watch基本格式
watch(监听的数据，作用函数，监听配置)

watch(data,
      (newData, oldData) => {},
      { immediate: true, deep: true })
     
deep:true 开启深度监听
当data为reactive数据时必开启，不然不触发监听
------
immediate:true 页面渲染后立刻触发监听
最好所有watch都配置，不然会出现watch监听不触发问题

 	const data = ref('开始')
    const stu = reactive({
      name: '张三',
      age: '12'
    })
    watch(data, (newName, oldName) => {
      console.log("newName", newName);
    }{ immediate:true });
    // reactive对象可以直接监听，但是oldName值会有问题
    watch(stu, (newName, oldName) => {
      console.log("newName", newName);
    },{ deep: true,immediate:true });
    // 监听reactive对象某个属性变化
    watch(() => stu.name,(newVal,oldVal)=>{
      console.log(newVal,'新')
      console.log(oldVal,'旧')
    },{ deep: true,immediate:true })
    // 注意子组件的props也是reactive对象
    // props监听
    props: {
      index: {
        type: Number,
      default: 0
      }
    },
    watch(() => props.index,(newVal,oldVal)=>{
      console.log(newVal,'新')
      console.log(oldVal,'旧')
    },{ deep: true,immediate:true })