Bootstrap4 菜鸟教程

Bootstrap4 Flex（弹性）布局

Bootstrap4 通过 flex 类来控制页面的布局。

弹性盒子(flexbox)

Bootstrap 3 与 Bootstrap 4 最大的区别就是 Bootstrap 4  使用弹性盒子来布局，而不是使用浮动来布局。

弹性盒子是 CSS3 的一种新的布局模式，更适合响应式的设计，如果你还不了解 flex，可以阅读我们的 CSS3 弹性盒子(Flex Box) 教程

注意：IE9 及其以下版本不支持弹性盒子，所以如果你需要兼容 IE8-9，请使用 Bootstrap 3。

以下示例使用 d-flex 类创建一个弹性盒子容器，并设置三个弹性子元素：

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>Flex</h2>
  <p>使用 d-flex 类创建一个弹性盒子容器，并设置三个弹性子元素：</p>
  <div class="d-flex p-3 bg-secondary text-white">  
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

创建显示在同一行上的弹性盒子容器可以使用 d-inline-flex 类:

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>行内 Flex</h2>
  <p>创建显示在同一行上的弹性盒子容器可以使用 d-inline-flex 类:</p>
  <div class="d-inline-flex p-3 bg-secondary text-white">  
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

水平方向

.flex-row 可以设置弹性子元素水平显示，这是默认的。

使用 .flex-row-reverse 类用于设置右对齐显示，即与 .flex-row  方向相反。

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>水平方向</h2>
  <p>使用 .flex-row 类设置弹性子元素水平显示：</p>
  <div class="d-flex flex-row bg-secondary mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <p>.flex-row-reverse 设置右对齐方向:</p>
  <div class="d-flex flex-row-reverse bg-secondary">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

垂直方向

.flex-column 类用于设置弹性子元素垂直方向显示,  .flex-column-reverse 用于翻转子元素：

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>垂直方向</h2>
  <p>.flex-column 类用于设置弹性子元素垂直方向显示：</p>
  <div class="d-flex flex-column mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <p>.flex-column-reverse 类用于设置弹性子元素垂直方向翻转显示：</p>
  <div class="d-flex flex-column-reverse">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

内容排列

.justify-content-* 类用于修改弹性子元素的排列方式，* 号允许的值有：start (默认), end, center, between 或 around:

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>内容排列方式</h2>
  <p> .justify-content-* 类用于修改弹性子元素的排列方式，* 号允许的值有：start (默认), end, center, between 或 around:</p>
  <div class="d-flex justify-content-start bg-secondary mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <div class="d-flex justify-content-end bg-secondary mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <div class="d-flex justify-content-center bg-secondary mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <div class="d-flex justify-content-between bg-secondary mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <div class="d-flex justify-content-around bg-secondary mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

等宽

.flex-fill 类强制设置各个弹性子元素的宽度是一样的:

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>等宽</h2>
  <p>.flex-fill 类强制设置各个弹性子元素的宽度是一样的:</p>
  <div class="d-flex mb-3">
    <div class="p-2 flex-fill bg-info">Flex item 1</div>
    <div class="p-2 flex-fill bg-warning">Flex item 2</div>
    <div class="p-2 flex-fill bg-primary">Flex item 3</div>
  </div>
  <p>不使用 .flex-fill 类示例:</p>
  <div class="d-flex mb-3 bg-secondary">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

扩展

.flex-grow-1  用于设置子元素使用剩下的空间。以下示例中前面两个子元素只设置了它们所需要的空间，最后一个获取剩余空间。 :

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>扩展</h2>
  <p>.flex-grow-1 用于设置子元素使用剩下的空间：</p>
  <div class="d-flex mb-3">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 flex-grow-1 bg-primary">Flex item 3</div>
  </div>
  <p>不使用 .flex-grow-1 示例:</p>
  <div class="d-flex mb-3 bg-secondary">
    <div class="p-2 bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

提示: 使用 .flex-shrink-1 用于设置子元素的收缩规则。

排序

.order 类可以设置弹性子元素的排序，从 .order-1 到 .order-12，数字越低权重越高( .order-1 排在 .order-2 之前) :

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>排序</h2>
  <p>.order 类可以设置弹性子元素的排序，从 .order-1 到 .order-12，数字越低权重越高:</p>
  <div class="d-flex mb-3">
    <div class="p-2 order-3 bg-info">Flex item 1</div>
    <div class="p-2 order-2 bg-warning">Flex item 2</div>
    <div class="p-2 order-1 bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

外边距

.mr-auto 类可以设置子元素右外边距为 auto，即 margin-right: auto!important;，.ml-auto 类可以设置子元素左外边距为 auto，即 margin-left: auto!important;:

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>外边距</h2>
  <p>.mr-auto 类可以设置子元素右外边距为 auto，.ml-auto 类可以设置子元素左外边距为 auto，</p>
  <div class="d-flex mb-3 bg-secondary">
    <div class="p-2 mr-auto bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 bg-primary">Flex item 3</div>
  </div>
  <div class="d-flex mb-3 bg-secondary">
    <div class="p-2  bg-info">Flex item 1</div>
    <div class="p-2 bg-warning">Flex item 2</div>
    <div class="p-2 ml-auto bg-primary">Flex item 3</div>
  </div>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

包裹

弹性容器中包裹子元素可以使用以下三个类： .flex-nowrap (默认), .flex-wrap 或 .flex-wrap-reverse。设置 flex 容器是单行或者多行。

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>包裹</h2>
  <p>弹性容器中包裹弹性子元素可以使用以下三个类： .flex-nowrap (默认), .flex-wrap 或 .flex-wrap-reverse:</p>
  <p><code>.flex-wrap:</code></p>
  <div class="d-flex flex-wrap bg-light">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
  <p><code>.flex-wrap-reverse:</code></p>
  <div class="d-flex flex-wrap-reverse bg-light">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
  <p><code>.flex-nowrap:</code></p>
  <div class="d-flex flex-nowrap bg-light">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
    <div class="p-2 border">Flex item 26</div>
    <div class="p-2 border">Flex item 27</div>
    <div class="p-2 border">Flex item 28</div>
    <div class="p-2 border">Flex item 29</div>
    <div class="p-2 border">Flex item 30</div>
    <div class="p-2 border">Flex item 31</div>
    <div class="p-2 border">Flex item 32</div>
    <div class="p-2 border">Flex item 33</div>
    <div class="p-2 border">Flex item 34</div>
    <div class="p-2 border">Flex item 35</div>
  </div>
  <br>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

内容对齐

我们可以使用 .align-content-* 来控制在垂直方向上如何去堆叠子元素，包含的值有：.align-content-start (默认), .align-content-end, .align-content-center, .align-content-between, .align-content-around 和 .align-content-stretch。

这些类在只有一行的弹性子元素中是无效的。

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>内容对齐</h2>
  <p>我们可以使用 .align-content-* 来控制子元素在垂直方向的对齐方式。</p>
  <p><strong>注意:</strong> 这个示例在小型设备上效果不好。这些类在只有一行的弹性子元素中是无效的。</p>
  <p>.align-content-start (默认):</p>
  <div class="d-flex flex-wrap align-content-start bg-light" style="height:300px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
  <p>.align-content-end:</p>
  <div class="d-flex flex-wrap align-content-end bg-light" style="height:300px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
  <p>.align-content-center:</p>
  <div class="d-flex flex-wrap align-content-center bg-light" style="height:300px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
  <p>.align-content-around:</p>
  <div class="d-flex flex-wrap align-content-around bg-light" style="height:300px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
  <p>.align-content-stretch:</p>
  <div class="d-flex flex-wrap align-content-stretch bg-light" style="height:300px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
    <div class="p-2 border">Flex item 4</div>
    <div class="p-2 border">Flex item 5</div>
    <div class="p-2 border">Flex item 6</div>
    <div class="p-2 border">Flex item 7</div>
    <div class="p-2 border">Flex item 8</div>
    <div class="p-2 border">Flex item 9</div>
    <div class="p-2 border">Flex item 10</div>
    <div class="p-2 border">Flex item 11</div>
    <div class="p-2 border">Flex item 12</div>
    <div class="p-2 border">Flex item 13 </div>
    <div class="p-2 border">Flex item 14</div>
    <div class="p-2 border">Flex item 15</div>
    <div class="p-2 border">Flex item 16</div>
    <div class="p-2 border">Flex item 17</div>
    <div class="p-2 border">Flex item 18</div>
    <div class="p-2 border">Flex item 19</div>
    <div class="p-2 border">Flex item 20</div>
    <div class="p-2 border">Flex item 21</div>
    <div class="p-2 border">Flex item 22</div>
    <div class="p-2 border">Flex item 23</div>
    <div class="p-2 border">Flex item 24</div>
    <div class="p-2 border">Flex item 25</div>
  </div>
  <br>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

子元素对齐

如果要设置单行的子元素对齐可以使用 .align-items-* 类来控制，包含的值有：.align-items-start, .align-items-end, .align-items-center, .align-items-baseline, 和 .align-items-stretch (默认)。

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>子元素对齐</h2>
  <p>如果要设置单行的子元素对齐可以使用 .align-content-* 类来控制。</p>
  <p>.align-items-start:</p>
  <div class="d-flex align-items-start bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-items-end:</p>
  <div class="d-flex align-items-end bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-items-center:</p>
  <div class="d-flex align-items-center bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-items-baseline:</p>
  <div class="d-flex align-items-baseline bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-items-stretch (默认):</p>
  <div class="d-flex align-items-stretch bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

指定子元素对齐

如果要设置指定子元素对齐对齐可以使用 .align-self-* 类来控制，包含的值有：.align-self-start, .align-self-end, .align-self-center, .align-self-baseline, 和 .align-self-stretch (默认)。

<!DOCTYPE html>
<html>
<head>
  <title>Bootstrap 示例</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css">
  <script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js"></script>
  <script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js"></script>
  <script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
  <h2>Align Self</h2>
  <p>如果要设置指定子元素对齐对齐可以使用 .align-self-* 类来控制。</p>
  <p>.align-self-start:</p>
  <div class="d-flex bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border align-self-start">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-self-end:</p>
  <div class="d-flex bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border align-self-end">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-self-center:</p>
  <div class="d-flex bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border align-self-center">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-self-baseline:</p>
  <div class="d-flex bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border align-self-baseline">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
  <p>.align-self-stretch (默认):</p>
  <div class="d-flex bg-light" style="height:150px">
    <div class="p-2 border">Flex item 1</div>
    <div class="p-2 border align-self-stretch">Flex item 2</div>
    <div class="p-2 border">Flex item 3</div>
  </div>
  <br>
</div>
</body>
</html>
测试看看 ‹/›

运行后效果如下:

图片.png

响应式 flex 类

我们可以根据不同的设备，设置 flex 类，从而实现页面响应式布局，以下表格中的 * 号可以的值有：sm, md, lg 或 xl, 对应的是小型设备、中型设备，大型设备，超大型设备。

类示例实现
弹性容器
.d-*-flex根据不同的屏幕设备创建弹性盒子容器尝试一下  
方向
.flex-*-row根据不同的屏幕设备在水平方向显示弹性子元素尝试一下  
.flex-*-column根据不同的屏幕设备在垂直方向显示弹性子元素尝试一下  
内容对齐
.justify-content-*-start根据不同屏幕设备在开始位置显示弹性子元素 (左对齐)尝试一下  
.justify-content-*-center根据不同屏幕设备在 flex 容器中居中显示子元素尝试一下  
.justify-content-*-around根据不同屏幕设备使用  "around" 显示弹性子元素尝试一下  
扩展
.flex-*-grow-0不同的屏幕设备不设置扩展 
.flex-*-grow-1不同的屏幕设备设置扩展 
收缩
.flex-*-shrink-0不同的屏幕设备不设置收缩 
.flex-*-shrink-1不同的屏幕设备设置收缩 
包裹
.flex-*-nowrap不同的屏幕设备不设置包裹元素尝试一下  
.flex-*-wrap-reverse不同的屏幕设备反转包裹元素尝试一下  
.align-content-*-end根据不同屏幕设备在结束位置堆叠元素尝试一下  
.align-content-*-around根据不同屏幕设备，使用 "around" 堆叠元素尝试一下  
元素对齐
.align-items-*-start根据不同屏幕设备，让元素在头部显示在同一行。尝试一下  
.align-items-*-center根据不同屏幕设备，让元素在中间位置显示在同一行。尝试一下  
.align-items-*-stretch根据不同屏幕设备，让元素延展高度并显示在同一行。尝试一下  
.align-self-*-end据不同屏幕设备，让单独一个子元素显示在尾部尝试一下  
.align-self-*-baseline据不同屏幕设备，让单独一个子元素显示在基线位置尝试一下  
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