data() {
  return {
   activeClass: -1, // 0为默认选择第一个，-1为不选择
  };
 },

<li :class="activeClass == index " v-for="(item,index) in itemList" :key="index" @click="getItem(index)">
     {{itme.text}}
    </li>

  getItme(index) {
   this.activeClass = index; // 把当前点击元素的index，赋值给activeClass
  },

.active {
 /* background: #eee; */
 color: #1e82d2;
 font-weight: bolder;
}

<div class="tab_basic">
  <span :class="{to_active:shows==1}" @click="today_a">日</span>
  <span :class="{to_active:shows==2}" @click="today_b">月</span>
  <span :class="{to_active:shows==3}" @click="today_c">年</span>
</div>
<style>
.to_active {background: #409eff!important;color: #fff;}
</style>
然后在method里定义
today_a(){
this.shows = 1;
},
today_b(){
this.shows = 2;
},
today_c(){
this.shows = 3;
},