伸缩侧边栏

## 效果图
### 展开
![](https://blog.coolcooltomato.com/uploads/article/侧边栏开.png)
### 收起
![](https://blog.coolcooltomato.com/uploads/article/侧边栏收.png)

## 实现原理
 - 展开：单击图标或空白展开主菜单栏，单击图标额外展开子菜单栏
 - 收起：单击空白收起主菜单栏和所有子菜单栏
 - 通过js改变元素的类，并在样式表中定义不同类的样式，实现样式的改变
 - 主菜单的类为.long时，width:25rem，类为.short时，width:7.5rem
 - 子菜单的类为.hide时，height:0，类为.show时，heigth由js根据子元素数量计算

## 源代码
### html
```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <script rel="script" src="./伸缩侧边栏.js"></script>
    <link rel="stylesheet" href="./伸缩侧边栏.css">
    <title>Waku</title>
</head>
<body>
<div id="list" class="short">
    <div id="list_but" onclick="list()"></div>
    <ul id="out_ul">
        <li class="title" onclick="in_list(this)">
            <img src="./light_home.png" class="light_icon">
            <a href="/popularity/index">首页</a>
        </li>
        <ul class="hide in_ul"></ul>
        <li id="ul_line"></li>
        <li class="title" onclick="in_list(this)">
            <img src="./light_content.png" class="light_icon">
            <span>文章</span>
        </li>
        <ul class="hide in_ul">
            <li><div></div><a href="/">文章1</a></li>
            <li><div></div><a href="/">文章2</a></li>
            <li><div></div><a href="/">文章3</a></li>
        </ul>
        <li class="title" onclick="in_list(this)">
            <img src="./light_image.png" class="light_icon">
            <span>图片</span>
        </li>
        <ul class="hide in_ul">
            <li><div></div><a href="/">图片1</a></li>
            <li><div></div><a href="/">图片3</a></li>
            <li><div></div><a href="/">图片2</a></li>
        </ul>
        <li class="title" onclick="in_list(this)">
            <img src="./light_video.png" class="light_icon">
            <span>视频</span>
        </li>
        <ul class="hide in_ul">
            <li><div></div><a href="/">视频1</a></li>
            <li><div></div><a href="/">视频3</a></li>
            <li><div></div><a href="/">视频2</a></li>
        </ul>
        <li class="title" onclick="in_list(this)">
            <img src="./light_qj.png" class="light_icon">
            <span>全景</span>
        </li>
        <ul class="hide in_ul">
            <li><div></div><a href="/">全景1</a></li>
            <li><div></div><a href="/">全景3</a></li>
            <li><div></div><a href="/">全景2</a></li>
        </ul>
    </ul>
</div>
</body>
</html>
```
### css
```css
html{
    font-size: 9px;
}
body{
    background: white;
    color: #707070;
}
body::-webkit-scrollbar {
    width: 0.5rem;
    height: 0.5rem;
    border-radius: 0.2rem;
}

body::-webkit-scrollbar-track {
    background-color: white;
    border-radius: 0.2rem;
}

body::-webkit-scrollbar-thumb {
    background-color: #555555;
    border-radius: 0.2rem;
}
.dark_icon{
    display: none;
}
.light_icon{
    display: unset;
}
/*---------颜色-----------*/
#list{
    background-color: white;
    box-shadow: 0 0 1rem #909090;
}
#ul_line{
    background-color: #AAAAAA;
}
.title:hover{
    background-color: #E0E0E0;
}
.title span{
    color: #707070;
}
.title a{
    color: #707070;
}
.in_ul li:hover{
    background-color: #E0E0E0;
}
.in_ul div{
    background-color: #707070;
}
.in_ul a{
    color: #707070;
}
.dark_icon{
    display: none;
}
.light_icon{
    display: unset;
}
/*---------形状-------------*/
/*左侧菜单栏*/
#list{
    height: 100%;
    left: 0;
    top: 0;
    position: fixed;
    transition: 0.4s ease-in-out;
    overflow: hidden;
}
.short{
    width: 7.5rem;
}
.long{
    width: 25rem;
}
#list_but{
    width: 100%;
    height: 15%;
    left: 0;
    top: 0;
    position: absolute;
}
#ul_line{
    width: 100%;
    height: 0.2rem;
    left: 0;
    position: relative;
    margin-top: 1rem;
    margin-bottom: 1rem;
}
#out_ul{
    width: 25rem;
    height: 70%;
    top: 15%;
    left: 0;
    position: absolute;
    list-style-type: none;
    margin: 0;
    padding: 0 0 0 0;
    overflow: hidden;
}
.title{
    width: 100%;
    height: 6rem;
    position: relative;
    cursor: pointer;
    transition: 0.2s ease-in-out;
}
.title img{
    width: 3.5rem;
    height: 3.5rem;
    left: 2rem;
    top: 50%;
    transform: translateY(-50%);
    position: absolute;
}
.title span{
    font-size: 1.8rem;
    left: 7.5rem;
    top: 50%;
    position: absolute;
    transform: translateY(-50%);
}
.title a{
    left: 7.5rem;
    top: 50%;
    transform: translateY(-50%);
    position: absolute;
    font-size: 1.8rem;
    text-decoration: none;
}
.hide{
    height: 0;
}
.in_ul{
    width: 100%;
    list-style-type: none;
    margin: 0;
    padding: 0 0 0 0;
    overflow: hidden;
    transition: 0.4s ease-in-out;
}
.in_ul li{
    width: 100%;
    height: 4rem;
    position: relative;
    transition: 0.2s ease-in-out;
}
.in_ul div{
    width: 1rem;
    height: 1rem;
    left: 3rem;
    top: 50%;
    position: absolute;
    transform: translateY(-50%);
    border-radius: 50%;
}
.in_ul a{
    left: 7.5rem;
    top: 50%;
    transform: translateY(-50%);
    position: absolute;
    font-size: 1.6rem;
    text-decoration: none;
}
```
### js
```javascript
/*单击空白*/
/*菜单动画*/
function list(){
    /*获取主菜单和所有子菜单*/
    let out_ul = document.getElementById('list');
    let in_ul = document.getElementsByClassName('in_ul');
    /*改变主菜单的类，通过css改变长度*/
    if (out_ul.className === 'short'){
        out_ul.className = 'long';
    }
    else {
        out_ul.className = 'short';
        /*将每个子菜单收起*/
        for (i of in_ul){
            i.className = "hide in_ul";
            i.style.height = 0;
        }
    }
}
/*单击图标*/
/*菜单展开*/
function in_list(self){
    /*获取图标对应的子菜单*/
    let in_ul = self.nextSibling.nextSibling;
    /*获取子菜单下的子元素个数（li标签）*/
    let ll = in_ul.children.length;
    /*获取主菜单*/
    let out_ul = document.getElementById('list');
    /*子菜单收起时，单击展开主菜单和该子菜单*/
    if (in_ul.className === "hide in_ul"){
        in_ul.className = "show in_ul";
        in_ul.style.height = ll * 4 + 'rem';
        out_ul.className = 'long';
    }
    /*子菜单展开时，单击收起该子菜单*/
    else {
        in_ul.className = "hide in_ul";
        in_ul.style.height = 0;
    }
}
```

伸缩侧边栏

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