<script>

export default {
  data() {
    return {
      screenWidth: document.body.clientWidth
    };
  },
  watch: {
    screenWidth(newVal) {
      console.log('newVal', newVal);
      // console.log('old', old);
      // this.boxWscreenWidthidth = document.body.clientWidth;
      // console.log('new', this.boxWidth);
    }
  },
  mounted() {
    const that = this;
    window.onresize = () => {
      window.screenWidth = document.body.clientWidth;
      that.screenWidth = window.screenWidth;
    };
    // console.log('that.screenWidth',that.screenWidth)
  }
};
</script>

先定义data中一个screenwidth属性,然后这个时候我们可以用watch监听到screenWidth,但是当改变窗口大小时,并不会打印改变之后的值,即只监听了一次,不能持续监听,于是在mounted中调用window.onresize方法,然后就可以持续监听了