element 2 级级联

html 代码：

            <el-col :span="10">
              <el-row type="flex" align="middle" >
                <el-col :span="13" class="domain">
                  <el-form-item prop="subject1" label="学科领域">
                    <el-select :value="form.subject1" clearable placeholder="请选择学科领域" class="selsect" @input="changeSubject">
                      <el-option v-for="(item, index) in subjectList" :key="index" :label="item.name" :value="item.value"></el-option>
                    </el-select>
                  </el-form-item>
                </el-col>
                <el-col :span="13" class="subject1">
                  <el-form-item prop="subject2">
                    <el-select v-model="form.subject2" clearable placeholder="请选择学科领域" class="selsect1">
                      <el-option v-for="(item, index) in subjectChildList" :key="index" :label="item.name" :value="item.value"></el-option>
                    </el-select>
                  </el-form-item>
                </el-col>
              </el-row>

数据初始化 ：

        subjectChildList: [], // 子集
        subjectList: [], // 父集

获取列表数据 ：

      async getSubjectList () {
        const { data } = await getSubjectList()
        if (data.code === 0) {
          this.allSubjectList = data.data.subjects
          const datas = data.data.subjects
          this.subjectList = toTree(datas, "value", "parentId", "children")
        }
      },

当父级变换时：

    // 修改父级subject
      changeSubject (event) {
        if (this.form.subject2 !== event) {
          this.form.subject1 = event
          this.$set(this.form, 'subject2', null)
          const findItem = this.subjectList.find(item => item.value === this.form.subject1)
          if (findItem && findItem.children && findItem.children.length) {
            this.subjectChildList = findItem.children
          } else {
            this.subjectChildList = []
          }
        }
      },

效果：