vue中watch记录变量变化的值
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<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
</head>
<body>
<div id="app">{{a}}</div>
<script>
var data = {a:1}
var vm = new Vue({
el: "#app",
data:data
})
//$watch是一个实例方法，可以观察变量的变化，放在变量变化之前，会记录变化后最新的值和之前的值
vm.$watch('a',function(newValue,oldValue){
//这个回调将在'vm.a'改变后调用
console.log(newValue,oldValue) //2 1
})
vm.$data.a='2'
</script>
</body>
</html>