This section describes how to trigger BFC and BFC functions (such as clear floating and prevent overlap of margin)
1. What is BFC? Box: basic unit of CSS layoutBox is the object and basic unit of CSS layout. In a straight view, a page is composed of many boxes. The element type and display attribute determine the type of the box. Different types of boxes are involved in different formatting context (a container that determines how to render the Document). Therefore, elements in the box are rendered in different ways. Let's see which boxes are available:
Formatting context is a concept in W3C css2.1 specification. It is a rendering area in the page and has a set of rendering rules that determine how its child elements will be located and the relationship and interaction with other elements. The most common formatting context includes block fomatting context (BFC) and inline formatting context (IFC ).
OnlyBFC AndIFC,Css3AddedGFC AndFFC。
The BFC (Block formatting context) literal translation is "block-level formatting context ". It is an independent rendering area with only block-level box involved. It specifies how the internal block-level box is laid out and has nothing to do with the outside of the area.
BFC layout rules:<style> body { width: 300px; position: relative; } .aside { width: 100px; height: 150px; float: left; background: #f66; } .main { height: 200px; background: #fcc; }</style><body> <div class="aside"></div> <div class="main"></div></body>
Page:
AccordingBFC3rd layout rules:
The left side of the margin box of each element is in contact with the left side of the border box containing the block (for formatting from left to right, otherwise the opposite is true ). This is true even if floating occurs.
Therefore, although the floating element aslide exists, the left side of main is still in contact with the left side of the contained block.
AccordingBFCArticle 4 of layout rules:
BFCDoes not matchfloat boxOverlap.
We can generateBFCTo achieve adaptive two-column layout.
.main { overflow: hidden;}
When the main generation is triggeredBFCAfter, this newBFCDoes not overlap with floating aside. Therefore, it is automatically narrowed according to the width of the included block and the width of the aside. The effect is as follows:
2. Clear internal floating
Code:
<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>
Page:
AccordingBFCArticle 6 of layout rules:
Computing
BFCThe floating element is also involved in the calculation.
To clear the internal floating point, we can trigger the par generation.BFCIn this case, when the height of a par is calculated, the floating element child in the PAR is also involved in the calculation.
Code:
.par { overflow: hidden;}
The effect is as follows:
?
Code:
<style> p { color: #f55; background: #fcc; width: 200px; line-height: 100px; text-align:center; margin: 100px; }</style><body> <p>Haha</p> <p>Hehe</p></body>
Page:
The distance between two P is PX, and a margin overlap is sent.
The second rule is as follows:
BoxThe vertical distance is determined by margin. Belong to the sameBFCTwo AdjacentBoxThe margin of will overlap.
We can wrap a layer of containers outside P and trigger this container to generateBFC. Then the two P do not belong to the sameBFC.
Code:
<style> .wrap { overflow: hidden; } p { color: #f55; background: #fcc; width: 200px; line-height: 100px; text-align:center; margin: 100px; }</style><body> <p>Haha</p> <div class="wrap"> <p>Hehe</p> </div></body>
The effect is as follows:
In fact, the above examples all reflectBFCArticle 5 of layout rules:
BFCIs an isolated independent container on the page. The child elements in the container do not affect the external elements. And vice versa.
BecauseBFCThe internal and external elements will never affect each other. Therefore, whenBFCWhen there is a floating outside, it should not be affectedBFCInternal box layout,BFCIt will narrow down without overlap with the floating. Similarly, whenBFCWhen there is a floating inside, in order not to affect the layout of external elements,BFCThe calculated height includes a floating height. This is also the case for avoiding the overlap of margin.
Front-end selection Digest: what is behind the magic of BFC
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