这里每次切换路由都会对路由表做一次深度遍历，路由多的画是否会影响性能呢？为什么不只在登录之后设定一次路由权限呢？

router.beforeEach(async(to, from, next) => {
  // start progress bar，顶部进度条
  NProgress.start()

  // set page title
  document.title = getPageTitle(to.meta.title)

  // determine whether the user has logged in
  const hasToken = getToken()

  if (hasToken) {
    if (to.path === '/login') {
      // if is logged in, redirect to the home page
      next({ path: '/' })
      NProgress.done() // hack: https://github.com/PanJiaChen/vue-element-admin/pull/2939
    } else {
      // determine whether the user has obtained his permission roles through getInfo
      const hasRoles = store.getters.roles && store.getters.roles.length > 0
      if (hasRoles) {
        next()
      } else {
        try {
          // get user info
          // note: roles must be a object array! such as: ['admin'] or ,['developer','editor']
          const { roles } = await store.dispatch('user/getInfo')

          // generate accessible routes map based on roles
          const accessRoutes = await store.dispatch('permission/generateRoutes', roles)

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

          // hack method to ensure that addRoutes is complete
          // set the replace: true, so the navigation will not leave a history record
          next({ ...to, replace: true })
        } catch (error) {
          // remove token and go to login page to re-login
          await store.dispatch('user/resetToken')
          Message.error(error || 'Has Error')
          next(`/login?redirect=${to.path}`)
          NProgress.done()
        }
      }
    }
  } else {
    /* has no token*/

    if (whiteList.indexOf(to.path) !== -1) {
      // in the free login whitelist, go directly
      next()
    } else {
      // other pages that do not have permission to access are redirected to the login page.
      next(`/login?redirect=${to.path}`)
      NProgress.done()
    }
  }
})