6.watch

<template>
  <div style="margin: 30px">
    {{ count }} <button @click="changeCount">clcik</button>
  </div>
</template>
<script>
import { ref, watch } from "vue";
export default { 
  setup(props) {
    const count =ref(0);
    const changeCount = ()=>{
      count.value = count.value+10;
    }
    watch(count,(newVale,oldValue)=> {
      console.log(newVale,oldValue)
    })
    return {
      count,
      changeCount
    };
  },
};
</script>