vue组件中watch props根据v-if动态判断并挂载DOM的问题

watch: {
 source (newVal, oldVal) {
 console.log(newVal, this.$refs.main) // [Array ...] undefined
 this.setOpts()
 }
 },

mounted () {
 console.log(this.source, this.$refs.main) // [] undefined
 this.setOpts()
 },

<template>
 <div>
 <div id="main" ref="main" style="width: 600px;height: 400px;"></div>
 </div>
</template>

export default {
 name: 'Chart',
 props: ['source'],
 mounted () {
 this.setOpts()
 },
 watch: {
 source () {
 this.setOpts()
 }
 },
 methods: {
 setOpts () {
 let myChart = this.$echarts.init(this.$refs.main)
 myChart.setOption({
 dataset: {
 dimensions: ['score', 'amount', 'product'],
 source: this.source
 },
 xAxis: { type: 'category' },
 yAxis: {},
 series: [
 {
 type: 'bar',
 encode: {
 x: 'product',
 y: 'amount'
 }
 }
 ]
 })
 }
 }
}

<template>
 <div>
 <Chart :source="chartData" v-if="flag"></Chart>
 <div v-else>none</div>
 </div>
</template>
import Chart from '../components/Chart'

export default {
 name: 'Home',
 components: { Chart },
 data () {
 return {
 chartData: [],
 flag: false
 }
 },
 methods: {
 getData () {
 setTimeout(() => {
 this.chartData = [
 [89.3, 58212, 'Matcha Latte'],
 [57.1, 78254, 'Milk Tea'],
 [74.4, 41032, 'Cheese Cocoa'],
 [50.1, 12755, 'Cheese Brownie'],
 [89.7, 20145, 'Matcha Cocoa'],
 [68.1, 79146, 'Tea'],
 [19.6, 91852, 'Orange Juice'],
 [10.6, 101852, 'Lemon Juice'],
 [32.7, 20112, 'Walnut Brownie']
 ]
 this.flag = true
 }, 2000)
 }
 },
 mounted () {
 this.getData()
 }
}