Watch侦听器

作用：监听到数据发生的变化

<template>
  <div>
    {{ msg }}
    <button @click="changeMsg">点我改变数据</button>
  </div>
</template>

<script>
  export default {
    data(){
      return{
        msg: "999"
      }
    },
    methods: {
      changeMsg(){
        this.msg = "我被改了"
      }
    },
    // 侦听器
    watch: {
      msg(newVal,oldVal){
        console.log("新的值",newVal);
        console.log("旧的值",oldVal);
      }
    }
  }
</script>

<style lang="less" scoped>

</style>