监听元素宽高变化

以Vue为例：

 mounted () {
   this.$nextTick(() => {
     const element = document.querySelector('div.dashboard-page')
     this.observer = new ResizeObserver((arr) => {
       const rect = arr[0].contentRect
       let width = rect.width // 宽度
       let height = rect.height // 高度
     })

     if (element) this.observer.observe(element)
   })
 },

 beforeDestroyed() {
   this.observer = null
 }