Vue watch中监听值的变化,判断后修改值方式

<mu-text-field v-model.number="weightNum" solo full-width ></mu-text-field>

watch: {
            //监听用户输入的重量值
            //如果超过最大重量，弹出提示，并且将输入框重量置于最大值
            //如果低于最小重量，弹出提示，并且将输入框重量置于最小值
            weightNum(val){
                if(val<this.weightLimit.minWeight){
                    this.$toast.showWarn({message: 'You can only enter between ' +this.weightLimit.minWeight+ ' and ' +this.weightLimit.maxWeight});
                    this.$data.weightNum = this.weightLimit.minWeight;
                }
                else if(val>this.weightLimit.maxWeight){
                    this.$toast.showWarn({message: 'You can only enter between ' +this.weightLimit.minWeight+ ' and ' +this.weightLimit.maxWeight});
                    this.$data.weightNum = this.weightLimit.maxWeight;
                }
            }        
        },

this.$data.weightNum = this.weightLimit.minWeight;

this.weightNum = this.weightLimit.minWeight;

 <script type="text/javascript">
            var vm = new Vue({
                el: app01,
                data:{
                    a:'test',
                }
            })
            // 检测数据'a'的改变，放数据改变时执行
            vm.$watch('a', function(newval, oldval){
                console.log(newval, oldval);
            })
            vm.$data.a = 'wdc'
        </script>