父组件

<template>
  <div>
    <!-- 顶部导航 -->
    <topNav
      title="XXX"
      @click="goBack"
      ref="child"
    ></topNav>
  </div>
 </template>
 <script>
import topNav from '../../components/topNavbar'
export default {
  components: { topNav },
  data() {
    return {
    }
  }
  methods: {
    goBack() {
      this.$refs.child.onClickLeft()
    }    
  }
}
</script>

子组件

<template>
  <div>
    <van-nav-bar
      :title="title"
      left-text="返回"
      left-arrow
      @click-left="onClickLeft"
    >
    </van-nav-bar>
  </div>
</template>
<script>
export default {
  name: 'topNav',
  props: {
    title: String
  },
  data() {
    return {

    }
  },
  methods: {
    onClickLeft() {
      console.log('我是子组件的方法')
      let that = this
      that.$router.go(-1)
    }
  }
}
</script>
<style lang="less" scoped>
</style>