Next-app-shell NPM

What is this about ?

The next-offline package provides integration with Workbox to ease the generation of the service worker, but it doesn't offer a true Progressive Web App experience: it caches pages as the user navigates through the website, but if the user goes offline and go to a page which hasn't been cached, it will fail.

Furthermore, the cache space is potentially important since the whole HTML files are cached. This can be a problem if we have, for example, a blog with hundreds of posts. In this case, we would prefer to cache only the app shell and load the data dynamically (eventually caching the API calls).

With next-app-shell, one app shell is generated for each NextJS page. This way, wherever your user goes, it will always be possible to show him something.

This plugin has only been tested with NextJS 9.4.2. As we had to do some kind of "reverse engineering" to make this work, new versions of NextJS may break the code. Please see the known issues below.

Installation

yarn add next-app-shell next-offline

npm install next-app-shell next-offline --save

Usage

const path = require('path');
const withOffline = require('next-offline');
const withAppShell = require('next-app-shell');

module.exports = withAppShell(
  withOffline({
      // Configure the service worker generation
      // See https://github.com/hanford/next-offline for the details
      generateSw: false,
      workboxOpts: {
        swSrc: path.resolve(__dirname, 'sw.js'),
        swDest: './sw.js'
      },
    
      // Configure the app shell files generation
      appShell : {
        nextPages: ['home', 'contact'],
        template: path.resolve(__dirname, 'app-shell.ejs')
      }
    })
);

Service worker configuration

In the service worker file, you need to map your routes to the app shell files, so that when the user goes to a certain route, he arrive .

With Workbox it can be done quite easily with the registerNavigationRoute method.

importScripts('https://storage.googleapis.com/workbox-cdn/releases/3.6.3/workbox-sw.js');

const pages = ['home', 'contact'];

// Prepare all the generated app shells
const appShells = pages.map(pageKey => ({
  url: `/app-shell/${pageKey}.html`,
  revision: Date.now().toString()
}));

// Depending on your routes, you may need to adapt the regex
appShells.forEach(appShell => {
  workbox.routing.registerNavigationRoute(
    appShell.url,
    { whitelist: new RegExp(`^\/${pageKey}`, 'i') }
  );
});

// __precacheManifest is populated by workbox-webpack-plugin on build time
// We also add the appShells since the service worker won't work without them
workbox.precaching.precacheAndRoute([...self.__precacheManifest, ...appShells]);

Parameters

All the parameters are inside a appShell object (see example above)

nextPages string[]

Array of page keys. They should be the same as the files in Next's /pages directory.

This parameter is mandatory.

template string

Path to the .ejs template file used to generate the app shell files.

If none is provided, the default template will be used.

filenameGenerator pageKey => string

A function to generate the filename of the destination app shell file, given a page key.

Defaults to pageKey => app-shell/${pageKey}.html

htmlWebpackPluginOptions object

Any other option that you want to pass to the HtmlWebpackPlugin which is used to generate the app shell file.

Known issues

When the service worker is activated, the getInitialProps method is never called. This means you need to do as if it doesn't exist.
We add a <meta name="next-head-count" content="1" /> in the header of each app shell, otherwise Next breaks. We currently don't know what this number is about and how we can generate it automatically for each page.