vue.js怎么在v-for中使v-bind:class跟着变量值改变，我现在只在页面刚渲染时才执行一次，但不会随着变量值变换动态渲染class

  <li v-for="item in items" v-on:click="clickOneItem(item.item_id)" v-bind:class="{active: isItemSelected(item.item_id)}">     <input type="checkbox">     <img v-bind:src="item.img_url">      <div class="fyh-textcon">         <p>{{ item.title }}</p>          <p class="">价格：<span>¥ {{ item.price }}</span></p>     </div>   </li>                                              <script>                  ...      methods: {             isItemSelected: function (item_id) {                 var self = this;                 if (undefined == self.fetchedItems[item_id]) {                     return false;                 }                 return self.fetchedItems[item_id].isSeclected;              } }                  ...           </script>