<template>
 <section class="box">
 <div class="head">
 head
 </div>
 <div class="content">
 <div class="left" ref="left">
 <ul>
  <li v-for="(item, index) in left" :key="item" :class="{current: currentIndex == index}" @click="selectItem(index, $event)">
  <span class="left-item">{{item}}</span>
  </li>
 </ul>
 </div>
 <div class="right" ref="right">
 <ul>
  <li class="right-item right-item-hook" v-for="item in right" :key="item.name">
  <h3>{{item.name}}</h3>
  <ul>
  <li v-for="num in item.content" :key="num.name">
  <div>{{item.name+num}}</div>
  </li>
  </ul>
  </li>
 </ul>
 </div>
 </div>
 </section>
</template>
<script>
import BScroll from 'better-scroll'
export default {
 data () {
 return {
 left: ['a', 'b', 'c', 'd', 'e', 'f'],
 right: [
 {
  name: 'a',
  content: ['1', '2', '3', '4', '5']
 },
 {
  name: 'b',
  content: ['1', '2', '3', '4', '5']
 },
 {
  name: 'c',
  content: ['1', '2', '3', '4', '5']
 },
 {
  name: 'd',
  content: ['1', '2', '3', '4', '5']
 },
 {
  name: 'e',
  content: ['1', '2', '3', '4', '5']
 },
 {
  name: 'f',
  content: ['1', '2', '3', '4', '5']
 },
 ],
 listHeight: [],
 scrollY: 0, //實時獲取當前y軸的高度
 clickEvent: false
 }
 },
 methods: {
 _initScroll () {
 //better-scroll的實現原理是監聽了touchStart,touchend事件，所以阻止了默認的事件（preventDefault）
 //所以在這里做點擊的話，需要在初始化的時候傳遞屬性click,派發一個點擊事件
 //在pc網頁瀏覽模式下，點擊事件是不會阻止的，所以可能會出現2次事件，所以為了避免2次，可以在綁定事件的時候把$event傳遞過去
 this.lefts = new BScroll(this.$refs.left, {
 click: true
 })
 this.rights = new BScroll(this.$refs.right, {
 probeType: 3 //探針的效果，實時獲取滾動高度
 })
 //rights這個對象監聽事件，實時獲取位置pos.y
 this.rights.on('scroll', (pos) => {
 this.scrollY = Math.abs(Math.round(pos.y))
 })
 },
 _getHeight () {
 let rightItems = this.$refs.right.getElementsByClassName('right-item-hook')
 let height = 0
 this.listHeight.push(height)
 for(let i = 0; i < rightItems.length; i++){
 let item = rightItems[i]
 height += item.clientHeight
 this.listHeight.push(height)
 }
 },
 selectItem(index,event){
 this.clickEvent = true
 //在better-scroll的派發事件的event和普通瀏覽器的點擊事件event有個屬性區別_constructed
 //瀏覽器原生點擊事件沒有_constructed所以當時瀏覽器監聽到該屬性的時候return掉
 if(!event._constructed){
 return
 }else{
 let rightItems = this.$refs.right.getElementsByClassName('right-item-hook')
 let el = rightItems[index]
 this.rights.scrollToElement(el, 300)
 }
 }
 },
 mounted () {
 this.$nextTick(() => {
 this._initScroll()
 this._getHeight()
 })
 },
 computed: {
 currentIndex () {
 for(let i = 0; i < this.listHeight.length; i ++){
 let height = this.listHeight[i]
 let height2 = this.listHeight[i + 1]
 //當height2不存在的時候，或者落在height和height2之間的時候，直接返回當前索引
 //>=height，是因為一開始this.scrollY=0,height=0
 if(!height2 || (this.scrollY >= height && this.scrollY < height2)){
  return i
 }
 if(this.listHeight[this.listHeight.length - 1] - this.$refs.right.clientHeight <= this.scrollY){
  if(this.clickTrue){
  return this.currentNum
  }else{
  return (this.listHeight.length - 1)
  }
 }
 }
 //如果this.listHeight沒有的話，就返回0
 return 0
 }
 }
}
</script>
<style scoped>
.content{
 display: flex;
 position: absolute;
 top:100px;
 bottom:100px;
 width:100%;
 overflow: hidden;
 background: #eee;
}
.left{
 flex: 0 0 80px;
 width:80px;
 background-color: #f3f5f7;
}
 .left li{
 width: 100%;
 height: 100%;
 }
 .current{
 background-color: red;
 }
 .left-item{
 display: block;
 width:100%;
 height:100px;
 line-height: 50px;
 text-align: center;
 border-bottom:1px solid yellow;
 }
 .right{
 flex: 1;
 }
 .right-item li{
 width:100%;
 height:100px;
 line-height:100px;
 text-align: center;
 border-bottom: 1px solid yellow;
 }
 *{
 list-style: none;
 margin: 0;
 padding: 0;
 }
</style>