code: function preload(pageDom) {
    const self = this;
    return new Promise((resolve, reject) => {
      let ajaxPreloadPromiseTasks = [];
      /**
       * Ajax预处理
       */
      ajaxPreloadPromiseTasks.push(AjaxPreloadPromise.call(self, pageDom));
      Promise.all(ajaxPreloadPromiseTasks).then(() => {
        resolve();
      }).catch((error) => {
        reject(error);
      });
    });
  }
docs: 预加载pageDom中所有要预加载的页面
  @access private
  @param {HtmlElement} pageDom
  @return {Promise}