VUE二级联动，改变一级下拉框，清空二级下拉框

在清空二级下拉框时，直接使用this.formData.xxx = '' 会出现清空失败的问题。

使用this.$set(this.formData, 'xxx', '')解决

        <el-form-item label="所属地市" prop="blongId1">
          <el-select v-model="bmixForm.blongId1" placeholder="请选择所属地市" style="width: 300px" @change="getAreaList2">
            <el-option v-for="(item, index) in blongId1List" :key="index" :label="item.NAME"
                       :value="item.ID"></el-option>
          </el-select>
        </el-form-item>
        <el-form-item label="所属分区" prop="blongId2">
          <el-select v-model="bmixForm.blongId2" placeholder="请选择所属分区" style="width: 300px">
            <el-option v-for="(item, index) in bmixFormBlongId2List" :key="index" :label="item.NAME"
                       :value="item.ID"></el-option>
          </el-select>
        </el-form-item>

    getAreaList2(){
      this.$set(this.bmixForm, 'blongId2', '')
      this.getAreaListByCity2(this.bmixForm.blongId1);
    }