uni-app 擷取元素尺寸

比如有個view元素如下

<view style="height:100px;width:100%;background:black;" res="topbox"></view>      

可以這樣擷取

getRefs(){
        let that = this
        return new Promise((resolve,reject)=>{
          resolve(that.$refs)
        })
      }

that.getRefs().then(res=>{
          that.top_height = res.topbox.$el.offsetHeight
        })      

this.$refs.topbox.offsetHeight