<template>
<div class="container">

  <p>{{count}}</p>

  <p>{{double}}</p>
  
  <button class="btn btn-primary" @click="add" type="submit">+</button>

</div>
  
</template>

<script lang="ts">
import { defineComponent, computed ,ref} from 'vue';
import 'bootstrap/dist/css/bootstrap.min.css'
import axios from 'axios'

// 就会提供些额外的方法
export default defineComponent({
  name: 'App',
  // setup 方 法还不能为空，真的很奇葩的事情
  setup(){
    const count = ref(100);
    const double = computed(()=>{
      return count.value* 2;
    })
    const add = ()=>{
      count.value ++;
    }
    return {
      count,
      double,
      add
    }
  }
});
</script>

<style>
#app {
  font-family: Avenir, Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 60px;
}
</style>

效果：

没有任何问题， 点击加号，都会发生变化

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watch 监听咋写？？