<view class='abox'>  <view wx:for="{{title}}" class='{{currentIndex==index?"tabTrue":""}}' bindtap='changeTab' data-aa='{{index}}'>   {{item}}    </view>  <view class='b' style="left:{{left}}px"></view></view>

.abox{ display: flex; flex-direction: row; width: 100%; justify-content: space-around; position: relative; padding-bottom: 20rpx;}.tabTrue{ color: red;}.b{ background: red; height: 4rpx; width:64rpx; position: absolute; bottom: 0; transition: all .3s linear;}

Page({ data: {  title: ["首頁","掘金","思否","知乎"],  currentIndex:"0",  left:"" }, changeTab:function(e){  //console.log(e)  this.setData({   currentIndex: e.currentTarget.dataset.aa  })  this.changeline(e)   }, changeline:function(){  const query = wx.createSelectorQuery()  var _this = this  query.select('.tabTrue').boundingClientRect()  query.exec(function (res) {   _this.setData({    left: res[0].left   })   //console.log(res[0].left)  }) }, onLoad: function () {    this.changeline(1)   }})