<div id="watch">
    <input type="text" v-model="msg">
    {{msg}}
    <button @click="update"></button>
</div>
<
<script>
    var all={el:‘#watch‘,
        data:{msg:‘fddgssdg‘},
        methods:{
        update:function () {
            this.msg=‘我草your mother‘;
        }
    }};
var v=new Vue(all);
var callback=function (val,oldVal) {
    alert(‘new val===‘+val);
    alert(‘old val===‘+oldVal);
}
v.$watch(‘msg‘,callback);
</script>