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Building an
Offline Page for
theguardian.com
Oliver Joseph Ash – London Web Perf, March 2016
@OliverJAsh
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web vs native
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• 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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Experience:
• offline: nothing
• server down: nothing
• poor connection: white screen of death
• good connection: new content
Website
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We can do better!
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/developer-blog
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How it works
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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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www.theguardian.com
https://
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if (navigator.serviceWorker) {
navigator.serviceWorker.register('/service-worker.js');
}
1. Create and register the service worker
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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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3. Intercept requests with fetch
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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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• 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); });Slide 22
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HTML
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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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Network first
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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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Assets
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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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Cache first
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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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4. Updating the crossword
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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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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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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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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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“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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Problems and caveats
• Browser bugs in both Chrome and Firefox
• CDN cache
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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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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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Thank you
Like what we’re doing?
developers.theguardian.com
@OliverJAsh