Vue+Element实现多选框选项上限提示与限定

//vue文件夹内
<el-form :model="form" class="form-inline">
              
              <el-form-item label="基础指标:">
                <el-checkbox-group v-model="form.saleIndex">
                  <el-checkbox label="累计会员数" name="saleIndex" checked>el-checkbox>
                  <el-checkbox label="累计会员占比" name="saleIndex" checked>el-checkbox>
                el-checkbox-group>
              el-form-item>
              <el-form-item label="成交指标:">
                
                <el-checkbox-group v-model="form.serviceIndex">
                  <el-checkbox label="成交会员数" name="saleIndex" checked>el-checkbox>
                  <el-checkbox label="成交会员占比" name="saleIndex" checked>el-checkbox>
                  <el-checkbox label="支付订单数" name="saleIndex" checked>el-checkbox>
                  <el-checkbox label="客单价" name="saleIndex">el-checkbox>
                  <el-checkbox label="支付金额" name="saleIndex">el-checkbox>
                  <el-checkbox label="支付金额占比" name="saleIndex">el-checkbox>
                  <el-checkbox label="人均消费频次" name="saleIndex">el-checkbox>
                el-checkbox-group>
              el-form-item>
              <span id="maxSelectSpan">最多选择8项span>
              <span class="form-inline-select">已选择 {{selectNum}}/8 个指标span>
            el-form>