element加固态（Element Plus实现Affix 固钉）

<template>
  <li ref="root" class="el-affix" :style="rootStyle">
    <li :class="{'el-affix--fixed': state.fixed}" :style="affixStyle">
      <slot></slot>
    </li>
  </li>
</template>

// 部分核心代码，代码顺序有所调整
setup(props, { emit }) {
    // target容器 ref
    const target = ref(null) 
    // 固钉ref，与template中的ref属性配合，得到HTML元素
    const root = ref(null)
    // 滚动容器ref
    const scrollContainer = ref(null)
    
    // 固钉状态
    const state = reactive({
      fixed: false,
      height: 0, // height of root
      width: 0, // width of root
      scrollTop: 0, // scrollTop of documentElement
      clientHeight: 0, // clientHeight of documentElement
      transform: 0,
    })
    
    onMounted(() => {
      // 根据传入的target确定 target容器
      if (props.target) {
        target.value = document.querySelector(props.target)
        if (!target.value) {
          throw new Error(`target is not existed: ${props.target}`)
        }
      } else {
        target.value = document.documentElement
      }
      
      // 根据固钉元素，向上寻找滚动容器
      scrollContainer.value = getScrollContainer(root.value)
      // 监听滚动容器的scroll事件
      on(scrollContainer.value, 'scroll', onScroll)
      // 监听固钉元素的resize事件
      addResizeListener(root.value, updateState)
    })
    
    // 滚动容器的scroll事件的响应函数
    const onScroll = () => {
      // 更新固钉状态
      updateState()
      
      emit('scroll', {
        scrollTop: state.scrollTop,
        fixed: state.fixed,
      })
    }
    
    // 更新固钉状态函数
    const updateState = () => {
      const rootRect = root.value.getBoundingClientRect()
      const targetRect = target.value.getBoundingClientRect()
      state.height = rootRect.height
      state.width = rootRect.width
      state.scrollTop = scrollContainer.value === window ? document.documentElement.scrollTop : scrollContainer.value.scrollTop
      state.clientHeight = document.documentElement.clientHeight

      if (props.position === 'top') {
        if (props.target) {
          const difference = targetRect.bottom - props.offset - state.height
          // targetRect.bottom > 0 对应的是让固钉始终保持在容器内，超过范围则隐藏
          state.fixed = props.offset > rootRect.top && targetRect.bottom > 0
          // 用于处理场景：滚动过程中，target容器可视区域不足以显示整个固钉，则固钉应相应偏移，只展示部分
          state.transform = difference < 0 ? difference : 0
        } else {
          state.fixed = props.offset > rootRect.top
        }
      } else {
        if (props.target) {
          const difference = state.clientHeight - targetRect.top - props.offset - state.height
          state.fixed = state.clientHeight - props.offset < rootRect.bottom && state.clientHeight > targetRect.top
          state.transform = difference < 0 ? -difference : 0
        } else {
          state.fixed = state.clientHeight - props.offset < rootRect.bottom
        }
      }
    }
    // 监测固钉fixed状态变化，并对外emit change事件
    watch(() => state.fixed, () => {
      emit('change', state.fixed)
    })
    
    // 计算属性，通过固钉的状态自动更新固钉的样式
    const affixStyle = computed(() => {
      if (!state.fixed) {
        return
      }
      const offset = props.offset ? `${props.offset}px` : 0
      const transform = state.transform ? `translateY(${state.transform}px)` : ''

      return {
        height: `${state.height}px`,
        width: `${state.width}px`,
        top: props.position === 'top' ? offset : '',
        bottom: props.position === 'bottom' ? offset : '',
        transform: transform,
        zIndex: props.zIndex,
      }
    })
}