Serving Jekyll Sites Offline

When I put this new blog together with Jekyll I added a service worker to the build process, generating the required URL’s to cache using liquids templating language. I was then able to include those generated links in my service workers code to save me needing to manaully update the list over time.

Creating a list of pages in the Jekyll site

In a previous post I show a simple service worker setup where some URL’s are specified for caching. When building the site with Jekyll, it is good to not have to keep a list of every page up to date in the service workers set of routes to cache, so we can instead use liquid within our service workers file to generate the routes we need to cache at build time.

The following captures a set of resources into a variable called asset_urls for inserting into the routes we want to cache JavaScript variable later.

{% capture asset_urls %}

{% for page in site.html_pages %}
'{{ page.url | remove: '.html' }}',
{% endfor %}

{% for post in site.posts %}
'{{ post.url }}',
{% endfor %}

{% for file in site.static_files %}
{% if file.path contains '/images' or file.path contains 'manifest' or file.path contains 'css' %}
'{{ file.path }}',
{% endif %}
{% endfor %}

{% endcapture %}

The above script tracks pages which are our main routes like / and /contact. It also tracks each post like the one you are reading right now and adds it to the list. Finally it also adds static files, like images and other Jekyll pages without front matter that are not processed. We can then push this as mentioned into a list of routes we want to cache with the service worker.

var urlsToCache = [
  {{ asset_urls | normalize_whitespace }}

We can also leverage some more liquid templating to help update our service worker each time we rebuild the site. We can use the available liquid tags {{ site.time | date_to_xmlschema }} as part of the caching key, to keep our cache name unique each time we rebuild the site, so when publishing the old cache is removed and replaced with the new list of resources.

var CACHE_NAME = "{{ site.url }}-site-{{ site.time | date_to_xmlschema }}';

Some static resources had to be added manually with the above setup, so a few resources were included in the list already.

var urlsToCache = [
  {{ asset_urls | normalize_whitespace }},
  '/manifest.json',   // manually added
  '/js/validate.js',  // manually added
  '/css/main.css'     // manually added

Installing the generated routes

These routes are injected inside the declared variable urlsToCache and then when we go to cache these URL’s with a service worker we pass these urlsToCache to the install step and they are added to our cache.

// Perform install steps
self.addEventListener('install', function(event) {
        .then(function(cache) {
            return cache.addAll(urlsToCache)

The install step inserts these resources into the browsers cache and subsequent loads of these resources intercepted by the service worker will use the cached asset over reaching out to the network, so the site will start working offline.

Wrapping up

This is just a short demonstration of how you can make use of the liquid templating made available to build out the resources to cache dynamically for a Jekyll site. As more posts get added to the blog they’ll automatically become part of the cached list of assets without manual work keeping the service worker up to date.

jekyll service-worker liquid
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