index.vue

<template>
  <div>
    parent {{ data.test }}
    <TextDemo :pass_string="data.test" @change="onChange" />
  </div>
</template>
<script>
// pages/demo/index.vue

import TextDemo from '@/components/TextDemo'

export default {
  components: {
    TextDemo,
  },
  data(){
    return {
      data:{
        test: 'hhhh',
      },
    }
  },
  methods:{
    onChange(e){
      console.log('got', e)
      this.data.test = e
    },
  }
  
}
</script>
<style lang="stylus">
</style>

TextDemo.vue

<template>
  <div >
    child: {{pass_string}}
    <textarea 
      v-model="pass_string" 
      @input="onChange"
      style="background:red;"
      ></textarea>
  </div>
</template>
<script>
// components/TextDemo.vue
export default {
  props:['pass_string'],
  methods:{
    onChange(e){
      console.log(e)
      this.$emit("change", e.mp.detail.value)
    },
  }
}
</script>
<style lang="stylus">
</style>