在VUE监听窗口中如何解决变化事件的问题

data: { 
 screenWidth: document.body.clientWidth // 这里是给到了一个默认值 （这个很重要）
  }

mounted () { 
    const that = this window.onresize = function() { 
    return function(){ 
      window.screenWidth = document.body.clientWidth;
      that.screenWidth = window.screenWidth
      })()
     } 
    }

watch: { 
   screenWidth (val) { 
      this.screenWidth = val
      } 
    }

watch: { 
   screenWidth (val) { 
     if (!this.timer) { 
       this.screenWidth = val 
       this.timer = true 
        let that = this 
   setTimeout(function () { 
        // that.screenWidth = that.$store.state.canvasWidth 
  console.log(that.screenWidth) 
        that.init() 
        that.timer = false 
       }, 400) 
      } 
     } 
   }