2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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## 前言
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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CSS3中的 flex 属性，在布局方面做了非常大的改进，使得我们对**多个元素之间**的布局排列变得十分灵活，适应性非常强。其强大的伸缩性和自适应性，在网页开中可以发挥极大的作用。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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### flex 初体验
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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我们先来看看下面这个最简单的布局：
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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上面这张图中的布局是我们都熟悉的：默认文档流中，在一个父容器里放置多个块级的子元素，那么，这些子元素会默认从上往下排列。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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在此基础之上，如果我给父容器仅仅加一个 `display: flex`属性，此时，这些子元素的布局会摇身一变：
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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没错，子元素们会**在水平方向上，从左至右排列**，就是这么神奇。到此为止，你已经掌握了关于 flex 的一半的知识。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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### flex 布局的优势
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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1、**flex 布局的子元素不会脱离文档流**，很好地遵从了“流的特性”。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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但你如果用 float 来做布局，float 属性的元素会脱离文档流，而且会涉及到各种 BFC、清除浮动的问题。浮动相关的问题，比较麻烦，所以也成了面试必问的经典题目。但有了 flex 布局之后，这些问题都不存在的。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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2、**flex 是一种现代的布局方式，是 W3C 第一次提供真正用于布局的 CSS 规范**。 flex 非常提供了丰富的属性，非常灵活，让布局的实现更佳多样化，且方便易用。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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flex 唯一的缺点就在于，它不支持低版本的 IE 浏览器。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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### flex 的兼容性问题
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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2018-02-23 12:23:47 +08:00
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2019-10-13 14:05:52 +08:00
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上图中可以看到， flex 布局不支持 IE9 及以下的版本；IE10及以上也只是部分支持。如果你的页面不需要处理 IE浏览器的兼容性问题，则可以放心大胆地使用 flex 布局。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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但是，比如网易新闻、淘宝这样的大型网站，面对的是海量用户，即便使用低版本浏览器的用户比例很少，但绝对基数仍然是很庞大的。因此，这些网站为了兼容低版本的 IE 浏览器，暂时还不敢尝试使用 flex 布局。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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### 概念：弹性盒子、子元素
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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在讲 flex 的知识点之前，我们事先约定两个概念：
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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- **弹性盒子**：指的是使用 `display:flex` 或 `display:inline-flex` 声明的**父容器**。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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- **子元素/弹性元素**：指的是父容器里面的子元素们（父容器被声明为 flex 盒子的情况下）。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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### 概念：主轴和侧轴
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在上面的“初体验”例子中，我们发现，弹性盒子里面的子元素们，默认是从左至右排列的，这个方向，代表的就是主轴的方向。
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在此，我们引入主轴和侧轴的概念。
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如上图所示：
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- 主轴：flex容器的主轴，默认是水平方向，从左向右。
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- 侧轴：与主轴垂直的轴称作侧轴，默认是垂直方向，从上往下。
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PS：主轴和侧轴并不是固定不变的，可以通过 `flex-direction` 更换方向，我们在后面会讲到。
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## 弹性盒子
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### 声明定义
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使用 `display:flex` 或 `display:inline-flex` 声明一个**父容器**为弹性盒子。此时，这个父容器里的子元素们，会遵循弹性布局。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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备注：一般是用 `display:flex`这个属性。`display:inline-flex`用得较少。
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:30:30 +08:00
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### flex-direction 属性
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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`flex-direction`：用于设置盒子中**子元素**的排列方向。属性值可以是：
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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| 属性值 | 描述 |
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|:-------------|:-------------|
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| row | 从左到右水平排列子元素（默认值） |
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|column|从上到下垂直排列子元素|
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| row-reverse |从右向左排列子元素 |
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|column-reverse|从下到上垂直排列子元素|
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2018-02-23 12:23:47 +08:00
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2019-10-08 17:26:18 +08:00
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备注：如果我们不给父容器写`flex-direction`这个属性，那么，子元素默认就是从左到右排列的。
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2018-02-23 12:23:47 +08:00
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代码演示：
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```html
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<!DOCTYPE html>
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<html>
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<head lang="en">
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<meta charset="UTF-8">
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<title></title>
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<style>
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*{
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margin: 0;
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padding: 0;
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list-style: none;
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}
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body{
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background-color: #eee;
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font-family: "Microsoft Yahei";
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font-size:22px;
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}
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h3{
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font-weight: normal;
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}
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section{
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width: 1000px;
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margin:40px auto;
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}
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ul{
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background-color: #fff;
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border: 1px solid #ccc;
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}
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ul li{
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width: 200px;
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height: 200px;
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background-color: pink;
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margin:10px;
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}
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section:nth-child(1) ul{
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overflow: hidden; /* 清除浮动 */
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}
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section:nth-child(1) ul li{
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float: left;
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}
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/* 设置伸缩盒子*/
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section:nth-child(2) ul{
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display: flex;
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}
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section:nth-child(3) ul{
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/* 设置伸缩布局*/
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display: flex;
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/* 设置主轴方向*/
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flex-direction: row;
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}
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section:nth-child(4) ul{
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/* 设置伸缩布局*/
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display: flex;
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/* 设置主轴方向 :水平翻转*/
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flex-direction: row-reverse;
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}
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section:nth-child(5) ul{
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/* 设置伸缩布局*/
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display: flex;
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/* 设置主轴方向 :垂直*/
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flex-direction: column;
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}
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section:nth-child(6) ul{
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/* 设置伸缩布局*/
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display: flex;
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/* 设置主轴方向 :垂直*/
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flex-direction: column-reverse;
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}
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</style>
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</head>
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<body>
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<section>
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<h3>传统布局</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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<section>
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<h3>伸缩布局 display:flex</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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<section>
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<h3>主轴方向 flex-direction:row</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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<section>
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<h3>主轴方向 flex-direction:row-reverse</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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<section>
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<h3>主轴方向 flex-direction:column</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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<section>
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<h3>主轴方向 flex-direction:column-reverse</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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</body>
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</html>
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```
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2019-10-08 17:30:30 +08:00
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### flex-wrap 属性
|
2018-02-23 12:23:47 +08:00
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|
2019-10-08 17:30:30 +08:00
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`flex-wrap`：控制子元素溢出时的换行处理。
|
2019-10-08 17:26:18 +08:00
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|
2019-10-08 17:30:30 +08:00
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### justify-content 属性
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`justify-content`：控制子元素在主轴上的排列方式。
|
2019-10-08 17:26:18 +08:00
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## 弹性元素
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### justify-content 属性
|
2018-02-23 12:23:47 +08:00
|
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- `justify-content: flex-start;` 设置子元素在**主轴上的对齐方式**。属性值可以是：
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- `flex-start` 从主轴的起点对齐（默认值）
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- `flex-end` 从主轴的终点对齐
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- `center` 居中对齐
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- `space-around` 在父盒子里平分
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- `space-between` 两端对齐 平分
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|
2019-10-08 17:26:18 +08:00
|
|
|
|
代码演示：（在浏览器中打开看效果）
|
2018-02-23 12:23:47 +08:00
|
|
|
|
|
|
|
|
|
|
```html
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|
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<!DOCTYPE html>
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<html>
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<head lang="en">
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<meta charset="UTF-8">
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<title></title>
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<style>
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*{
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margin: 0;
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padding: 0;
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list-style:none;}
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body{
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background-color: #eee;
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font-family: "Microsoft Yahei";
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}
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section{
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width: 1000px;
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margin:50px auto;
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}
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section h3{
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font-size:22px;
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font-weight: normal;
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}
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ul{
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border: 1px solid #999;
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background-color: #fff;
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display: flex;
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}
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ul li{
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width: 200px;
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height: 200px;
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background: pink;
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margin:10px;
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}
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section:nth-child(1) ul{
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/* 主轴对齐方式：从主轴开始的方向对齐*/
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justify-content: flex-start;
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}
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section:nth-child(2) ul{
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/* 主轴对齐方式：从主轴结束的方向对齐*/
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justify-content: flex-end;
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}
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section:nth-child(3) ul{
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/* 主轴对齐方式：居中对齐*/
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justify-content: center;
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}
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section:nth-child(4) ul{
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/* 主轴对齐方式：在父盒子中平分*/
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justify-content: space-around;
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}
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section:nth-child(5) ul{
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|
/* 主轴对齐方式：两端对齐 平分*/
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|
|
justify-content: space-between;
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}
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</style>
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|
</head>
|
|
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|
|
<body>
|
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|
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|
|
<section>
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|
<h3>主轴的对齐方式：justify-content:flex-start</h3>
|
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|
|
<ul>
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|
|
<li>1</li>
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<li>2</li>
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|
<li>3</li>
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|
</ul>
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|
</section>
|
|
|
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|
<section>
|
|
|
|
|
|
<h3>主轴的对齐方式：justify-content:flex-end</h3>
|
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|
|
<ul>
|
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|
|
<li>1</li>
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|
<li>2</li>
|
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|
|
<li>3</li>
|
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|
|
</ul>
|
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|
</section>
|
|
|
|
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|
|
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|
|
<section>
|
|
|
|
|
|
<h3>主轴的对齐方式：justify-content:center</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>主轴的对齐方式：justify-content:space-round</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>主轴的对齐方式：justify-content:space-bettwen</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
<li>4</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
</body>
|
|
|
|
|
|
</html>
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
|
2019-10-08 17:30:30 +08:00
|
|
|
|
### align-items 属性
|
2018-02-23 12:23:47 +08:00
|
|
|
|
|
2019-10-08 17:30:30 +08:00
|
|
|
|
`align-items`：设置子元素在**侧轴上的对齐方式**。属性值可以是：
|
2018-02-23 12:23:47 +08:00
|
|
|
|
- `flex-start` 从侧轴开始的方向对齐
|
|
|
|
|
|
- `flex-end` 从侧轴结束的方向对齐
|
|
|
|
|
|
- `baseline` 基线 默认同flex-start
|
2018-03-13 23:12:27 +08:00
|
|
|
|
- `center` 中间对齐
|
2018-02-23 12:23:47 +08:00
|
|
|
|
- `stretch` 拉伸
|
|
|
|
|
|
|
|
|
|
|
|
代码演示：
|
|
|
|
|
|
|
|
|
|
|
|
```html
|
|
|
|
|
|
<!DOCTYPE html>
|
|
|
|
|
|
<html>
|
|
|
|
|
|
<head lang="en">
|
|
|
|
|
|
<meta charset="UTF-8">
|
|
|
|
|
|
<title></title>
|
|
|
|
|
|
<style>
|
|
|
|
|
|
*{
|
|
|
|
|
|
margin: 0;
|
|
|
|
|
|
padding: 0;
|
|
|
|
|
|
list-style:none;
|
|
|
|
|
|
}
|
|
|
|
|
|
body{
|
|
|
|
|
|
background-color: #eee;
|
|
|
|
|
|
font-family: "Microsoft Yahei";
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
section{
|
|
|
|
|
|
width: 1000px;
|
|
|
|
|
|
margin:50px auto;
|
|
|
|
|
|
}
|
|
|
|
|
|
section h3{
|
|
|
|
|
|
font-size:22px;
|
|
|
|
|
|
font-weight: normal;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
ul{
|
|
|
|
|
|
border: 1px solid #999;
|
|
|
|
|
|
background-color: #fff;
|
|
|
|
|
|
display: flex;
|
|
|
|
|
|
height:500px;
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
ul li{
|
|
|
|
|
|
width: 200px;
|
|
|
|
|
|
height: 200px;
|
|
|
|
|
|
background: pink;
|
|
|
|
|
|
margin:10px;
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(1) ul{
|
|
|
|
|
|
/* 侧轴对齐方式 ：从侧轴开始的方向对齐*/
|
|
|
|
|
|
align-items:flex-start;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(2) ul{
|
|
|
|
|
|
/* 侧轴对齐方式 ：从侧轴结束的方向对齐*/
|
|
|
|
|
|
align-items:flex-end;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(3) ul{
|
|
|
|
|
|
/* 侧轴对齐方式 ：居中*/
|
|
|
|
|
|
align-items:center;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(4) ul{
|
|
|
|
|
|
/* 侧轴对齐方式 ：基线 默认同flex-start*/
|
|
|
|
|
|
align-items:baseline;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(5) ul{
|
|
|
|
|
|
/* 侧轴对齐方式 ：拉伸*/
|
|
|
|
|
|
align-items:stretch;
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(5) ul li{
|
|
|
|
|
|
height:auto;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
|
|
<body>
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>侧轴的对齐方式:align-items ：flex-start</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>侧轴的对齐方式：align-items:flex-end</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>侧轴的对齐方式：align-items:center</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>侧轴的对齐方式：align-itmes:baseline</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
|
|
|
|
|
|
<section>
|
|
|
|
|
|
<h3>侧轴的对齐方式：align-itmes: stretch</h3>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>1</li>
|
|
|
|
|
|
<li>2</li>
|
|
|
|
|
|
<li>3</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
</body>
|
|
|
|
|
|
</html>
|
|
|
|
|
|
```
|
|
|
|
|
|
|
2019-10-08 17:26:18 +08:00
|
|
|
|
### `flex`属性：设置子盒子的权重
|
2018-02-23 12:23:47 +08:00
|
|
|
|
|
|
|
|
|
|
代码演示：
|
|
|
|
|
|
|
|
|
|
|
|
```html
|
|
|
|
|
|
<!DOCTYPE html>
|
|
|
|
|
|
<html>
|
|
|
|
|
|
<head lang="en">
|
|
|
|
|
|
<meta charset="UTF-8">
|
|
|
|
|
|
<title></title>
|
|
|
|
|
|
<style>
|
|
|
|
|
|
*{
|
|
|
|
|
|
margin: 0;
|
|
|
|
|
|
padding: 0;
|
|
|
|
|
|
list-style:none;
|
|
|
|
|
|
}
|
|
|
|
|
|
body{
|
|
|
|
|
|
background-color: #eee;
|
|
|
|
|
|
font-family: "Microsoft Yahei";
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
section{
|
|
|
|
|
|
width: 1000px;
|
|
|
|
|
|
margin:50px auto;
|
|
|
|
|
|
}
|
|
|
|
|
|
section h3{
|
|
|
|
|
|
font-size:22px;
|
|
|
|
|
|
font-weight: normal;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
ul{
|
|
|
|
|
|
border: 1px solid #999;
|
|
|
|
|
|
background-color: #fff;
|
|
|
|
|
|
display: flex;
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
ul li{
|
|
|
|
|
|
width: 200px;
|
|
|
|
|
|
height: 200px;
|
|
|
|
|
|
background: pink;
|
|
|
|
|
|
margin:10px;
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(1) ul li:nth-child(1){
|
|
|
|
|
|
flex:1;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(1) ul li:nth-child(2){
|
|
|
|
|
|
flex:1;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(1) ul li:nth-child(3){
|
|
|
|
|
|
flex:8;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(2) ul li:nth-child(1){
|
|
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(2) ul li:nth-child(2){
|
|
|
|
|
|
flex:1;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
section:nth-child(2) ul li:nth-child(3){
|
|
|
|
|
|
flex:4;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
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</style>
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</head>
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<body>
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<section>
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<h3>伸缩比例:flex</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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<section>
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<h3>伸缩比例:flex</h3>
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<ul>
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<li>1</li>
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<li>2</li>
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<li>3</li>
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</ul>
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</section>
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</body>
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</html>
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```
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2019-10-08 17:26:18 +08:00
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## 相关链接
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### CSS Flexbox 可视化手册
|
2019-08-21 21:09:21 +08:00
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2019-08-21 21:27:39 +08:00
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可视化的截图如下：（请点开链接，查看大图）
|
2019-08-21 21:09:21 +08:00
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|
2019-08-21 21:27:39 +08:00
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<http://img.smyhvae.com/20190821_2101.png>
|
2019-08-21 21:09:21 +08:00
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相关文章：
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- 【英文原版】 CSS Flexbox Fundamentals Visual Guide：<https://medium.com/swlh/css-flexbox-fundamentals-visual-guide-1c467f480dac>
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- 【中文翻译】CSS Flexbox 可视化手册：<https://zhuanlan.zhihu.com/p/56046851>
|
2018-02-23 12:23:47 +08:00
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|
2019-10-08 17:26:18 +08:00
|
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### flex 相关的推荐文章
|
2018-02-23 12:23:47 +08:00
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|
2019-08-21 21:09:21 +08:00
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|
- flex 效果在线演示：<https://demos.scotch.io/visual-guide-to-css3-flexbox-flexbox-playground/demos/>
|
2018-02-23 12:23:47 +08:00
|
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|
2019-08-21 21:09:21 +08:00
|
|
|
|
- A Complete Guide to Flexbox | 英文原版：<https://css-tricks.com/snippets/css/a-guide-to-flexbox/>
|
2018-02-23 12:23:47 +08:00
|
|
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|
2019-08-21 21:09:21 +08:00
|
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|
- CSS3 Flexbox 布局完全指南 | 中文翻译：<https://www.html.cn/archives/8629>
|
2018-02-23 12:23:47 +08:00
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|
2019-10-08 17:26:18 +08:00
|
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|
### flex 相关的教程
|
2018-02-23 12:23:47 +08:00
|
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|
2019-10-08 17:30:30 +08:00
|
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|
- [后盾人 flex 教程](http://houdunren.gitee.io/note/css/10%20%E5%BC%B9%E6%80%A7%E5%B8%83%E5%B1%80.html)
|
2018-02-23 12:23:47 +08:00
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|
2019-10-08 17:26:18 +08:00
|
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|
## 技巧：使用 margin 自动撑满剩余空间
|
2018-02-23 12:23:47 +08:00
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