# ServiceWorker::Rails
[](https://travis-ci.org/rossta/serviceworker-rails)
Turn your Rails app into a Progressive Web App. Use [Service Worker](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API) with the Rails asset pipeline.
## Why?
The Rails asset pipeline makes a number of assumptions about what's best for deploying JavaScript, including asset digest fingerprints and long-lived cache headers - mostly to increase "cacheability". Rails also assumes a single parent directory, `/public/assets`, to make it easier to look up the file path for a given asset.
Service worker assets must play by different rules. Consider these behaviors:
* Service workers may only be active from within the scope from which they are
served. So if you try to register a service worker from a Rails asset pipeline
path, like `/assets/serviceworker-abcd1234.js`, it will only be able to interact
with requests and responses within `/assets/`**. This is not what we want.
* [MDN states](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API#Download_install_and_activate) browsers check for updated service worker scripts in the background every 24 hours (possibly less). Rails developers wouldn't be able to take advantage of this feature since the fingerprint strategy means assets at a given url are immutable. Beside fingerprintings, the `Cache-Control` headers used for static files served from Rails also work against browser's treatment of service workers.
We want Sprockets to compile service worker JavaScript from ES6/7, CoffeeScript, ERB, etc. but must remove the caching and scoping mechanisms offered by Rails asset pipeline defaults. This is where `serviceworker-rails` comes in.
*Check out the [blog post](https://rossta.net/blog/service-worker-on-rails.html)
for more background.*
### Demo
See various examples of using Service Workers in the demo Rails app, [Service Worker Rails Sandbox](https://serviceworker-rails.herokuapp.com/). The [source code](https://github.com/rossta/serviceworker-rails-sandbox) is also on GitHub.
## Features
* Maps service worker endpoints to Rails assets
* Adds appropriate response headers to service workers
* Renders compiled source in production and development
## Installation
Add this line to your application's Gemfile:
```ruby
gem 'serviceworker-rails'
```
And then execute:
$ bundle
Or install it yourself as:
$ gem install serviceworker-rails
To set up your Rails project for use with a Service Worker, you either use the
Rails generator and edit the generated files as needed, or you can follow the
manual installation steps.
### Automated setup
After bundling the gem in your Rails project, run the generator from the root of
your Rails project.
```
$ rails g serviceworker:install
```
The generator will create the following files:
* `config/initializers/serviceworker.rb` - for configuring your Rails app
* `app/assets/javascripts/serviceworker.js.erb` - a blank Service Worker
script with some example strategies
* `app/assets/javascripts/serviceworker-companion.js` - a snippet of JavaScript
necessary to register your Service Worker in the browser
* `app/assets/javascripts/manifest.json.erb` - a starter web app manifest
pointing to some default app icons provided by the gem
* `public/offline.html` - a starter offline page
It will also make the following modifications to existing files:
* Adds a sprockets directive to `application.js` to require
`serviceworker-companion.js`
* Adds `serviceworker.js` and `manifest.json` to the list of compiled assets in
`config/initializers/assets.rb`
* Injects tags into the `head` of `app/views/layouts/application.html.erb` for
linking to the web app manifest
**NOTE** Given that Service Worker operates in a separate browser thread, outside the context of your web pages, you don't want to include `serviceworker.js` script in your `application.js`. So if you have a line like `require_tree .` in your `application.js` file, you'll either need to move your `serviceworker.js` to another location or replace `require_tree` with something more explicit.
To learn more about each of the changes or to perform the set up yourself, check
out the manual setup section below.
### Manual setup
Let's add a `ServiceWorker` to cache some of your JavaScript and CSS assets. We'll assume you already have a Rails application using the asset pipeline built on Sprockets.
#### Add a service worker script
Create a JavaScript file called `app/assets/javascripts/serviceworker.js.erb`:
```javascript
// app/assets/javascripts/serviceworker.js.erb
console.log('[Service Worker] Hello world!');
function onInstall(event) {
event.waitUntil(
caches.open('cached-assets-v1').then(function prefill(cache) {
return cache.addAll([
'<%= asset_path "application.js" %>',
'<%= asset_path "application.css" %>',
'/offline.html',
// you get the idea ...
]);
})
);
}
function onActivate(event) {
console.log('[Serviceworker]', "Activating!", event);
event.waitUntil(
caches.keys().then(function(cacheNames) {
return Promise.all(
cacheNames.filter(function(cacheName) {
// Return true if you want to remove this cache,
// but remember that caches are shared across
// the whole origin
return key.indexOf('v1') !== 0;
}).map(function(cacheName) {
return caches.delete(cacheName);
})
);
})
);
}
self.addEventListener('install', onInstall)
self.addEventListener('activate', onActivate)
```
For use in production, instruct Sprockets to precompile service worker scripts separately from `application.js`, as in the following example:
#### Register the service worker
You'll need to register the service worker with a companion script in your main page JavaScript, like `application.js`. You can use the following:
```javascript
// app/assets/javascripts/serviceworker-companion.js
if (navigator.serviceWorker) {
navigator.serviceWorker.register('/serviceworker.js', { scope: './' })
.then(function(reg) {
console.log('[Page] Service worker registered!');
});
}
// app/assets/javascripts/application.js
// ...
//= require serviceworker-companion
```
#### Add a manifest
You may also want to create a `manifest.json` file to make your web app installable.
```javascript
// app/assets/javascripts/manifest.json
{
"name": "My Progressive Rails App",
"short_name": "Progressive",
"start_url": "/"
}
```
You'd then link to your manifest from the application layout:
```html
```
#### Configure the middleware
Next, add a new initializer as show below to instruct the `serviceworker-rails`
middleware how to route requests for assets by canonical url.
```ruby
# config/initializers/serviceworker.rb
Rails.application.configure do
config.serviceworker.routes.draw do
match "/serviceworker.js"
match "/manifest.json"
end
end
```
#### Precompile the assets
```ruby
# config/initializers/assets.rb
Rails.application.configure do
config.assets.precompile += %w[serviceworker.js manifest.json]
end
```
#### Test the setup
At this point, restart your Rails app and reload a page in your app in Chrome or Firefox. Using dev tools, you should be able to determine.
1. The page requests a service worker at `/serviceworker.js`
2. The Rails app responds to the request by compiling and rendering the file in `app/assets/javascripts/serviceworker.js.erb`.
3. The console displays messages from the page and the service worker
4. The application JavaScript and CSS assets are added to the browser's request/response [Cache](https://developer.mozilla.org/en-US/docs/Web/API/Cache).
#### Using the cache
So far so good? At this point, all we've done is pre-fetched assets and added them to the cache, but we're not doing anything with them yet.
Now, we can use the service worker to intercept requests and either serve them from the cache if they exist there or fallback to the network response otherwise. In most cases, we can expect responses coming from the local cache to be much faster than those coming from the network.
```javascript
// app/assets/javascripts/serviceworker.js.erb
function onFetch(event) {
// Fetch from network, fallback to cached content, then offline.html for same-origin GET requests
var request = event.request;
if (!request.url.match(/^https?:\/\/example.com/) ) { return; }
if (request.method !== 'GET') { return; }
event.respondWith(
fetch(request) // first, the network
.catch(function fallback() {
caches.match(request).then(function(response) { // then, the cache
response || caches.match("/offline.html"); // then, /offline cache
})
})
);
// See https://jakearchibald.com/2014/offline-cookbook/#on-network-response for more examples
}
self.addEventListener('install', onInstall);
```
## Configuration
When `serviceworker-rails` is required in your Gemfile, it will insert a middleware into the Rails
middleware stack. You'll want to configure it by mapping serviceworker routes to
Sprockets JavaScript assets in an initializer, like the example below.
```ruby
# config/initializers/serviceworker.rb
Rails.application.configure do
config.serviceworker.routes.draw do
# maps to asset named 'serviceworker.js' implicitly
match "/serviceworker.js"
# map to a named asset explicitly
match "/proxied-serviceworker.js" => "nested/asset/serviceworker.js"
match "/nested/serviceworker.js" => "another/serviceworker.js"
# capture named path segments and interpolate to asset name
match "/captures/*segments/serviceworker.js" => "%{segments}/serviceworker.js"
# capture named parameter and interpolate to asset name
match "/parameter/:id/serviceworker.js" => "project/%{id}/serviceworker.js"
# insert custom headers
match "/header-serviceworker.js" => "another/serviceworker.js",
headers: { "X-Resource-Header" => "A resource" }
# anonymous glob exposes `paths` variable for interpolation
match "/*/serviceworker.js" => "%{paths}/serviceworker.js"
end
end
```
`Serviceworker::Rails` will insert a `Cache-Control` header to instruct browsers
not to cache your serviceworkers by default. You can customize the headers for all service worker routes if you'd like,
such as adding the experimental [`Service-Worker-Allowed`](https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#service-worker-allowed) header to set the allowed scope.
```ruby
config.serviceworker.headers["Service-Worker-Allowed"] = "/"
config.serviceworker.headers["X-Custom-Header"] = "foobar"
```
## Development
After checking out the repo, run `bin/setup` to install dependencies. Then, run `bin/rake` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
## Contributing
Bug reports and pull requests are welcome on GitHub at https://github.com/rossta/serviceworker-rails. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [Contributor Covenant](http://contributor-covenant.org) code of conduct.
## License
The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).