vue 父子组件间如何通过事件进行通信$emit和update:modelValue<meta name="viewpo

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title></title>
  <script src="https://unpkg.com/vue@next"></script>
</head>
<body>
  <div id="root"></div>
</body>
<script>
  const app = Vue.createApp({
    data() {
      return { count: 1 }
    },
    template: `
      <counter v-model="count" />
    `
  });

  app.component('counter', {
    props: ['modelValue'],
    methods: {
      handleClick() {
        this.$emit('update:modelValue', this.modelValue + 3);
      }
    },
    template: `
      <div @click="handleClick">{{modelValue}}</div>
    `
  });

  const vm = app.mount('#root');
</script>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>lesson</title>
  <script src="https://unpkg.com/vue@next"></script>
</head>
<body>
  <div id="root"></div>
</body>
<script>
  const app = Vue.createApp({
    data() {
      return {
        count: 'a',
      }
    },
    template: `
      <counter v-model.uppercase="count" />
    `
  });

  app.component('counter', {
    props: {
      'modelValue': String,
      'modelModifiers': {
        default: ()=> ({})
      }
    },
    methods: {
      handleClick() {
        let newValue = this.modelValue + 'b';
        if(this.modelModifiers.uppercase) {
          newValue = newValue.toUpperCase();
        }
        this.$emit('update:modelValue', newValue);
      },
    },
    template: `
      <div @click="handleClick">{{modelValue}}</div>
    `
  });

  const vm = app.mount('#root');
</script>
</html>