vue2实现高度过渡效果，函数式组件

const transitionStyle = '0.3s height ease-in-out'
const Transition = {
  // 进入前 设置el元素的transition,高度0
  beforeEnter (el) {
    el.style.transition = transitionStyle
    el.style.height = 0
  },
  // 进入中
  enter (el) {
    // 获取元素显示的高度
    if (el.scrollHeight !== 0) {
      el.style.height = `${el.scrollHeight}px`
    } else {   //没有高度就不显示
      el.style.height = ''
    }
    el.style.overflow = 'hidden'
  },
  // 进入结束
  afterEnter (el) {
    // 清除过度样式
    el.style.transition = ''
    el.style.height = ''
  },
 
  // 离开前
  beforeLeave (el) {
    el.style.height = `${el.scrollHeight}px`
    el.style.overflow = 'hidden'
  },
  // 离开中
  leave (el) {
    if (el.scrollHeight !== 0) {
      el.style.transition = transitionStyle
      el.style.height = 0
    }
  },
  // 离开结束
  afterLeave (el) {
    el.style.transition = ''
    el.style.height = ''
  }
}
 
export default {
  functional: true,   //指定函数组件
  render (h, {children}) { 
    return h('transition', { on: Transition}, children)
  }
}

<template>
<div>
    <p @click="show=!show">切换</p>
    <my-transition>
        <ul class='box'  v-if="show">
            <li>aaaaaaa</li>
            <li>bbbbbbb</li>
            <li>ccccccc</li>
            <li>ddddddd</li>
            <li>eeeeeee</li>
        </ul>
    </my-transition>
    </div>
</template>
<script>
// 自定义过度组件
import myTransition from "../utils/transition.js";
export default {
    data() {
        return {
            show:false
        }
    },
    components: {
        "my-transition": myTransition,
    },
}
</script>
<style lang="less" scoped>
.box{
    background: rgb(248, 248, 248);
    position: absolute;
    box-sizing: border-box;
    top: 50px;
    left: 40%;
}
</style>