el-tree懒加载 setCheckedKeys子节点 再getCheckedKeys为空数组?

Question

因为子节点一开始没有加载出来所以getCheckedKeys()为空,点击加载后能获取到值，内部应该有存了setCheckedKeys的值，想问下能否访问那个存了值的临时变量

<template>
  <div>
    <el-tree ref="tree" :props="props" :load="loadNode" lazy node-key="id" show-checkbox ></el-tree>
    <button @click="getCheckedKeys()">getCheckedKeys</button>
  </div>
</template>

<script>

export default {
  data() {
    return {
      props: {
        label: "name",
        children: "zones",
        isLeaf: "leaf"
      },
      nameList: ["zone", "buf", "bar"],
    };
  },
  methods: {
    loadNode(node, resolve) {
      if (node.level === 0) {
        return resolve([
          { name: "中国", id: 1 },
          { name: "美国", id: 2 }
        ]);
      }
      if (node.level > 1) return resolve([]);

      setTimeout(() => {
        const data = this.nameList.map((it, idx) => {
          return {
            name: it,
            leaf: true,
            id: Number(`${node.data.id}${idx + 1}`)
          };
        });

        resolve(data);
      }, 500);
    },
    getCheckedKeys(){
      console.log(this.$refs.tree.getCheckedKeys())
    }
  },
  mounted(){
    this.$nextTick( ()=> {
      console.log('我setCheckedKeys了')
      this.$refs.tree.setCheckedKeys([13])
  })
  }
};
</script>