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
   

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4. web vs native
   

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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
   

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6. Experience:
   • offline: nothing
   • server down: nothing
   • poor connection: white screen of death
   • good connection: new content
   Website
   

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7. We can do better!
   

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8. /developer-blog
   

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9. How it works
   

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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
    

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11. www.theguardian.com
    https://
    

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12. <br/>if (navigator.serviceWorker) {<br/>navigator.serviceWorker.register('/service-worker.js');<br/>}<br/>
    1. Create and register the service worker
    

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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
    

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18. 3. Intercept requests with fetch
    

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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
    

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21. • e.g., use templating to construct
    a HTML response from JSON
    Service worker: custom responses
    self.addEventListener('fetch', function (event) {
    var responseBody =
    'Hello, World!';
    var responseOptions = {
    headers: {
    'Content-Type': 'text/html'
    }
    };
    var response = new Response(
    responseBody,
    responseOptions
    );
    event.respondWith(response);
    });
    

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22. HTML
    

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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));
    }
    });
    

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24. Network first
    

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25. self.addEventListener('fetch', function (event) {
    var request = event.request;
    if (doesRequestAcceptHtml(request)) {
    event.respondWith(
    fetch(request).catch(function () {
    return caches.match('/offline-page');
    })
    );
    }
    });
    

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26. Assets
    

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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)
    );
    }
    });
    

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29. Cache first
    

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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);
    })
    );
    }
    });
    

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31. 4. Updating the crossword
    

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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);
    }
    });
    //
    

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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
    

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35. Offline-first
    • Instantly respond to navigations with a “shell”
    • Show stale content while revalidating
    • Improves the experience for users with poor
    connections
    

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38. Why? Is this valuable?
    • Fun
    • Insignificant usage due to HTTPS/browser support
    • … but good to plant the seed
    • Iron out browser bugs
    

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39. “If we only use features that work in IE8, we're
    condemning ourselves to live in an IE8 world.”
    — Nolan Lawson
    

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40. Problems and caveats
    • Browser bugs in both Chrome and Firefox
    • CDN cache
    

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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
    

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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
    

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43. Thank you
    Like what we’re doing?
    developers.theguardian.com
    @OliverJAsh
    

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