Bootstrap 4 栅格系统垂直布局

关键词：bootstrap 4，vertical alignment，grid systerm

问题：按照Bootstrap 4 官方文档的说明。应用.align-items-center可以使元素垂直居中：

Bootstrap 4 垂直居中示例.png

官方示例代码：

<div class="container">
  <div class="row align-items-start">
    <div class="col">
      One of three columns
    </div>
    <div class="col">
      One of three columns
    </div>
    <div class="col">
      One of three columns
    </div>
  </div>
  <div class="row align-items-center">
    <div class="col">
      One of three columns
    </div>
    <div class="col">
      One of three columns
    </div>
    <div class="col">
      One of three columns
    </div>
  </div>
  <div class="row align-items-end">
    <div class="col">
      One of three columns
    </div>
    <div class="col">
      One of three columns
    </div>
    <div class="col">
      One of three columns
    </div>
  </div>
</div>

将官方示例代码复制粘贴到新文档，在浏览器中打开：

官方示例代码实际的效果.png

垂直的居中的效果并没有实现。

解决办法：给要垂直居中的元素所在的row加一个高度，

<!doctype html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

    <title>Hello, world!</title>

    <style type="text/css">
      div {
        border-style: solid;
        border-width: 1px;
      }

      .row,.container {
        height: 30vh;
      }
    </style>
  </head>
  <body>

    <div class="container">
    <div class="row align-items-start">
      <div class="col">
        One of three columns
      </div>
      <div class="col">
        One of three columns
      </div>
      <div class="col">
        One of three columns
      </div>
    </div>
    <div class="row align-items-center">
      <div class="col">
        One of three columns
      </div>
      <div class="col">
        One of three columns
      </div>
      <div class="col">
        One of three columns
      </div>
    </div>
    <div class="row align-items-end">
      <div class="col">
        One of three columns
      </div>
      <div class="col">
        One of three columns
      </div>
      <div class="col">
        One of three columns
      </div>
    </div>
  </div>


    <!-- Optional JavaScript -->
    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
    <script src="https://cdn.bootcss.com/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
    <script src="https://cdn.bootcss.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
  </body>
</html>

总结：
将垂直居中的元素包裹在.row中，再用.container包裹，并给.row设置一个高度。

Bootstrap 4 栅格系统垂直布局

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