Vue2 监听属性改变watch的实例代码

<div id="app2">
 <label>幼儿园入学年龄（3～6）：</label><input type="number" v-model="child.age"> <button @click="older"> + </button> <button @click="younger"> - </button>
 <p v-show="hasErr">{{ errMsg }}</p>
</div>
<script>
 var app = new Vue({
 el:"#app2",
 data:{
 child:{age:2},
 hasErr:false,
 errMsg:""
 },
 methods:{
 older:function () {
 this.child.age ++;
 },
 younger:function () {
 this.child.age --;
 },
 hideErr:function () {
 var self = this;
 setTimeout(function () {
 self.hasErr = false;
 },3000);
 }
 },
 //构造器内的watch
 watch:{
 'child.age':function (newVal,oldVal) {
 if(newVal > 6){
 this.child.age = 6;
 this.errMsg = "不得大于6岁";
 this.hasErr = true;
 this.hideErr();
 }
 }
 }
 });
 
 app.$watch("child.age", function (newVal,oldVal) {
 if(newVal < 3){
 app.child.age = 3;
 app.errMsg = "不得小于3岁";
 app.hasErr = true;
 app.hideErr();
 }
 }, {deep:true, immediate:true}); //deep默认true immediate指示是否立即以表达式的当前值触发回调
 
</script>