监听属性属性关键词: watch
通过 watch 来响应数据的变化
例子
代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>vue</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<script src="Vue.js"></script>
</head>
<body>
<div id = "app">
输入：<input type=" text" v-model="theValue">
<p ref="change">
变化前：<br/>变化后:
</p>
</div>
<script>
let app = new Vue({
el: '#app',
data: {
theValue:'',
},
watch:{
theValue: function(newVal, oldVal){
this.$refs.change.innerHTML = `变化前：${oldVal}<br/>变化后: ${newVal}`
}
}
});
</script>
</body>
</html>