SOURCE

new Vue({
  el: '#app',
  components: {
    'inputField': {
      template: '<div><input v-model="newMessage" :placeholder="placehoder" @input="modify"></div>',
      props: ['placehoder'],
      data() {
        return {
          newMessage: ''
        }
      },
      methods: {
        modify() {
          console.log(this.newMessage)
          this.$emit('update:updateMessage', this.newMessage)
        }
      }
    }
  },
  data: {
    message: 'Hello'
  }
})

<script src="https://unpkg.com/vue">
</script>
<div id="app">
  <p>
    初始值： {{message}}
  </p>
  <br/>
  <p>
    子组件：textfield
  </p>
  <input-field placehoder="随便写点啥" :update-message.sync="message">
  </input-field>
</div>