直接上代码

<template>
  <div id="app">
    <div :style="{width: `${screenWidth}px`}">
      <router-view/>
    </div>
  </div>
</template>

<script>
export default {
  name: 'App',
  data () {
    return {
      screenWidth: document.body.clientWidth
    }
  },
  mounted () {
    const that = this
    window.onresize = () => {
      return (() => {
        window.screenWidth = document.body.clientWidth
        that.screenWidth = window.screenWidth
      })()
    }
  },
  watch: {
    screenWidth (val) {
      this.screenWidth = val
    }
  }
}
</script>