vue项目中如何实现在父组件点击按钮触发子组件

<template>
 <div id="app">
  <!--父组件-->
  <input v-model="msg" />
  <button v-on:click="notify">广播事件</button>
  <!--子组件-->
  <popup ref="child"></popup>
 </div>
</template>
 <script>
import popup from "@/components/popup";
export default {
 name: "app",
 data: function () {
  return {
   msg: "",
  };
 },
 components: {
  popup,
 },
 methods: {
  notify: function () {
   if (this.msg.trim()) {
    this.$refs.child.parentMsg(this.msg);
   }
  },
 },
};
</script>
 <style>
#app {
 font-family: "Avenir", Helvetica, Arial, sans-serif;
 -webkit-font-smoothing: antialiased;
 -moz-osx-font-smoothing: grayscale;
 text-align: center;
 color: #2c3e50;
 margin-top: 60px;
}
</style>

<template>
 <div>
  <ul>
   <li v-for="item in messages">父组件输入了：{{ item }}</li>
  </ul>
 </div>
</template>
 <style>
body {
 background-color: #ffffff;
}
</style>
 <script>
export default {
 name: "popup",
 data: function () {
  return {
   messages: [],
  };
 },
 methods: {
  parentMsg: function (msg) {
   this.messages.push(msg);
  },
 },
};
</script>