data () {
    return {
       clientHeight: document.documentElement.clientHeight
    }
}
mounted () {
    var n = navigator.userAgent
    if (n.indexOf('iPhone') <= -1) {
      window.onresize = () => {
        if (this.clientHeight > document.documentElement.clientHeight) {
          // 隱藏按鈕
          this.isShow = true
        } else {
          // 顯示按鈕 
          this.isShow = false
        }
      }
    }
  }