源代码地址-collapse-transition

version:element-plus 1.0.1-beta.0

<template>
  <transition v-on="on">
    <slot></slot>
  </transition>
</template>
<script lang='ts'>
import { addClass, removeClass } from '@element-plus/utils/dom'

export default {
  name: 'ElCollapseTransition',
  setup() {
    return {
      // https://cn.vuejs.org/v2/api/#transition
      on: {
        beforeEnter(el) {
          // 根元素class添加'collapse-transition'
          addClass(el, 'collapse-transition')
          // 第一次触发 初始化
          if (!el.dataset) el.dataset = {}

          el.dataset.oldPaddingTop = el.style.paddingTop
          el.dataset.oldPaddingBottom = el.style.paddingBottom

          el.style.height = '0'
          el.style.paddingTop = 0
          el.style.paddingBottom = 0
        },

        enter(el) {
          el.dataset.oldOverflow = el.style.overflow
          // 是否存在内容
          if (el.scrollHeight !== 0) {
            // 存在则读取高度和上下padding
            el.style.height = el.scrollHeight + 'px'
            el.style.paddingTop = el.dataset.oldPaddingTop
            el.style.paddingBottom = el.dataset.oldPaddingBottom
          } else {
            // 空的话就高度变成空
            // 其实就高度不一致，感觉可以上下padding提出判断外
            el.style.height = ''
            el.style.paddingTop = el.dataset.oldPaddingTop
            el.style.paddingBottom = el.dataset.oldPaddingBottom
          }

          // 设置超出隐藏
          el.style.overflow = 'hidden'
        },

        afterEnter(el) {
          // for safari: remove class then reset height is necessary
          removeClass(el, 'collapse-transition')
          el.style.height = ''
          // 恢复原先设置的overflow
          el.style.overflow = el.dataset.oldOverflow
        },

        beforeLeave(el) {
          if (!el.dataset) el.dataset = {}
          el.dataset.oldPaddingTop = el.style.paddingTop
          el.dataset.oldPaddingBottom = el.style.paddingBottom
          el.dataset.oldOverflow = el.style.overflow

          el.style.height = el.scrollHeight + 'px'
          el.style.overflow = 'hidden'
        },

        leave(el) {
          if (el.scrollHeight !== 0) {
            // for safari: add class after set height, or it will jump to zero height suddenly, weired
            addClass(el, 'collapse-transition')
            el.style.height = 0
            el.style.paddingTop = 0
            el.style.paddingBottom = 0
          }
        },

        afterLeave(el) {
          removeClass(el, 'collapse-transition')
          el.style.height = ''
          el.style.overflow = el.dataset.oldOverflow
          el.style.paddingTop = el.dataset.oldPaddingTop
          el.style.paddingBottom = el.dataset.oldPaddingBottom
        },
      },
    }
  },
}
</script>