vue ref如何获取子组件属性值

<script>
  ....
  import CustomerModal from './modules/CustomerModal'
  export default {
    name: "CustomerList",
    mixins:[JeecgListMixin],
    components: {
      JDate,
      CustomerModal,
      JDictSelectTag
    },
    ...
  }
</script>

<customer-modal ref="modalForm" @ok="modalFormOk"></customer-modal>
// ref属性值指定了从$refs中获取组件的名称

this.$refs.modalForm.add();

this.$refs.modalForm.title = "新增";

<component-a :num.sync="number">这是子组件</component-a>

<template>
  <div>
    <p @click="change">子属性{{num}}</p>
  </div>
</template>
<script>
    export default {
        name: "ComponentA",
        props: {
          num: Number
        },
        methods: {
          change(){
            this.$emit('update:num', 10)
          }
        }
    }
</script>