watch就是监听某个变量是否有变化，变化后就执行对应的操作

<template>
  <div>
    <a href="https://vitejs.dev" target="_blank"></a>
    <H2>欢迎光临红浪漫洗浴中心</H2>
    <div>请选择一位美女</div>
    <button 
    v-for="(item,index) in girls" 
    v-bind:key="index"
    @click="selectGirlFun(index)"
    >
      {{index}}:{{item}}
    </button>
    <div>你选择了【{{selectGirl}}】为你服务</div>
    <button @click="overAction">点餐完毕</button>
    <div>{{overText}}</div>
  </div>
</template>
 
<script lang="ts">
import {
  reactive,
  toRefs,
  ref,
  watch,
  } from "vue";

// 接口定义下面各种类型 (规范，不写页面也可以运行，因为没有申明类型ts会默认类型推断)
interface DataProps{
  girls: string[];
  selectGirl: string;
  selectGirlFun:(index:number)=>void

}

export default({
  name:"APP",

  setup(){
    const data=reactive({
      girls:["大脚","刘颖","小红"],
      selectGirl:"",
      selectGirlFun :(index:number)=>{
          data.selectGirl=data.girls[index];
    }
    });

  //使用toRefs 让变量和 方法可以在模版直接调用
  const refData=toRefs(data)
  const overText=ref("红浪漫")

  const overAction =()=>{
    overText.value="点餐完成 |"+overText.value
  }


  // // 监听单个值的方法
  // watch(overText,(newValue,oldValue)=>{
  //   console.log(`new----->${newValue}`)
  //   console.log(`new----->${oldValue}`)
  //   document.title=newValue;
  // })


   // 监听多个值的方法
   //监听json里面的值要()=>不然前端还有警告提示
  watch([overText,()=>data.selectGirl],(newValue,oldValue)=>{
    console.log(`new----->${newValue}`) //数组[overText,data.selectGirl] 展示
    console.log(`new----->${oldValue}`)
    document.title=newValue[0];
  })



    return{
      ...refData,
      overText,
      overAction,
  }
  },

  });
</script>


<style scoped>
.logo {
  height: 6em;
  padding: 1.5em;
  will-change: filter;
}
.logo:hover {
  filter: drop-shadow(0 0 2em #646cffaa);
}
.logo.vue:hover {
  filter: drop-shadow(0 0 2em #42b883aa);
}
</style>

https://www.bilibili.com/video/BV1L5411j7vj?p=9&vd_source=caabcbd2a759a67e2a3de8acbaaf08ea