code: function isIos() {
    const ua = navigator.userAgent;
    const ipad = ua.match(/(iPad).*OS\s([\d_]+)/);
    const ipod = ua.match(/(iPod)(.*OS\s([\d_]+))?/);
    const iphone = !ipad && ua.match(/(iPhone\sOS)\s([\d_]+)/);
    const os = {};
    if (iphone && !ipod) {
      os.ios = true;
      os.iphone = true;
      os.version = iphone[2].replace(/_/g, '.');
    }
    if (ipad) {
      os.ios = true;
      os.ipad = true;
      os.version = ipad[2].replace(/_/g, '.');
    }
    if (ipod) {
      os.ios = true;
      os.ipod = true;
      os.version = ipod[3] ? ipod[3].replace(/_/g, '.') : null;
    }
    return os.ios;
  }
docs: 对系统的判断来自：https://github.com/madrobby/zepto/blob/master/src/detect.js