在Bootstrap 4的不同屏幕上的中间对齐一个flex项目
要在Bootstrap4的不同屏幕上的中心对齐弹性项目，请使用.align-self-*-center类。
在小，中，大等不同的屏幕上进行设置-
在小屏幕的中央对齐一个弹性项目
<div class="d-flex bg-primary" style="height:200px"> <div class="p-2 border">A-one</div> <div class="p-2 border">B-one</div> <div class="p-2 border align-self-sm-center">C-one</div> <div class="p-2 border">D-one</div> </div>
在中间屏幕的中央对齐一个弹性项目
<div class="d-flex bg-primary" style="height:200px"> <div class="p-2 border">A-one</div> <div class="p-2 border">B-one</div> <div class="p-2 border align-self-md-center">C-one</div> <div class="p-2 border">D-one</div> </div>
将弹性项目在大屏幕的中央对齐
<div class="d-flex bg-primary" style="height:200px"> <div class="p-2 border">A-one</div> <div class="p-2 border">B-one</div> <div class="p-2 border align-self-lg-center">C-one</div> <div class="p-2 border">D-one</div> </div>
让我们看一个示例，以在不同屏幕的中心对齐弹性项目-
示例
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
<h1>Align Specific Flex Item in the center</h1>
<div class="d-flex bg-primary" style="height:200px">
<div class="p-2 border">A-one</div>
<div class="p-2 border">B-one</div>
<div class="p-2 border align-self-center">C-one</div>
<div class="p-2 border">D-one</div>
</div>
<h2>Small Screen Size</h2>
<div class="d-flex bg-primary" style="height:200px">
<div class="p-2 border">A-one</div>
<div class="p-2 border">B-one</div>
<div class="p-2 border align-self-sm-center">C-one</div>
<div class="p-2 border">D-one</div>
</div>
<h2>Medium Screen Size</h2>
<div class="d-flex bg-secondary" style="height:200px">
<div class="p-2 border">A-one</div>
<div class="p-2 border">B-one</div>
<div class="p-2 border align-self-md-center">C-one</div>
<div class="p-2 border">D-one</div>
</div>
<h2>Large Screen Size</h2>
<div class="d-flex bg-info" style="height:200px">
<div class="p-2 border">A-one</div>
<div class="p-2 border">B-one</div>
<div class="p-2 border align-self-lg-center">C-one</div>
<div class="p-2 border">D-one</div>
</div>
</div>
</body>
</html>