Vue 的觀察屬性（$watch）
與computer屬性類似，用於觀察量的變化，然後進行相應的處理。（從Angular而來）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<script src="https://unpkg.com/vue/dist/vue.js"></script>
<title>Document</title>
</head>
<body>
<div id = "myApp">
<p>任天堂釋出的新主機價格是：{{price}}元，含稅價格為{{priceInTax}}元，摺合人名幣為：{{priceChinaRMB}}元</p>
<button @click = "btnClick(10000)" >加價10000</button>
</div>
<script>
var myApp = new Vue({
el:"#myApp",
data:{
price:0,
priceInTax:0,
priceChinaRMB:0,
},
watch:{
price:function(newVal,oldVal){
console.log(newVal,oldVal);
this.priceInTax = Math.round(this.price * 1.08);
this.priceChinaRMB = Math.round(this.priceInTax / 16.75);
},
},
methods:{
btnClick:function(newPrice){
this.price += newPrice;
},
},
});
myApp.price = 29980;
</script>
</body>
</html>