Building an Offline Page for theguardian.com — London Web Perf, March 2016

You’re on a train to work and you open up the Guardian app on your phone. A tunnel surrounds you, but the app still works in very much the same way as it usually would—despite your lack of internet connection, you still get the full experience, only the content shown will be stale. If you tried the same for the Guardian website, however, it wouldn’t load at all. Native apps have long had the tools to deal with these situations, in order to deliver rich user experiences whatever the user’s situation may be. With service workers, the web is catching up.

Transcript

1. Building an Offline Page for theguardian.com Oliver Joseph Ash – London Web Perf, March 2016
 @OliverJAsh
4. web vs native
5. • Content is cached • Experience: • offline: stale content remains • server down: stale content remains • poor connection: stale while revalidate • good connection: stale while revalidate Native app
6. Experience: • offline: nothing • server down: nothing • poor connection: white screen of death • good connection: new content Website
7. We can do better!
8. /developer-blog
9. How it works
10. What is a service worker? • Script that runs in the background • Useful for features with no user interaction, e.g.: • listen to push events, useful for pushing notifications • intercept and handle network requests • future: • background sync • geofencing • Trusted origins only (HTTPS, localhost) • Chrome and Firefox stable
11. www.theguardian.com https://
12. <script> if (navigator.serviceWorker) { navigator.serviceWorker.register('/service-worker.js'); } </script> 1. Create and register the service worker
13. 1 2
16. var version = 1; var staticCacheName = 'static' + version; var updateCache = function () { return caches.open(staticCacheName) .then(function (cache) { return cache.addAll([ '/offline-page', '/assets/css/main.css', '/assets/js/main.js' ]); }); }; self.addEventListener('install', function (event) { event.waitUntil(updateCache()); }); 2. Get ready with install • install event: get ready! • Cache the assets needed later • Version the cache
17. 1 2
18. 3. Intercept requests with fetch
19. const staticCacheName = 'static'; const version = 1; const updateCache = () => ( caches.open(staticCacheName + version) .then(cache => cache.addAll([ '/offline-page.html', '/assets/css/main.css', '/assets/js/main.js' ]); ); ); self.addEventListener('install', function (event) { event.waitUntil(updateCache()); }); self.addEventListener('fetch', function (event) { event.respondWith(fetch(event.request)); }); • Default: just fetch • Override default • Intercept network requests to: • fetch from the network • read from the cache • construct your own response fetch events
21. • e.g., use templating to construct a HTML response from JSON Service worker: custom responses self.addEventListener('fetch', function (event) { var responseBody = '<h1>Hello, World!</h1>'; var responseOptions = { headers: { 'Content-Type': 'text/html' } }; var response = new Response( responseBody, responseOptions ); event.respondWith(response); });
22. HTML
23. var doesRequestAcceptHtml = function (request) { return request.headers.get('Accept') .split(',') .some(function (type) { return type === ‘text/html’; }); }; self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith(fetch(request)); } });
24. Network first
25. self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith( fetch(request).catch(function () { return caches.match('/offline-page'); }) ); } });
26. Assets
28. self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith( fetch(request).catch(function () { return caches.match('/offline-page'); }) ); } else { event.respondWith( caches.match(request) ); } });
29. Cache first
30. self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith( fetch(request).catch(function () { return caches.match('/offline-page'); }) ); } else { event.respondWith( caches.match(request).then(function (response) { response || fetch(request); }) ); } });
31. 4. Updating the crossword
32. var version = 1; var cacheNameSuffix = 'static' + '-' + version; var getISODate = function () { return new Date().toISOString().split('T')[0]; }; var getCacheName = function () { return getISODate() + '-' + cacheNameSuffix; }; var isLatestCacheName = function (key) { return key === getCacheName(); }; var deleteOldCaches = function () { var getOldCacheKeys = function () { return caches.keys().then(function (keys) { return keys.filter(function (key) { return !isLatestCacheName(key); }); }); }; return getOldCacheKeys().then(function (oldCacheKeys) { return Promise.all(oldCacheKeys.map(function (key) { return caches.delete(key); })); }); }; var isCacheUpdated = function () { return caches.keys().then(function (keys) { return keys.some(isLatestCacheName); }); }; isCacheUpdated().then(function (isUpdated) { if (!isUpdated) { updateCache().then(deleteOldCaches); } }); // <normal fetch handling code>
34. Future usages of service worker on theguardian.com • Offline page will increase in significance • Offline page can be extended • show stale content, like native apps • show personalised content, downloaded ahead of time • show content that has been “saved for later” • Go fully offline-first
35. Offline-first • Instantly respond to navigations with a “shell” • Show stale content while revalidating • Improves the experience for users with poor connections
38. Why? Is this valuable? • Fun • Insignificant usage due to HTTPS/browser support • … but good to plant the seed • Iron out browser bugs
39. “If we only use features that work in IE8, we're condemning ourselves to live in an IE8 world.” — Nolan Lawson
40. Problems and caveats • Browser bugs in both Chrome and Firefox • CDN cache
41. Conclusion • Service workers allow us to: • define rich experiences for users who are offline • improve performance for users with poor connections • It’s easy to build an offline page • Offline page is a good place to start
42. Further reading • https://www.theguardian.com/info/developer-blog/2015/nov/04/ building-an-offline-page-for-theguardiancom • https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md • https://www.theguardian.com/service-worker.js
43. Thank you Like what we’re doing? developers.theguardian.com @OliverJAsh