vue中watch记录变量变化的值

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    <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>
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<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>


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