How does flexbox in CSS3 implement horizontal, vertical centering and three-column equal-height layout?-HTML Tutorial-php.cn

In the past few days, I have been making up for the basic knowledge of CSS and CSS3. When I opened the web page, I discovered that there is such a thing on the default homepage of Firefox.

I didn’t understand the first css attribute. Ever since, I started looking for information and reading various books. These days, I am writing a blog post about my understanding of the CSS3 retractable layout box (flexbox) model, with the purpose of giving a brief introduction to flexbox.

The following content will be divided into the following subsections:

1. Concepts you need to master about flexbox in css3

2. Flexbox realizes horizontal and vertical center alignment

3. Three columns of equal height are adaptive, and the footer area is glued to the bottom layout

1. What you need to know about flexbox in css3

　Because three-column equal-height adaptive layout and horizontal and vertical center alignment require some understanding of the basic concepts of flexbox in CSS3, I will give a brief introduction to the concept of flexbox to pave the way for subsequent examples. I have always believed that no matter what knowledge you learn, understanding the concepts is very important.

　a: Flexible container: refers to setting an element to flex or inline-box (standard version) through the display attribute. This container is a flexible container.

　b: Flexible item: A flexable item is a child element of the flex container. The contents of a flex container have more than zero flex items - each child element of the flex container becomes a flex item (including text, called an anonymous flex item).

　c: Telescopic flow direction: refers to the main axis direction in the telescopic container, which can be understood as the direction of the x-axis. The direction of the scalable flow is mainly set through the flex-direction attribute (standard version), and the default value is row.

　d: Flexible line wrapping: Flexible items sometimes overflow the flexible container in the flexible container. In the flex container properties, the flex-wrap property is mainly used to set whether the flex container wraps. The default value is nowrap.

　e: Scalability: Defining a flex item can change the width or height of the flex container to fill the available space. You can allocate extra space in a flex container to its flex items or shrink them to prevent the flex items from overflowing.

2. Flexbox realizes horizontal and vertical center alignment

<span style="color: #800000;">html, body </span>{<span style="color: #ff0000;">
  height</span>:<span style="color: #0000ff;"> 100%</span>;<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 100%</span>;
}<span style="color: #800000;">
body </span>{<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -moz-box</span>;<span style="color: #ff0000;">
  -moz-box-orient</span>:<span style="color: #0000ff;"> vertical</span>;<span style="color: #ff0000;">
  -moz-box-align</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  -moz-box-pack</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -webkit-box</span>;<span style="color: #ff0000;">
  -webkit-box-orient</span>:<span style="color: #0000ff;"> vertical</span>;<span style="color: #ff0000;">
  -webkit-box-align</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  -webkit-box-pack</span>:<span style="color: #0000ff;"> center</span>;
}<span style="color: #800000;">
.content </span>{<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 300px</span>;<span style="color: #ff0000;">
  height</span>:<span style="color: #0000ff;"> 300px</span>;<span style="color: #ff0000;">
  background-color</span>:<span style="color: #0000ff;"> lightblue</span>;<span style="color: #ff0000;">
  text-align</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -moz-box</span>;<span style="color: #ff0000;">
  -moz-box-orient</span>:<span style="color: #0000ff;"> vertical</span>;<span style="color: #ff0000;">
  -moz-box-align</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  -moz-box-pack</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -webkit-box</span>;<span style="color: #ff0000;">
  -webkit-box-orient</span>:<span style="color: #0000ff;"> vertical</span>;<span style="color: #ff0000;">
  -webkit-box-align</span>:<span style="color: #0000ff;"> center</span>;<span style="color: #ff0000;">
  -webkit-box-pack</span>:<span style="color: #0000ff;"> center</span>;
}

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<span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="content"</span><span style="color: #0000ff;">><</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>水平垂直居中对齐<span style="color: #0000ff;"></</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>

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　First, set the width and height of html and body to 100%. The purpose is to make them have the same width and height. Otherwise, side main axis alignment (box-pack) and side axis alignment (box-align) will not allow the scalable items to be distributed and retracted before Extra space for the container.

　 Then, make the body a scalable container, set the display attribute to box, set box-pack and box-align to control the main axis alignment and cross-axis alignment, and set their attribute values to center.

　Finally, the .content element becomes a flex container, so that its inner text block becomes an anonymous flex item. At this time, the .content element is both a scalable container and a scalable project. When used as a scalable container, the h1 element is its scalable item; when used as a scalable item, the body is its scalable container. Also set box-align and box-pack for .content to control cross-axis alignment and main-axis alignment.

　The rendering is as follows, both the .cotent element and h1 are aligned horizontally and vertically in the center.

3. Three columns of equal height are adaptive, and the footer area is glued to the bottom layout

　 There are many methods for three-column layout, which can be achieved by combining float + percentage width, or using inline-block with percentage, but it is difficult to achieve a layout where the footer is adhered to the bottom of the browser's visual window. Here we only introduce the CSS3 three-column equal-height layout.

　Any layout effect cannot be achieved without HTML structure.

    <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="header"</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"><</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>头部<span style="color: #0000ff;"></</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>
    <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>
    <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="page"</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="main"</span><span style="color: #0000ff;">></span>
            <span style="color: #0000ff;"><</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>主内容<span style="color: #0000ff;"></</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="sidebar-left"</span><span style="color: #0000ff;">></span>
            <span style="color: #0000ff;"><</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>左边栏<span style="color: #0000ff;"></</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="sidebar-right"</span><span style="color: #0000ff;">></span>
            <span style="color: #0000ff;"><</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>右边栏<span style="color: #0000ff;"></</span><span style="color: #800000;">h1</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>
    <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>
    <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="footer"</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"><</span><span style="color: #800000;">p</span><span style="color: #0000ff;">></span>页脚<span style="color: #0000ff;"></</span><span style="color: #800000;">p</span><span style="color: #0000ff;">></span>
    <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>

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　 Assume that the width of the header and footer is 100%, the width of the left and right columns is 200px, and the main content is adaptive to the width.

<span style="color: #800000;">body </span>{<span style="color: #ff0000;">
  -moz-box-sizing</span>:<span style="color: #0000ff;"> border-box</span>;<span style="color: #ff0000;">
  -webkit-box-sizing</span>:<span style="color: #0000ff;"> border-box</span>;<span style="color: #ff0000;">
  box-sizing</span>:<span style="color: #0000ff;"> border-box</span>;
}<span style="color: #800000;">
#header, #footer </span>{<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 100%</span>;<span style="color: #ff0000;">
  padding</span>:<span style="color: #0000ff;"> 10px</span>;<span style="color: #ff0000;">
  background-color</span>:<span style="color: #0000ff;"> #ccc</span>;
}<span style="color: #800000;">
#footer </span>{<span style="color: #ff0000;">
  margin-top</span>:<span style="color: #0000ff;"> 10px</span>;
}<span style="color: #800000;">
#sidebar-left, #sidebar-right </span>{<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 200px</span>;<span style="color: #ff0000;">
  padding</span>:<span style="color: #0000ff;"> 10px</span>;<span style="color: #ff0000;">
  background-color</span>:<span style="color: #0000ff;"> #f36</span>;
}

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　The purpose of setting box-sizing for the body here is to make the width of the box model = content width + border + padding, to avoid the need to calculate the width of the width when setting the padding value.

Next, use the scalable layout box model box (the old version can still be used) to make the #page element a scalable container. Set box-flex to make its child elements scalable and adaptive to the remaining space of the scalable container.

<span style="color: #800000;">#page </span>{<span style="color: #ff0000;">
  margin-top</span>:<span style="color: #0000ff;"> 10px</span>;<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 100%</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -moz-box</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -webkit-box</span>;
}<span style="color: #800000;">
#main </span>{<span style="color: #ff0000;">
  background-color</span>:<span style="color: #0000ff;"> #e66</span>;<span style="color: #ff0000;">
  padding</span>:<span style="color: #0000ff;"> 10px</span>;<span style="color: #ff0000;">
  margin</span>:<span style="color: #0000ff;"> 0 10px</span>;<span style="color: #ff0000;">
  -moz-box-flex</span>:<span style="color: #0000ff;"> 1</span>;<span style="color: #ff0000;">
  -webkit-box-flex</span>:<span style="color: #0000ff;"> 1</span>;
}

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　　上面代码中，使用为#page元素设置了display属性让其成为一个伸缩容器。这里要注意的是必须设置width。如果没有设置width，这里的#main元素的box-flex属性会失效，原因在于父容器没有宽度，自然无法填充伸缩容器的额外空间，（这里的额外空间指的是#page元素所占的面积，而不单单指宽度）。为#main元素设置box-flex属性是让其自适应伸缩容器的额外宽度。因为webkit内核的浏览器（Chrome，Safari）和Gecko内核（Firefox）不支持box-flex属性和box属性，所以必须添加厂商前缀。

　　上面实例中，需要修改一下主内容和左边栏，右边栏的排列方式，使用box-ordinal-group属性。

<span style="color: #800000;">#sidebar-right </span>{<span style="color: #ff0000;">
  -moz-box-ordinal-group</span>:<span style="color: #0000ff;"> 3</span>;<span style="color: #ff0000;">
  -webkit-box-ordinal-group</span>:<span style="color: #0000ff;"> 3</span>;
}<span style="color: #800000;">
#main </span>{<span style="color: #ff0000;">
  background-color</span>:<span style="color: #0000ff;"> #e66</span>;<span style="color: #ff0000;">
  padding</span>:<span style="color: #0000ff;"> 10px</span>;<span style="color: #ff0000;">
  margin</span>:<span style="color: #0000ff;"> 0 10px</span>;<span style="color: #ff0000;">
  -moz-box-flex</span>:<span style="color: #0000ff;"> 1</span>;<span style="color: #ff0000;">
  -moz-box-ordinal-group</span>:<span style="color: #0000ff;"> 2</span>;<span style="color: #ff0000;">
  -webkit-box-flex</span>:<span style="color: #0000ff;"> 1</span>;<span style="color: #ff0000;">
  -webkit-box-ordinal-group</span>:<span style="color: #0000ff;"> 2</span>;
}

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　　上面代码中，使用了box-ordinal-group属性，这个属性是用于修改伸缩项目在伸缩容器中的显示顺序，默认值为1，也就是按照DOM文档流出现的先后顺序进行排序。下面重置了box-ordinal-group属性之后的效果。

　　至此，这个页面就已经做好了。但是出现了一个问题，就是页脚区域不会黏附在浏览器窗口可视区域底部，这让用户体验非常糟糕。

　　使用css3的flexbox属性实现就很简单。最关键的技巧就是让body元素变成一个伸缩容器，并且使用伸缩性属性box-flex让页脚区域之前的div具有伸缩性（也就是#page元素）。也就是说，页脚区域前的div会变成一个伸缩项目，会根据伸缩容器的高度自适应填充伸缩容器的额外空间，也就是自动拉伸页脚区域前的div填充浏览器可视区域中的所有空间。

　　如果希望整个页面的布局要和浏览器窗口可视区域一样高，

　　首先必须设置html和body元素的高度和浏览器窗口可视区域高度一样高。如果少了body高度的设置，body本身就没有高度，当然伸缩项目的伸缩性也就无法体现。

<span style="color: #800000;">html, body </span>{<span style="color: #ff0000;">
  height</span>:<span style="color: #0000ff;"> 100%</span>;
}

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　　其次，让body元素自身成为一个伸缩容器，并且设置伸缩流方向（box-orient）为vertical（旧版本中的属性）。

<span style="color: #800000;">body </span>{<span style="color: #ff0000;">
  -moz-box-sizing</span>:<span style="color: #0000ff;"> border-box</span>;<span style="color: #ff0000;">
  -webkit-box-sizing</span>:<span style="color: #0000ff;"> border-box</span>;<span style="color: #ff0000;">
  box-sizing</span>:<span style="color: #0000ff;"> border-box</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -moz-box</span>;<span style="color: #ff0000;">
  -moz-box-orient</span>:<span style="color: #0000ff;"> vertical</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -webkit-box</span>;<span style="color: #ff0000;">
  -webkit-box-orient</span>:<span style="color: #0000ff;"> vertical</span>;<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 100%</span>;
}

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　　最后，设置页脚区域前的div（#page元素）中的box-flex属性，让其根据伸缩容器（这里是指body）的高度自适应伸缩容器body的额外空间，也就是自动拉伸#page元素的高度。这样就会是页脚一直在浏览器可视窗口底部显示。

<span style="color: #800000;">#page </span>{<span style="color: #ff0000;">
  margin-top</span>:<span style="color: #0000ff;"> 10px</span>;<span style="color: #ff0000;">
  width</span>:<span style="color: #0000ff;"> 100%</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -moz-box</span>;<span style="color: #ff0000;">
  -moz-box-flex</span>:<span style="color: #0000ff;"> 1</span>;<span style="color: #ff0000;">
  -moz-box-align</span>:<span style="color: #0000ff;"> stretch</span>;<span style="color: #ff0000;">
  display</span>:<span style="color: #0000ff;"> -webkit-box</span>;<span style="color: #ff0000;">
  -webkit-box-flex</span>:<span style="color: #0000ff;"> 1</span>;<span style="color: #ff0000;">
  -webkit-box-align</span>:<span style="color: #0000ff;"> stretch</span>;
}

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　　上面代码中，#page元素本身是一个伸缩容器，现在变成伸缩项目。在伸缩布局盒模型中，伸缩项目在侧轴的对齐方式box-align（旧版本）默认值为stretch，（css中可不写box-align属性）致使#page元素的三个伸缩项目都会自动拉伸，不管内容高度有多少都具有伸缩容器#page的高度，从而实现三列等高布局并且页脚黏附浏览器可视区域底部的效果。最后附上效果图。

完。

感谢大家的阅读。