vue-element-admin 超过两级嵌套路由无法缓存的解决办法

解决方法如下：

1. 增加路由扁平化处理方法（src/store/modules/permission.js）：
   

   /**
   * 生成扁平化机构路由(仅两级结构)
   * @param {允许访问的路由Tree} accessRoutes
   * 路由基本机构:
   * {
   * name: String,
   * path: String,
   * component: Component,
   * redirect: String,
   * children: [
   * ]
   * }
   */
   function generateFlatRoutes(accessRoutes) {
   let flatRoutes = [];

   for (let item of accessRoutes) {
   let childrenFflatRoutes = [];
   if (item.children && item.children.length > 0) {
   childrenFflatRoutes = castToFlatRoute(item.children, "");
   }

   // 一级路由是布局路由,需要处理的只是其子路由数据
   flatRoutes.push({
   name: item.name,
   path: item.path,
   component: item.component,
   redirect: item.redirect,
   children: childrenFflatRoutes
   });
   }

   return flatRoutes;
   }

   /**
   * 将子路由转换为扁平化路由数组（仅一级）
   * @param {待转换的子路由数组} routes
   * @param {父级路由路径} parentPath
   */
   function castToFlatRoute(routes, parentPath, flatRoutes = []) {
   for (let item of routes) {
   if (item.children && item.children.length > 0) {
   if (item.redirect && item.redirect !== 'noRedirect') {
   flatRoutes.push({
   name: item.name,
   path: (parentPath + "/" + item.path).substring(1),
   redirect: item.redirect,
   meta: item.meta
   });
   }
   castToFlatRoute(item.children, parentPath + "/" + item.path, flatRoutes);
   } else {
   flatRoutes.push({
   name: item.name,
   path: (parentPath + "/" + item.path).substring(1),
   component: item.component,
   meta: item.meta
   });
   }
   }

   return flatRoutes;
   }

   // 在原有的生成可访问路由方法中，增加路由扁平化处理，并返回扁平化处理后的结果，具体修改参看分割线部分：
   const actions = {
   generateRoutes({
   commit
   }, permissionKeys) {
   return new Promise(resolve => {
   let accessedRoutes = filterAsyncRoutes(asyncRoutes, permissionKeys, false)
   // -------------------------- 分隔线 ----------------------------
   let flatRoutes = generateFlatRoutes(accessedRoutes)

   commit('SET_ROUTES', accessedRoutes)
   resolve(flatRoutes)
   // -------------------------- /分隔线 ----------------------------
   })
   }
   }

   2. 为了方便后续维护，还是修改下vue-router钩子函数的变量名称吧（也可以不改）：
      

      // 拉取用户权限数据
      const { permissions } = await store.dispatch('user/getInfo')

      // 这里把原来的 accessRoutes 修改成了 flatRoutes
      const flatRoutes = await store.dispatch('permission/generateRoutes', permissions)

      // dynamically add accessible routes
      router.addRoutes(flatRoutes)