我们在日常的开发中，经常会遇到这样一个问题，就是如何实现居中水平垂直居中对齐。并且在面试中也会出现这样的问题，但是我们往往回答的不是很全部，而导致没有得到面试加分。接下来我们通过不同的方式来实现，让我们成功破解这道面试。

1、定宽高

一、绝对定位和负magin值

<template>
    <div id="app">
        <div class="box">
            <div class="children-box"></div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    position: relative;
}
.children-box {
    position: absolute;
    width: 100px;
    height: 100px;
    background: yellow;
    left: 50%;
    top: 50%;
    margin-left: -50px;
    margin-top: -50px; 
}
</style>

二、绝对定位 + transform

<template>
    <div id="app">
        <div class="box">
            <div class="children-box"></div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    position: relative;
}
.children-box {
    position: absolute;
    width: 100px;
    height: 100px;
    background: yellow;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%); 
}
</style>

三、绝对定位 + left/right/bottom/top + margin

<template>
    <div id="app">
        <div class="box">
            <div class="children-box"></div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    position: relative;
}
.children-box {
    position: absolute;
    display: inline;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0px;
    background: yellow;
    margin: auto;
    height: 100px;
    width: 100px;
}
</style>

四、flex布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box"></div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: flex;
    justify-content: center;
    align-items: center;
}
.children-box {
    background: yellow;
    height: 100px;
    width: 100px;
}
</style>

五、grid布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box"></div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: grid;
}
.children-box {
    width: 100px;
    height: 100px;
    background: yellow;
    margin: auto;
}
</style>

六、table-cell + vertical-align + inline-block/margin: auto

<template>
    <div id="app">
        <div class="box">
            <div class="children-box"></div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: table-cell;
    text-align: center;
    vertical-align: middle;
}
.children-box {
    width: 100px;
    height: 100px;
    background: yellow;
    display: inline-block;// 可以换成margin: auto;
}
</style>

2、不定宽高

一、绝对定位 + transform

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">111111</div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    position: relative;
}
.children-box {
   position: absolute;
   background: yellow;
   left: 50%;
   top: 50%;
   transform: translate(-50%, -50%);
}
</style>

二、table-cell

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">111111</div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: table-cell;
    text-align: center;
    vertical-align: middle;
}
.children-box {
   background: yellow;
   display: inline-block;
}
</style>

三、flex布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">11111111</div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: flex;
    justify-content: center;
    align-items: center;
}
.children-box {
    background: yellow;
}
</style>

四、flex变异布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">11111111</div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: flex;
}
.children-box {
    background: yellow;
    margin: auto;
}
</style>

五、grid + flex布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">11111111</div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: grid;
}
.children-box {
    background: yellow;
    align-self: center;
    justify-self: center;
}
</style>

六、gird + margin布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">11111111</div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    display: grid;
}
.children-box {
    background: yellow;
    margin: auto;
}
</style>

七、writing-mode属性布局

<template>
    <div id="app">
        <div class="box">
            <div class="children-box">
                <p>11111</p>
            </div>
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    writing-mode: vertical-lr;
    text-align: center;
}

.box>.children-box {
    writing-mode: horizontal-tb;
    display: inline-block;
    text-align: center;
    width: 100%;
}

.box>.children-box>p {
    display: inline-block;
    margin: auto;
    text-align: left;
    background: yellow;
}
</style>

writing-mode 属性定义了文本在水平或垂直方向上如何排布。兼容性上还有些小瑕疵，但大部分浏览器已经支持。

3、番外(图片定高|不定高水平垂直居中)

一、table-cell

<template>
    <div id="app">
        <div class="box">
            <img src="https://ss1.baidu.com/70cFfyinKgQFm2e88IuM_a/forum/pic/item/242dd42a2834349b406751a3ceea15ce36d3beb6.jpg">
        </div>
        </div>
</template>
<style type="text/css">
.box {
     height: 200px;
    width: 200px;
    display: table-cell;
    text-align: center;
    border: 1px solid #ccc;
    vertical-align: middle;
}
</style>

二、 ::after

<template>
    <div id="app">
        <div class="box">
            <img src="https://ss1.baidu.com/70cFfyinKgQFm2e88IuM_a/forum/pic/item/242dd42a2834349b406751a3ceea15ce36d3beb6.jpg">
        </div>
    </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid red;
    text-align: center;
}

.box::after {
    content: '';
    display: inline-block;
    vertical-align: middle;
    height: 100%;
}
img {
    vertical-align: middle;
}
</style>

三、 ::before

<template>
    <div id="app">
        <div class="box">
            <img src="https://ss1.baidu.com/70cFfyinKgQFm2e88IuM_a/forum/pic/item/242dd42a2834349b406751a3ceea15ce36d3beb6.jpg">
        </div>
        </div>
</template>
<style type="text/css">
.box {
    width: 200px;
    height: 200px;
    border: 1px solid #ccc;

    text-align: center;
    font-size: 0;
}

.box::before {
    display: inline-block;
    vertical-align: middle;
    content: '';
    height: 100%;
}

img {
    vertical-align: middle;
}
</style>

四、总结

这里总结了很多种垂直水平居中的方法，但是肯定不是最完全的，还有很多其他中方式。但是总体的思路可以总结为以下几点。

一、内联元素居中布局

水平居中

行内元素可设置：text-align: center;
flex布局设置父元素：display: flex; justify-content: center;

垂直居中

单行文本父元素确认高度：height === line-height
多行文本父元素确认高度：disaply: table-cell; vertical-align: middle;

二、块级元素居中布局

水平居中

定宽: margin: 0 auto;
不定宽：参考上诉例子中不定宽高例子。

垂直居中

position: absolute设置left、top、margin-left、margin-to(定高)；
position: fixed设置margin: auto(定高)；
display: table-cell；
transform: translate(x, y)；
flex(不定高，不定宽)；
grid(不定高，不定宽)，兼容性相对比较差；