虚拟列表 ​

<template>
  <div>
    <virtual-list style="height: 360px; overflow-y: auto;" // make list scrollable
      :data-key="'uid'"
      :data-sources="items"
      :data-component="itemComponent"
      :estimate-size="50" // 每个item的预估大小
    />
  </div>
</template>

<script>
  import Item from './Item'
  import VirtualList from 'vue-virtual-scroll-list'

  export default {
    name: 'root',
    data () {
      return {
        itemComponent: Item,
        items: [{uid: 'unique_1', text: 'abc'}, {uid: 'unique_2', text: 'xyz'}, ...]
      }
    },
    components: { 'virtual-list': VirtualList }
  }
</script>

<template>
  <div ref="list" :style="{height}" class="infinite-list-container" @scroll="scrollEvent($event)">
    <div ref="phantom" class="infinite-list-phantom"></div>
    <div ref="content" class="infinite-list">
      <div class="infinite-list-item" ref="items" :id="item._index" :key="item._index" v-for="item in visibleData">
        <slot ref="slot" :item="item.item"></slot>
      </div>
    </div>
  </div>
</template>


<script>
export default {
  name:'VirtualList',
  props: {
    //所有列表数据
    listData:{
      type:Array,
      default:()=>[]
    },
    //预估高度
    estimatedItemSize:{
      type:Number,
      required: true
    },
    //缓冲区比例
    bufferScale:{
      type:Number, 
      default:1
    },
    //容器高度 100px or 50vh
    height:{
      type:String,
      default:'100%'
    }
  },
  computed:{
    _listData(){
      return this.listData.map((item,index)=>{
        return {
          _index:`_${index}`,
          item
        }
      })
    },
    visibleCount(){
      return Math.ceil(this.screenHeight / this.estimatedItemSize);
    },
    aboveCount(){
      return Math.min(this.start,this.bufferScale * this.visibleCount)
    },
    belowCount(){
      return Math.min(this.listData.length - this.end,this.bufferScale * this.visibleCount);
    },
    visibleData(){
      let start = this.start - this.aboveCount;
      let end = this.end + this.belowCount;
      return this._listData.slice(start, end);
    }
  },
  created(){
    this.initPositions();
    window.vm = this;
  },
  mounted() {
    this.screenHeight = this.$el.clientHeight;
    this.start = 0;
    this.end = this.start + this.visibleCount;
  },
  updated(){
    this.$nextTick(function () {
      if(!this.$refs.items || !this.$refs.items.length){
        return ;
      }
      //获取真实元素大小，修改对应的尺寸缓存
      this.updateItemsSize(); 
      //更新列表总高度
      let height = this.positions[this.positions.length - 1].bottom;
      this.$refs.phantom.style.height = height + 'px'
      //更新真实偏移量
      this.setStartOffset();
    })
  },
  data() {
    return {
      //可视区域高度
      screenHeight:0,
      //起始索引
      start:0,
      //结束索引
      end:0,
    };
  },
  methods: {
    initPositions(){
      this.positions = this.listData.map((d,index)=>({
          index,
          height:this.estimatedItemSize,
          top:index * this.estimatedItemSize,
          bottom:(index+1) * this.estimatedItemSize
        })
      );
    },
    //获取列表起始索引
    getStartIndex(scrollTop = 0){
      //二分法查找
      return this.binarySearch(this.positions,scrollTop)
    },
    binarySearch(list,value){
      let start = 0;
      let end = list.length - 1;
      let tempIndex = null;

      while(start <= end){
        let midIndex = parseInt((start + end)/2);
        let midValue = list[midIndex].bottom;
        if(midValue === value){
          return midIndex + 1;
        }else if(midValue < value){
          start = midIndex + 1;
        }else if(midValue > value){
          if(tempIndex === null || tempIndex > midIndex){
            tempIndex = midIndex;
          }
          end = end - 1;
        }
      }
      return tempIndex;
    },
    //获取列表项的当前尺寸
    updateItemsSize(){
      let nodes = this.$refs.items;
      nodes.forEach((node)=>{
        let rect = node.getBoundingClientRect();
        let height = rect.height;
        let index = +node.id.slice(1)
        let oldHeight = this.positions[index].height;
        let dValue = oldHeight - height;
        //存在差值
        if(dValue){
          this.positions[index].bottom = this.positions[index].bottom - dValue;
          this.positions[index].height = height;
          for(let k = index + 1;k<this.positions.length; k++){
            this.positions[k].top = this.positions[k-1].bottom;
            this.positions[k].bottom = this.positions[k].bottom - dValue;
          }
        }
      })
    },
    //获取当前的偏移量
    setStartOffset(){
      let startOffset;
      if(this.start >= 1){
        let size = this.positions[this.start].top - (this.positions[this.start - this.aboveCount] ? this.positions[this.start - this.aboveCount].top : 0);
        startOffset = this.positions[this.start - 1].bottom - size;
      }else{
        startOffset = 0;
      }
      this.$refs.content.style.transform = `translate3d(0,${startOffset}px,0)`
    },
    //滚动事件
    scrollEvent() {
      //当前滚动位置
      let scrollTop = this.$refs.list.scrollTop;
      // let startBottom = this.positions[this.start - ]
      //此时的开始索引
      this.start = this.getStartIndex(scrollTop);
      //此时的结束索引
      this.end = this.start + this.visibleCount;
      //此时的偏移量
      this.setStartOffset();
    }
  }
};
</script>


<style scoped>
.infinite-list-container {
  overflow: auto;
  position: relative;
  -webkit-overflow-scrolling: touch;
}

.infinite-list-phantom {
  position: absolute;
  left: 0;
  top: 0;
  right: 0;
  z-index: -1;
}

.infinite-list {
  left: 0;
  right: 0;
  top: 0;
  position: absolute;
}

.infinite-list-item {
  padding: 5px;
  color: #555;
  box-sizing: border-box;
  border-bottom: 1px solid #999;
  /* height:200px; */
}

</style>

<VirtualList :listData="dataList" :estimatedItemSize="100" height="200px" v-slot="slotProps">
  <p>
    <span style="color:red">{{slotProps.item.id}}</span>
    {{slotProps.item.value}}
  </p>
</VirtualList>

<script setup>
  import { reactive } from 'vue'
  import VirtualList from '../../../components/VirtualList.vue'

  const dataList = reactive([])
  for (let id = 0; id < 1000; id++) {
    dataList.push({
      id,
      value: "长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本长文本" // 
    })
  }
</script>