希望获取每一个子元素的高度
html

    <div class="foods-wrapper" ref="foodsWrapper">
      <ul>
         <li v-for="item in goods" class="food-list food-list-hook">
           <h1 class="title">{{item.name}}</h1>
         </li>
      </ul>    
    </div>

vue逻辑

    mounted() {
      this.$nextTick(() => {
        this._calculateHeight()
      })
    },
    methods: {
      _calculateHeight() {
        let foodList = this.$refs.foodsWrapper.getElementsByClassName('food-list-hook')
        console.log(foodList)
        console.log(foodList.length)// 0
      }

_calculatrHeight中得到的foodList打印出来如下，length长度为9，但是并不能使用，打印foodList.Length长度为0，求指点