Vue监听表单变化

需求：当表单中未填写任何项时，提交按钮置灰，只要填写其中一项，则可点击按钮。

<el-button type="primary" :disabled="isButtonGray" @click="handleSave('ruleForm')"> 保存 </el-button>

data() {
    return {
        formItem: {
            commodityCode: null,
            name: '',
            price: ''
        },
        preForm: null
    }
},
watch: {
    resetData: function() {
      this.$refs.ruleForm.resetFields()
    },
    formItem: {
      handler:function(nowVal,oldVal){
        var $this = this;
        for(let i in $this.formItem){
          if(nowVal[i] != $this.preForm[i]) {
              $this.isButtonGray = false;
              break;
          }else {
              $this.isButtonGray = true;
          }
        }
      },
      deep:true
    }
},
mounted() {
   this.preForm = Object.assign({}, this.formItem)
},