<a class="dropdown-item" href="#">Submenu item 1</a>
<a class="dropdown-item" href="#">Submenu item 2</a>
</ul>
</div>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown3" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown 2
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown3">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Contact</a>
</li>
</ul>
</div>
</nav>
在第一个下拉菜单中，添加了子菜单，由于 Bootstrap 不支持多层级的下拉菜单，因此需要使用嵌套的 .dropdown-menu 来实现子菜单。
按钮下拉菜单
与导航条中的下拉菜单不同，按钮下拉菜单是在按钮上触发的。下面是一个简单的按钮下拉菜单实例：
Copy Code<div class="btn-group">
<button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown
</button>
<div class="dropdown-menu">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Something else here</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Separated link</a>
</div>
</div>
其中，.btn-group 指定了按钮组，.btn-secondary 指定了按钮样式，data-toggle="dropdown" 表示点击按钮时展开菜单，.dropdown-menu 指定了菜单内容。
结语
通过上述实例，我们可以看到 Bootstrap 下拉菜单的实现方法非常简单，但对于页面设计和用户体验来说十分重要，希望本文能够对大家有所帮助。