watch 选项主要用于数据监控，有两种书写方式：
一种是写在构造器内部，另一种是在构造器外部使用
下面以监控天气为例，代码如下：
- 每次加减5°C，在watch内部做 if 判断
<!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">
<title>watch option</title>
<script src="../assets/js/vue.js"></script>
</head>
<body>
<h1>watch option</h1>
<hr>
<div id="app">
<p>今日温度：{{temperature}}°C</p>
<p>建议：{{proposal}}</p>
<p><button @click="add">add</button>
<button @click="reduce">reduce</button></p>
</div>
<script>
var proposalArr = ['T恤短袖', '夹克长裙', '棉衣羽绒服']
var app = new Vue({
el: '#app',
data: {
temperature: 14,
proposal: '夹克长裙'
},
methods: {
add() {
this.temperature += 5
},
reduce() {
this.temperature -= 5
}
},
// watch: {
// temperature(newVal, oldVal) {
// if (newVal > 26) {
// this.proposal = proposalArr[0]
// } else if (newVal < 26 && newVal > 0) {
// this.proposal = proposalArr[1]
// } else {
// this.proposal = proposalArr[2]
// }
// }
// }
})
app.$watch('temperature', function (newVal, oldVal) {
if (newVal > 26) {
this.proposal = proposalArr[0]
} else if (newVal < 26 && newVal > 0) {
this.proposal = proposalArr[1]
} else {
this.proposal = proposalArr[2]
}
})
</script>
</body>
</html>
