Bootstrap4 滚动监听(Scrollspy)
滚动监听（Scrollspy）组件，会根据滚动的位置，自动更新导航条的目标。
滚动监听创建
向您想要监听的元素（通常是 body）添加 data-spy="scroll" 。
然后添加 data-target 属性，它的值为导航栏的 id 或 class (.navbar)。这样就可以联系上可滚动区域。
注意可滚动项元素上的 id （<div id="section1">） 必须匹配导航栏上的链接选项 （<a href="#section1">)。
可选项data-offset 属性用于计算滚动位置时，距离顶部的偏移像素。 默认为 10 px。
实例：
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap4 实例</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script>
<script src="https://cdn.bootcss.com/popper.js/1.12.5/umd/popper.min.js"></script>
<script src="https://cdn.bootcss.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<style>
body {
position: relative;
}
</style>
</head>
<body data-spy="scroll" data-target=".navbar" data-offset="50">
<nav class="navbar navbar-expand-sm bg-dark navbar-dark fixed-top">
<ul class="navbar-nav">
<li class="nav-item">
<a class="nav-link" href="#section1">Section 1</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#section2">Section 2</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#section3">Section 3</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbardrop" data-toggle="dropdown">
Section 4
</a>
<div class="dropdown-menu">
<a class="dropdown-item" href="#section41">Link 1</a>
<a class="dropdown-item" href="#section42">Link 2</a>
</div>
</li>
</ul>
</nav>
<div id="section1" class="container-fluid bg-success" style="padding-top:70px;padding-bottom:70px">
<h1>Section 1</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section2" class="container-fluid bg-warning" style="padding-top:70px;padding-bottom:70px">
<h1>Section 2</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section3" class="container-fluid bg-secondary" style="padding-top:70px;padding-bottom:70px">
<h1>Section 3</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section41" class="container-fluid bg-danger" style="padding-top:70px;padding-bottom:70px">
<h1>Section 4 Submenu 1</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section42" class="container-fluid bg-info" style="padding-top:70px;padding-bottom:70px">
<h1>Section 4 Submenu 2</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
</body>
</html>设置相对定位
使用 data-spy="scroll" 的元素需要将其 CSS position 属性设置为 "relative" 才能起作用。
实例：垂直滚动监听
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap4 实例</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script>
<script src="https://cdn.bootcss.com/popper.js/1.12.5/umd/popper.min.js"></script>
<script src="https://cdn.bootcss.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<style>
body {
position: relative;
}
ul.nav-pills {
top: 20px;
position: fixed;
}
div.col-8 div {
height: 500px;
}
</style>
</head>
<body data-spy="scroll" data-target="#myScrollspy" data-offset="1">
<div class="container-fluid">
<div class="row">
<nav class="col-sm-3 col-4" id="myScrollspy">
<ul class="nav nav-pills flex-column">
<li class="nav-item">
<a class="nav-link active" href="#section1">Section 1</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#section2">Section 2</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#section3">Section 3</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#">Section 4</a>
<div class="dropdown-menu">
<a class="dropdown-item" href="#section41">Link 1</a>
<a class="dropdown-item" href="#section42">Link 2</a>
</div>
</li>
</ul>
</nav>
<div class="col-sm-9 col-8">
<div id="section1" class="bg-success">
<h1>Section 1</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section2" class="bg-warning">
<h1>Section 2</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section3" class="bg-secondary">
<h1>Section 3</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section41" class="bg-danger">
<h1>Section 4-1</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
<div id="section42" class="bg-info">
<h1>Section 4-2</h1>
<p>学编程，从W3Cschool开始！</p>
</div>
</div>
</div>
</div>
</body>
</html>