<div class="main">
    <div v-for="(dom) in longListEl" :key="dom.id" class="longList">
        <div :id="dom.id" class="contain">
            <div style="width: calc(100% - 20px);position: relative;">
                <div :style="{height:`${expectationCellHeight}px`}" v-for="(item,index) in dom.data" :key="item.id">
                    {{ item.title }}
                </div>
            </div>
        </div>
    </div>
</div>

export default {
    data() {
        return {
            longListData:[],
            expectationCellHeight:40,
            longListEl:[{id:'contain',data:[]},{id:'contain1',data:[]}],
            longListDataManages:[]
        };
    },
    mounted() {
        for(let index = 0;index < 250000;index ++){
            this.longListData.push({id:index,title:('我是第' + index + '個(gè)')})
        }
        this.$nextTick(()=>{
            this.longListEl.forEach((dom)=>{
                let containEl = document.getElementById(dom.id)
                if(containEl){
                    this.longListDataManages.push(new LongListDataManage(containEl,this.longListData,this.expectationCellHeight,(data)=>{
                        dom.data = data
                    }))
                }
            })
        })
    },
    methods: {
    },
};

function LongListDataManage(el,totalData,expectationCellHeight,refreshDataCallBack){
    this.el = el//需要操作的dom
    this.startIndex = 0//數(shù)據(jù)源的開始索引
    this.endIndex = 0//數(shù)據(jù)源的結(jié)束索引
    this.containElHeight = el ? el.offsetHeight : 0//可視區(qū)域高度
    this.totalData = totalData//全部數(shù)據(jù)源
    this.refreshDataCallBack = refreshDataCallBack//數(shù)據(jù)刷新回調(diào)
    this.expectationCellHeight = expectationCellHeight//預(yù)期的單元cell高度
    this.expectationCellFullScreenNum = 0//預(yù)期可視區(qū)域展示的單元條數(shù)
    this.expectationTotalHeight = 0//預(yù)期滑動的最大高度
    this.scrollTop = 0//當(dāng)前滾動距離
    this.beginRefreshDataDistance = expectationCellHeight//滑動觸發(fā)刷新數(shù)據(jù)最小距離，這里僅為一個(gè)單元格高度
    // 開始填充數(shù)據(jù)
    this.begin = function(){
        this.initData()
        this.getCurrentShowDatas()
        this.addListenerScroll()
    }
    // 初始化參數(shù)
    this.initData = function(){
        //總高度設(shè)定
        this.expectationTotalHeight = this.totalData.length * this.expectationCellHeight
        this.el.style.height = this.expectationTotalHeight + 'px'
        //數(shù)據(jù)源的結(jié)束索引設(shè)定（向上取整）
        this.endIndex = this.expectationCellFullScreenNum = Math.ceil((this.containElHeight / this.expectationCellHeight) + 0.1)
    }
    // 開始獲取當(dāng)前需要展示的區(qū)間
    this.getCurrentShowDatas = function(){
        //截取數(shù)據(jù)源開始跟結(jié)束節(jié)點(diǎn)之間的數(shù)據(jù)
        let currentData = this.totalData.slice(this.startIndex,this.endIndex)
        //給vue提供數(shù)據(jù)
        this.refreshDataCallBack(currentData)
    }
    // 監(jiān)聽滾動
    this.addListenerScroll = function(){
        //父節(jié)點(diǎn)
        let parentNode = this.el.parentNode
        //第一個(gè)子節(jié)點(diǎn)（改變其top值實(shí)現(xiàn)始終在父節(jié)點(diǎn)的可視區(qū)域內(nèi)）
        let firstChild = this.el.firstChild
        let scrollingEvent = ()=>{
            let scrollTop = parentNode.scrollTop
            //滑動的距離超過規(guī)定的閥值或者滑動距離為0進(jìn)行數(shù)據(jù)源重置
            if(Math.abs(scrollTop - this.scrollTop) >= this.beginRefreshDataDistance || scrollTop <= 0){
                //更改數(shù)據(jù)源區(qū)間索引
                this.startIndex = Math.ceil(scrollTop / this.expectationCellHeight)
                this.endIndex = this.startIndex + this.expectationCellFullScreenNum
                //上下平移可視區(qū)域展示dom，實(shí)現(xiàn)屏幕相對位置不變
                firstChild.style.top = scrollTop + 'px'
                //回傳數(shù)據(jù)源
                this.getCurrentShowDatas()
                this.scrollTop = scrollTop
            }
        }
        //開始監(jiān)聽滾動
        parentNode.addEventListener("scroll", scrollingEvent)
    }
}