效果展示

效果实现

方法一

准备工作

首先把导航栏效果做出来

HTML

<div class="nav">
    <div class="top">
        <div class="title">Title1</div>
        <div class="list">
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
        </div>
    </div>
    <div class="top">
        <div class="title">Title2</div>
        <div class="list">
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
        </div>
    </div>
    <div class="top">
        <div class="title">Title3</div>
        <div class="list">
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
        </div>
    </div>
</div>

CSS

.nav{
    width: 1200px;
    height: 72px;
    display: flex;
    background-color: #DDD;
    justify-content: center;
}
.top{
    margin: 0 20px 0 20px;
    background-color: #EEE;
    width: 10%;
    height: 100%;
}
.title{
    display: flex;
    height: 100%;
    font-family: "微软雅黑";
    font-size: 36px;
    justify-content: center;
    align-items: center;
}

再给下拉框设置样式

.list{
    width: 100%;
    background: #EEE;
}
.list>a{
    display: flex;
    justify-content: center;
    padding: 10px 0 10px 0;
    font-family: "微软雅黑";
    font-size: 24px;
    text-decoration: none;
    color: #000;
}

现在效果应该是这样的

控制下拉框的显示与隐藏

隐藏下拉框

.list{
    opacity: 0;
    transform: scaleY(0);
}

显示下拉框

.top:hover>.list{
    opacity: 1;
    transform: scaleY(1);
}

添加动画效果

下拉效果

.list{
    opacity: 0;
    transform: scaleY(0);
    transform-origin: top center;
    transition: opacity 0.5s ease-out,transform 0.5s;
}
.top:hover>.list{
    opacity: 1;
    transform: scaleY(1);
}

淡出淡入效果

.list{
    opacity: 0;
    transform: scaleY(0);
    transform-origin: top center;
    transition: opacity 0.5s ease-out,transform 0s 0.5s;
}
.top:hover>.list{
    opacity: 1;
    transform: scaleY(1);
    transition: opacity 0.5s ease-out,transform 0s;
}

方法二

准备工作

HTML

<div class="nav">
    <div class="top">
        <div class="title t1">
            <a href="" class="first">Title1</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
        </div>
    </div>
    <div class="top">
        <div class="title t2">
            <a href="" class="first">Title2</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
        </div>
    </div>
    <div class="top">
        <div class="title t3">
            <a href="" class="first">Title3</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
            <a href="">xxxxxx</a>
        </div>
    </div>
</div>

CSS

.nav{
    width: 100vw;
    height: 72px;
    display: flex;
    margin-top: 200px;
    background-color: #DDD;
    justify-content: center; 
}
.top{
    margin: 0 20px 0 20px;
    background-color: #EEE;
    width: 10%;
    height: 100%;
    position: relative;
}
.title>a{
    display: block;
    font-family: "微软雅黑";
    text-decoration: none;
    color: #000;
    text-align: center;
    background: #EEE;
}
.title>.first{
    height: 72px;
    line-height: 72px;
    font-size: 36px;
}
.title>a:not(.first){
    height: 52px;
    line-height: 52px;
    font-size: 24px;
}

无动画效果的显示与隐藏

隐藏下拉框

.title{
    height: 72px;
    overflow: hidden;
}

显示下拉框

.title:hover{
    overflow: visible;
}

有动画效果的显示与隐藏

隐藏下拉框

.title{
    height: 72px;
    overflow: hidden;
    transition: height 0.5s ease-out;
}

显示下拉框

.title:hover{
/* 52px为下拉框项的高度, 72px为下拉框Title项的高度 */
    height: calc(var(--n) * 52px + 72px);
}
/* n为下拉框项的数量 */
.t1{
    --n:3;
}
.t2{
    --n:5;
}
.t3{
    --n:8;
}

效果