CSS3引入了一种新的布局模式——Flexbox布局，即伸缩布局盒（Flexible Box）模型
<div id="cnblogs_post_body" class="blogpost-body blogpost-body-html"><p>CSS2定义了四种布局：块布局、行内布局、表格布局盒定位布局。</p><p>CSS3引入了一种新的布局模式——Flexbox布局，即伸缩布局盒（Flexible Box）模型，此布局对于设计比较复杂的页面非常有用，特别是“垂直居中”布局，非常容易实现。</p><p>Flexbox布局语法有三种：</p><p>旧版本：2009年版本，使用display:box或者display:inline-box; [目前仅Safari浏览器支持]</p><p>混合版本：2011年版本，使用display:flexbox或者display:inline-flexbox; [仅IE10支持]</p><p>最新版本：使用display:flex或者display:inline-flex. [目前的主流浏览器均支持，不过在Webkit内核浏览器需要使用前缀“-webkit-”]</p><p>对于Flex布局，最初是通过看绿叶学习网，学的是flexbox旧版本，感觉很简单；然后又看了大漠老师的《图解CSS3-核心技术与案例实战》，其中详细介绍了三种布局，但我理解的不深，有些迷糊。最后通过网上找到了阮一峰翻译的一篇文章，直观易懂，终于明晰了Flex布局。</p><p>关于Flexbox布局旧版本（display:box/inline-box）版本，请参考<a title="弹性盒子模型" href="http://www.lvyestudy.com/css3/css3_13.1.aspx" target="_blank" rel="noopener">绿叶学习网教程</a>。</p><p>以下内容主要介绍Flexbox布局最新版本。（内容删减自<a title="Flex 布局教程：语法篇" href="http://www.ruanyifeng.com/blog/2015/07/flex-grammar.html" target="_blank" rel="noopener">阮一峰博客</a>）</p><p>——————————————————————————–</p><h2>一、Flex布局是什么？<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h2><p>Flex是Flexible Box的缩写，意为”弹性布局”，用来为盒状模型提供最大的灵活性。</p><p>任何一个容器都可以指定为Flex布局。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box<span class="token punctuation">{<span class="token property"><span class="hljs-attribute">display<span class="token punctuation">: flex<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p>行内元素也可以使用Flex布局。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box<span class="token punctuation">{<span class="token property"><span class="hljs-attribute">display<span class="token punctuation">: inline-flex<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p>Webkit内核的浏览器，必须加上-webkit前缀。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box<span class="token punctuation">{<span class="token property"><span class="hljs-attribute">display<span class="token punctuation">: -webkit-flex<span class="token punctuation">; <span class="token comment"><span class="hljs-comment">/* Safari */<span class="token property"><span class="hljs-attribute">display<span class="token punctuation">: flex<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></pre></blockquote><p>注意，设为Flex布局以后，子元素的float、clear和vertical-align属性将失效。</p><h2>二、基本概念<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h2><p>采用Flex布局的元素，称为Flex容器（flex container），简称”容器”。它的所有子元素自动成为容器成员，称为Flex项目（flex item），简称”项目”。</p><p><p>容器默认存在两根轴：水平的主轴（main axis）和垂直的交叉轴（cross axis）。主轴的开始位置（与边框的交叉点）叫做main start，结束位置叫做main end；交叉轴的开始位置叫做cross start，结束位置叫做cross end。</p><p>项目默认沿主轴排列。单个项目占据的主轴空间叫做main size，占据的交叉轴空间叫做cross size。</p><h2>三、容器的属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h2><p>以下6个属性设置在容器上。</p><blockquote><ul><li>flex-direction</li><li>flex-wrap</li><li>flex-flow</li><li>justify-content</li><li>align-items</li><li>align-content</li></ul></blockquote><h3>3.1 flex-direction属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>flex-direction属性决定主轴的方向（即项目的排列方向）。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex-direction<span class="token punctuation">: row | row-reverse | column | column-reverse<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>它可能有4个值。</p><blockquote><ul><li>row（默认值）：主轴为水平方向，起点在左端。</li><li>row-reverse：主轴为水平方向，起点在右端。</li><li>column：主轴为垂直方向，起点在上沿。</li><li>column-reverse：主轴为垂直方向，起点在下沿。</li></ul></blockquote><h3>3.2 flex-wrap属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>默认情况下，项目都排在一条线（又称”轴线”）上。flex-wrap属性定义，如果一条轴线排不下，如何换行。</p><p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box<span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex-wrap<span class="token punctuation">: nowrap | wrap | wrap-reverse<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p>它可能取三个值。</p><p>（1）nowrap（默认）：不换行。</p><p><p>（2）wrap：换行，第一行在上方。</p><p><p>（3）wrap-reverse：换行，第一行在下方。</p><p><h3>3.3 flex-flow<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>flex-flow属性是flex-direction属性和flex-wrap属性的简写形式，默认值为row nowrap。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex-flow<span class="token punctuation">: <span class="token ignore"><flex-direction> || <span class="token ignore"><flex-wrap><span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></span></code></pre></blockquote><h3>3.4 justify-content属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>justify-content属性定义了项目在主轴上的对齐方式。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">justify-content<span class="token punctuation">: flex-start | flex-end | center | space-between | space-around<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>它可能取5个值，具体对齐方式与轴的方向有关。下面假设主轴为从左到右。</p><blockquote><ul><li>flex-start（默认值）：左对齐</li><li>flex-end：右对齐</li><li>center： 居中</li><li>space-between：两端对齐，项目之间的间隔都相等。</li><li>space-around：每个项目两侧的间隔相等。所以，项目之间的间隔比项目与边框的间隔大一倍。</li></ul></blockquote><h3>3.5 align-items属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>align-items属性定义项目在交叉轴上如何对齐。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">align-items<span class="token punctuation">: flex-start | flex-end | center | baseline | stretch<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>它可能取5个值。具体的对齐方式与交叉轴的方向有关，下面假设交叉轴从上到下。</p><blockquote><ul><li>flex-start：交叉轴的起点对齐。</li><li>flex-end：交叉轴的终点对齐。</li><li>center：交叉轴的中点对齐。</li><li>baseline: 项目的第一行文字的基线对齐。</li><li>stretch（默认值）：如果项目未设置高度或设为auto，将占满整个容器的高度。</li></ul></blockquote><h3>3.6 align-content属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>align-content属性定义了多根轴线的对齐方式。如果项目只有一根轴线，该属性不起作用。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.box <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">align-content<span class="token punctuation">: flex-start | flex-end | center | space-between | space-around | stretch<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>该属性可能取6个值。</p><blockquote><ul><li>flex-start：与交叉轴的起点对齐。</li><li>flex-end：与交叉轴的终点对齐。</li><li>center：与交叉轴的中点对齐。</li><li>space-between：与交叉轴两端对齐，轴线之间的间隔平均分布。</li><li>space-around：每根轴线两侧的间隔都相等。所以，轴线之间的间隔比轴线与边框的间隔大一倍。</li><li>stretch（默认值）：轴线占满整个交叉轴。</li></ul></blockquote><h3>四、项目的属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>以下6个属性设置在项目上。</p><blockquote><ul><li>order</li><li>flex-grow</li><li>flex-shrink</li><li>flex-basis</li><li>flex</li><li>align-self</li></ul></blockquote><h3>4.1 order属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>order属性定义项目的排列顺序。数值越小，排列越靠前，默认为0。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.item <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">order<span class="token punctuation">: <span class="token ignore"><integer><span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></code></pre></blockquote><p><h3>4.2 flex-grow属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>flex-grow属性定义项目的放大比例，默认为0，即如果存在剩余空间，也不放大。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.item <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex-grow<span class="token punctuation">: <span class="token ignore"><number><span class="token punctuation">; <span class="token comment"><span class="hljs-comment">/* default 0 */<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>如果所有项目的flex-grow属性都为1，则它们将等分剩余空间（如果有的话）。如果一个项目的flex-grow属性为2，其他项目都为1，则前者占据的剩余空间将比其他项多一倍。</p><h3>4.3 flex-shrink属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>flex-shrink属性定义了项目的缩小比例，默认为1，即如果空间不足，该项目将缩小。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.item <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex-shrink<span class="token punctuation">: <span class="token ignore"><number><span class="token punctuation">; <span class="token comment"><span class="hljs-comment">/* default 1 */<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>如果所有项目的flex-shrink属性都为1，当空间不足时，都将等比例缩小。如果一个项目的flex-shrink属性为0，其他项目都为1，则空间不足时，前者不缩小。</p><p>负值对该属性无效。</p><h3>4.4 flex-basis属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>flex-basis属性定义了在分配多余空间之前，项目占据的主轴空间（main size）。浏览器根据这个属性，计算主轴是否有多余空间。它的默认值为auto，即项目的本来大小。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.item <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex-basis<span class="token punctuation">: <span class="token ignore"><length> | auto<span class="token punctuation">; <span class="token comment"><span class="hljs-comment">/* default auto */<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></span></span></code></pre></blockquote><p>它可以设为跟width或height属性一样的值（比如350px），则项目将占据固定空间。</p><h3>4.5 flex属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>flex属性是flex-grow, flex-shrink 和 flex-basis的简写，默认值为0 1 auto。后两个属性可选。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.item <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">flex<span class="token punctuation">: none | [ <span class="token ignore"><<span class="token string"><span class="hljs-string">'flex-grow'> <span class="token ignore"><<span class="token string"><span class="hljs-string">'flex-shrink'>? || <span class="token ignore"><<span class="token string"><span class="hljs-string">'flex-basis'> ]<span class="token punctuation">}</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></pre></blockquote><p>该属性有两个快捷值：auto (1 1 auto) 和 none (0 0 auto)。</p><p>建议优先使用这个属性，而不是单独写三个分离的属性，因为浏览器会推算相关值。</p><h3>4.6 align-self属性<button class="cnblogs-toc-button" title="显示目录导航" aria-expanded="false"></button></h3><p>align-self属性允许单个项目有与其他项目不一样的对齐方式，可覆盖align-items属性。默认值为auto，表示继承父元素的align-items属性，如果没有父元素，则等同于stretch。</p><blockquote><pre class=" language-css"><code class="language-css hljs"><span class="token selector"><span class="hljs-selector-class">.item <span class="token punctuation">{<span class="token property"><span class="hljs-attribute">align-self<span class="token punctuation">: auto | flex-start | flex-end | center | baseline | stretch<span class="token punctuation">;<span class="token punctuation">}</span></span></span></span></span></span></span></span></code></pre></blockquote><p><p>该属性可能取6个值，除了auto，其他都与align-items属性完全一致。</p><p> 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