Vue侦听器

       Vue侦听器是根据组件状态做DOM更新或者异步更新其他级联状态的。计算属性的主要目标是根据已有数据计算出组件的状态，它是组件内部的计算，计算结果在组件内部应用。侦听器的主要目标是根据组件状态的变动，做级联的或者异步的操作或DOM更新，操作的影响范围比计算属性大得多。

<script setup lang="ts">
import {reactive, ref, watch} from "vue"

   const obj=reactive({name:"obj",count:0});
   
   // count is formal parameter, it's change will not saved to obj.count
   watch(()=>obj.count,(count)=>{
      count=count+3;
      console.log("watch by obj.count by getter ："+count);
   });

   // obj is real parameter , it's change will be saved to obj
   watch(obj,(oldValue,newValue)=>{
       console.log("watch by obj by deep way ："+newValue.count);
   });

   watch(name,(oldName,newName)=>{
        console.log("oldName is "+oldName+", newName is "+newName);
   });

</script>

<template>
      <div>
            <button type="button" @click="obj.count++">测试侦听器</button>
      </div>
</template>

<style>

</style>

侦听器的标准格式如下：

const data=ref({});

watch(data,(oldValue,newValue)=>{
      console.log(oldValue);
      console.log(newValue);
   },{
      immediate:true,  //default value is false, true value means watch callback function will be called immediately
      deep:true,    //default value is false, true value means watch is a deep watch
      flush:"post"   //default value is "pre", "post" means watch callback function can fetch data after Vue DOM updated completely
   });

侦听器默认情况下是懒执行、浅层侦听、在Vue组件更新之前被调用的。