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深入理解BFC_html/css_WEB-ITnose

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Jun 24, 2016 am 11:25 AM

× 目录 [1]定义 [2]触发 [3]作用

定义

　　在解释BFC之前，先说一下文档流。我们常说的文档流其实分为定位流、浮动流和普通流三种。而普通流其实就是指BFC中的FC。FC是formatting context的首字母缩写，直译过来是格式化上下文，它是页面中的一块渲染区域，有一套渲染规则，决定了其子元素如何布局，以及和其他元素之间的关系和作用。常见的FC有BFC、IFC，还有GFC和FFC。BFC是block formatting context，也就是块级格式化上下文，是用于布局块级盒子的一块渲染区域

触发条件

　　满足下列条件之一就可触发BFC

　　【1】根元素，即HTML元素

　　【2】float的值不为none

　　【3】overflow的值不为visible

　　【4】display的值为inline-block、table-cell、table-caption、flex

　　【5】position的值为absolute或fixed

作用

　　BFC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元素，反之亦然。它与普通的块框类似，但不同之处在于:

　　【1】可以阻止元素被浮动元素覆盖

　　通过改变内容为BFC背景为红色的盒子的属性值，使其成为BFC，以此阻止被绿色的浮动盒子覆盖

　　【2】可以包含浮动元素

　　通过改变高度塌陷的黑色边框的盒子的属性值，使其成为BFC，以此来包含绿色的浮动盒子

　　【3】属于同一个BFC的两个相邻块级子元素的上下margin会发生重叠，(设置writing-mode:tb-rl时，水平margin会发生重叠)。所以当两个相邻块级子元素分属于不同的BFC时可以阻止margin重叠

　　淡红色背景的块级盒子二的外面包一个div，通过改变此div的属性使红色盒子与绿色盒子分属于两个不同的BFC，以此来阻止margin重叠

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