需求：实付金额不能手动输入，并且等于购买数量✖优惠价➖平台补贴➖店铺补贴

把需要处理的这几个框绑上change事件等于同一个方法名

@change="handleChange"

        handleChange(value){
          let _this = this;
          let isNull = validatenull;
          _this.ruleForm.preferentialPrice = Number(Math.round(_this.ruleForm.goodsShelvesPrice * _this.ruleForm.buyNumber - _this.ruleForm.platformSubsidyPrice - _this.ruleForm.shopSubsidyPrice))
        },