Element Plus实现Affix 固钉

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