9-监控属性$watch.html

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
    <div id="app">
        <input v-model="num">
        <p>{{doubleNum}}</p>
    </div>

<script src="js/vue-2.1.3.js"></script>
<script>
    var vm = new Vue({
        el: '#app',
        data: {
            num: 123,
            doubleNum: 246
        },
        watch: {
            num: function(num) {
                this.doubleNum = num * 2;
            }
        }
    })
</script>
</body>
</html>