vue组件中watch props根据v-if动态判断并挂载DOM的问题

<Chart :source="chartData"></Chart>

import Chart from '../components/Chart'

export default {
 name: 'Home',
 components: { Chart },
 data () {
  return {
   chartData: []
  }
 },
 mounted () {
  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']
   ]
  }, 2000)
 }
}

<div v-if="source && source.length" id="main" ref="main" style="width: 600px;height: 400px;"></div>
<div vi-else>none</div>

export default {
 // ...
 watch: {
  source (newVal, oldVal) {
   this.setOpts()
  }
 },
 props: ['source'],
 methods: {
  setOpts () {
   let myChart = this.$echarts.init(this.$refs.main)
   myChart.setOption({
    dataset: {
     // ...
     source: this.source
    },
    // ...
   })
  }
 }
}

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()
 }
}