<div id = "app">
    <p style = "font-size:25px;">计数器: {{ counter }}</p>
    <button @click = "counter++" style = "font-size:25px;">点我</button>
</div>
<script type = "text/javascript">
var vm = new Vue({
    el: '#app',
    data: {
        counter: 1
    }
});
vm.$watch('counter', function(nval, oval) {              /*监听counter的变化,和函数(nval,oval),使用which去响应数据的变化*/
    alert('计数器值的变化 :' + oval + ' 变为 ' + nval + '!');
});
</script>