BFC(Block formatting contexts)初探_html/css_WEB-ITnose-HTML Tutorial-php.cn

前端工程师求职面试的时候，或多或少都会问到一点BFC(Block formatting contexts)相关的知识。在平时的开发中我也遇到过BFC方面的问题，但是没有好好整理过这方面的知识，所以别人问我的时候，我都不敢说自己知道这么个概念o(╯□╰)o。这两天，我仔细阅读了一些文章来填补这块知识漏洞。

什么是BFC？

在理解BFC是什么之前，需要了解Box、Formatting Context的概念

Box：CSS布局的基本单位

Box是CSS布局的对象和基本单位，直观点来说，就是一个页面是由很多个Box组成的。元素的类型和display属性，决定了这个Box的类型。不同类型的Box，会参与不同的Formatting Context(一个决定如何渲染文档的容器)，因此Box内的元素会以不同的方式渲染。盒子分类：

block-level box：display属性为block, list-item, table的元素，会生成block-level box。并且参与block formatting context；

inline-level box：display属性为inline, inline-block, inline-table的元素，会生成inline-level box。并且参与inline formatting context；

run-in box：display属性为run-in的元素，根据上下文决定对象是内联对象还是块级对象。CSS3属性！

Formatting Context

Formatting Context是W3C CSS2.1规范中的一个概念。它是页面中的一块渲染区域，并且有一套渲染规则，它决定了其子元素将如何定位，以及和其他元素的关系和相互作用。最常见的Formatting Context有Block Formatting Context(BFC)和Inline Formatting Context(IFC)。

BFC定义

BFC(Block Formatting Context)块级格式化上下文，它是一个独立的渲染区域，只有Block-level box参与，它规定了内部的Block-level Box如何布局，并且与这个区域外部毫不相干。

BFC布局规则

内部的Box会在垂直方向，一个接一个地放置

Box垂直方向的距离由margin决定，属于同一个BFC的两个相邻Box的margin会发生重叠

每个元素的margin box的左边，与包含块border box的左边相接触(对于从左往右的格式化，否则相反)

BFC的区域不会与float box重叠

BFC就是页面上的一个隔离的独立容器，容器里面的子元素不会影响到外面的元素。反之也如此

计算BFC的高度时，浮动元素也参与计算

哪些元素会生成BFC

根元素

float属性不为none

position为absolute或fixed

display为inline-block, table-cell, table-caption, flex, inline-flex

overflow不为visible

BFC作用与原理

1. 自适应两栏布局

1	`<style> body { width: 300px; position: relative; } .aside { width: 100px; height: 150px; float: left; background-color: #f66; } .main { height: 200px; background-color: #fcc; }</style><body> <div` `class="aside"></div> <div` `class="main"></div></body>`

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显示效果：

根据BFC布局规则第3条：

1	`每个元素的margin box的左边，与包含块border box的左边相接触(对于从左往右的格式化，否则相反)。即使存在浮动也是如此。`

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因此，虽然存在浮动的元素aside，但main的左边依然会与包含块的左边相接触

根据BFC布局规则第四条：

1	`BFC的区域不会与float box重叠`

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我们可以通过通过触发main生成BFC，来实现自适应两栏布局：

1	`.main { height: 200px; background-color: #fcc; overflow: hidden; }`

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当触发main生成BFC后，这个新的BFC不会与浮动的aside重叠。因此会根据包含块的宽度，和aside的宽度，自动变窄。效果如下：

2. 清除内部浮动

1	`<style> .par { border: 5px solid #fcc; width: 300px; } .child { border: 5px solid #f66; width: 100px; height: 100px; float: left; }</style><body> <div` `class="par"> <div` `class="child"></div> <div` `class="child"></div> </div></body>`

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显示效果：

根据BFC布局规则第六条：

1	`计算BFC的高度时，浮动元素也参与计算`

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为达到清除内部浮动，我们可以触发par生成BFC，那么par在计算高度时，par内部的浮动元素child也会参与计算

1	`.par { border: 5px solid #fcc; width: 300px; overflow: hidden; }`

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效果如下：

3. 防止垂直margin重叠

1	`<style> p { width: 200px; line-height: 100px; margin: 100px; font-weight: bold; color: #f55; background-color: #fcc; text-align: center; }</style><body> <p>heihei</p> <p>gagaga</p></body>`

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显示效果

两个p之间的距离为100px，发生了margin重叠

根据BFC布局规则第二条：

1	`Box垂直方向的距离由margin决定。属于同一个BFC的两个相邻Box的margin会发生重叠margin重叠规则：* 两个相邻的外边距都是正数时，折叠结果是它们两者之间较大的值* 两个相邻的外边距都是负数时，折叠结果是两者绝对值的较大值* 两个外边距一正一负时，折叠结果是两者的相加的和`

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我们可以在p外面包裹一层容器，并触发该容器生成一个BFC。那么两个P便不属于同一个BFC，就不会发生margin重叠了

1	`<style> .wrap { overflow: hidden; } p { width: 200px; line-height: 100px; margin: 100px; font-weight: bold; color: #f55; background-color: #fcc; text-align: center; }</style><body> <p>heihei</p> <div` `class="wrap"><p>gagaga</p></div></body>`

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显示效果：

总结

其实以上的几个例子都体现了BFC布局规则第五条：

1	`BFC就是页面上的一个隔离的独立容器，容器里面的子元素不会影响到外面的元素。反之也如此！`

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因为BFC内部的元素和外部的元素绝对不会互相影响，因此，当BFC外部存在浮动时，它不应该影响BFC内部Box的布局，BFC会通过变窄，而不与浮动有重叠。同样的，当BFC内部有浮动时，为了不影响外部元素的布局，BFC计算高度时会包括浮动的高度。避免margin重叠也是这样的一个道理。

参考

前端精选文摘：BFC 神奇背后的原理

我对BFC的理解

谈外margin collapsing(外边距叠加)

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