uniapp的form表单自定义验证规则

<uni-forms ref="form" :value="user" labelWidth="80px">
      <uni-forms-item name="oldPassword" label="旧密码">
        <uni-easyinput type="password" v-model="user.oldPassword" placeholder="请输入旧密码" />
      </uni-forms-item>
      <uni-forms-item name="newPassword" label="新密码">
        <uni-easyinput type="password" v-model="user.newPassword" placeholder="请输入新密码" />
      </uni-forms-item>
      <uni-forms-item name="confirmPassword" label="确认密码">
        <uni-easyinput type="password" v-model="user.confirmPassword" placeholder="请确认新密码" />
      </uni-forms-item>
      <button type="primary" @click="submit">提交</button>
    </uni-forms>

user: {
          oldPassword: undefined,
          newPassword: undefined,
          confirmPassword: undefined
        },
        rules: {
          oldPassword: {
            rules: [{
              required: true,
              errorMessage: '旧密码不能为空'
            }]
          },
          newPassword: {
            rules: [{
                required: true,
                errorMessage: '新密码不能为空',
              },
              {
                minLength: 6,
                maxLength: 20,
                errorMessage: '长度在 6 到 20 个字符'
              }
            ]
          },
          confirmPassword: {
            rules: [{
              required: true,
              errorMessage: '确认密码不能为空'
            }, {
              validateFunction: (rule, value, data) => data.newPassword === value,
              errorMessage: '两次输入的密码不一致'
            }]
          }
        }

submit() {
        this.$refs.form.validate().then(res => {
        updateUserPwd(this.user.oldPassword,this.user.newPassword).then(response =>           {
            this.$modal.msgSuccess("修改成功")
          })
        })
      }