简单，仅 CSS 解决方案：
.dropdown:hover>.dropdown-menu {
display: block;
}
单击时，它仍会切换到 show 类（当不再悬停时将保持打开状态）。
要绕过这个适当地是使用为基于指针的设备保留的事件和属性：jQuery 的mouseenter、mouseleave 和:hover。应该能够流畅、直观地工作，同时完全不会干扰下拉菜单在基于触摸的设备上的工作方式。试试看，让我知道它是否适合你：
完整的 jQuery 解决方案（touch 原封不动）：
Pre v4.1.2 解决方案（已弃用）：
$('body').on('mouseenter mouseleave','.dropdown',function(e){
var _d=$(e.target).closest('.dropdown');
if (e.type === 'mouseenter')_d.addClass('show');
setTimeout(function(){
_d.toggleClass('show', _d.is(':hover'));
$('[data-toggle="dropdown"]', _d).attr('aria-expanded',_d.is(':hover'));
},300);
});
$('body').on('mouseenter mouseleave','.dropdown',function(e){
var _d=$(e.target).closest('.dropdown');
if (e.type === 'mouseenter')_d.addClass('show');
setTimeout(function(){
_d.toggleClass('show', _d.is(':hover'));
$('[data-toggle="dropdown"]', _d).attr('aria-expanded',_d.is(':hover'));
},300);
});
/* this is not needed, just prevents page reload when a dd link is clicked */
$('.dropdown a').on('click tap', e => e.preventDefault())
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://code.jquery.com/jquery-3.1.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tether/1.4.0/js/tether.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js"></script>
<nav class="navbar navbar-toggleable-md navbar-light bg-faded">
<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarNavDropdown" aria-controls="navbarNavDropdown" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<a class="navbar-brand" href>Navbar</a>
<div class="collapse navbar-collapse" id="navbarNavDropdown">
<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">
<a class="nav-link" href>Features</a>
</li>
<li class="nav-item">
<a class="nav-link" href>Pricing</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href id="navbarDropdownMenuLink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown link
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdownMenuLink">
<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>
</li>
</ul>
</div>
</nav>
v4.1.2 shiplist 向this change 介绍了下拉菜单的工作原理，使上述解决方案不再有效。
以下是 最新 解决方案，用于在 v4.1.2 及更高版本中悬停时打开下拉菜单：
function toggleDropdown (e) {
const _d = $(e.target).closest('.dropdown'),
_m = $('.dropdown-menu', _d);
setTimeout(function(){
const shouldOpen = e.type !== 'click' && _d.is(':hover');
_m.toggleClass('show', shouldOpen);
_d.toggleClass('show', shouldOpen);
$('[data-toggle="dropdown"]', _d).attr('aria-expanded', shouldOpen);
}, e.type === 'mouseleave' ? 300 : 0);
}
$('body')
.on('mouseenter mouseleave','.dropdown',toggleDropdown)
.on('click', '.dropdown-menu a', toggleDropdown);
function toggleDropdown (e) {
const _d = $(e.target).closest('.dropdown'),
_m = $('.dropdown-menu', _d);
setTimeout(function(){
const shouldOpen = e.type !== 'click' && _d.is(':hover');
_m.toggleClass('show', shouldOpen);
_d.toggleClass('show', shouldOpen);
$('[data-toggle="dropdown"]', _d).attr('aria-expanded', shouldOpen);
}, e.type === 'mouseleave' ? 300 : 0);
}
$('body')
.on('mouseenter mouseleave','.dropdown',toggleDropdown)
.on('click', '.dropdown-menu a', toggleDropdown);
/* not needed, prevents page reload for SO example on menu link clicked */
$('.dropdown a').on('click tap', e => e.preventDefault())
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js"></script>
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Link</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 disabled" href="#">Disabled</a>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
</form>
</div>
</nav>
重要提示：如果使用 jQuery 解决方案，删除 CSS 很重要（否则单击 .dropdown-toggle 或单击菜单选项时下拉菜单不会关闭）。