Vue 3 父组件 setup 中执行子组件方法

Vue 3 父组件调用子组件方法，可以在生命周期函数中直接调用：
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta
      name="viewport"
      content="width=device-width, initial-scale=1.0, maximum-scale=1, minimum-scale=1, user-scalable=no"
    />
    <title>父组件调用子组件方法</title>
    <script src="https://cdn.bootcdn.net/ajax/libs/vue/3.2.37/vue.global.js"></script>
  </head>
  <div id="root"></div>

  <body></body>
  <script>
    const app = Vue.createApp({
      template: `
    <div>
    父页面
      <child ref="sonRef"/>
      <button @click="handleClick">test</button>
    </div>
  `,
      setup() {
        const { ref, onMounted } = Vue;
        const sonRef = ref();

        const handleClick = () => {
          sonRef.value.song();
        };

        onMounted(() => {
          console.log("这里执行子组件方法", sonRef.value.song());
        });

        return { sonRef, handleClick };
      },
    });

    app.component("child", {
      template: `
      <div>
        子页面
      </div>
    `,
      setup() {
        const song = () => alert("hello world");

        return {
          song,
        };
      },
    });
    app.mount("#root");
  </script>
</html>