SOURCE

const vue = new Vue({
    el: '#app',
    data() {
        return {
            listData: ['list1', 'list2', 'list3']
        }
    },
    methods: {
        addItem() {
            this.listData.push(Date.now());
            this.listData.push(Date.now());
            this.listData.push(Date.now());

            // $nextTick -> 获取最新DOM
            this.$nextTick(() => {
                const list = this.$refs.list;
                alert('最新DOM: ' + list.childNodes.length) 
            })

            const list = this.$refs.list;
            alert(list.childNodes.length) 
        }
    }
})

<div id="app">
    <ul ref="list">
        <li v-for="(item, index) in listData" :key="index">
            {{item}}
        </li>
    </ul>
    <button @click="addItem">btn</button>
</div>

body {
    font-size: 24px
}