如何理解Vue的.sync修饰符的使用

<div id="app">  <div>{{bar}}</div>  <my-comp :foo.sync="bar"></my-comp>  <!-- <my-comp :foo="bar" @update:foo="val => bar = val"></my-comp> --></div><script>  Vue.component('my-comp', {    template: '<div @click="increment">点我+1</div>',    data: function() {      return {copyFoo: this.foo}    },    props: ['foo'],    methods: {      increment: function() {        this.$emit('update:foo', ++this.copyFoo);      }    }  });  new Vue({    el: '#app',    data: {bar: 0}  });</script>