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Overview
鈼� What is a Service Worker?
鈼� What existed before the Service Worker?
鈼� Lifecycle of a Service Worker
鈼� Use cases
鈼� A fair assessment (a.k.a. the bad)

A service worker is a script that is run
by your browser in the background,
separate from a web page

鈼� It's a JavaScript Worker, so it can't access the DOM
directly
鈼� Service worker is a programmable network proxy,
allowing you to control how network requests from your
page are handled.
鈼� It will be terminated when not in use, and restarted
when it's next needed
鈼� Service workers are asynchronous and make extensive
use of promises
鈼� Service workers require content to be served via https

What existed before the Service Worker?

CACHE MANIFEST
assets/6/script/mainmin.js
assets/6/style/mainmin.css
assets/6/style/fonts/pro.ttf
assets/6/style/imgs/sprites1.png
<html manifest="offline.appcache">

1. The service worker URL is fetched
and registered
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/trained-to-thrill/sw.js')
.then(function(reg) {
console.log('success!', reg);
}, function(err) {
console.log('error!', err);
});
}

2. If successful, the service worker is
executed in a ServiceWorkerGlobalScope;
this is basically a special kind of worker
context, running off the main script execution
thread, with no DOM access.
3. The service worker is now ready to
process events.

4. Installation of the worker is
attempted when service worker-
controlled pages are accessed
subsequently. An Install event is
always the first one sent to a service
worker
5. Same procedure as installing a
native app 鈥� making everything
available for use offline.

6. When the oninstall handler completes,
the service worker is considered installed.
self.oninstall = function(event) {
event.waitUntil(
caches.open('trains-static-v14').then(function(cache) {
return cache.addAll([
'/trained-to-thrill/',
'/trained-to-thrill/static/css/all.css',
'/trained-to-thrill/static/js/page.js',
'/trained-to-thrill/static/imgs/logo.svg',
'/trained-to-thrill/static/imgs/icon.png'
]);
})
);
};

7. When the service worker is installed, it
then receives an activate event. The
primary use of onactivate is for cleanup of
resources used in previous versions of a
Service Worker script.
self.onactivate = function(event) {
// remove caches beginning "trains-" that aren't in
// expectedCaches
event.waitUntil(
caches.keys().then(function(cacheNames) {
return Promise.all(
cacheNames.map(function(cacheName) {
if (!/^trains-/.test(cacheName)) {
return;
}
if (expectedCaches.indexOf(cacheName) == -1) {
return caches.delete(cacheName);
}}));}));};

8. The Service Worker will now control pages,
but only those opened after the register() is
successful. i.e. a document starts life with or
without a Service Worker and maintains that
for its lifetime. So documents will have to be
reloaded to actually be controlled.
this.addEventListener('fetch', function(event) {
var cachedResponse = caches.match(event.request).catch
(function() {
return event.default().then(function(response) {
return caches.get('v1').then(function(cache) {
cache.put(event.request, response.clone());
return response;
});
});
}).catch(function() {
return caches.match('/sw-test/gallery/myLittleVader.jpg');
});
event.respondWith(cachedResponse);
});

Cache assets specific to the version of
the app

Give the user a "Read later" or "Save
for offline" button. When it's clicked,
fetch what you need from the network &
pop it in the cache.

On network response: Frequently
updating resources such as a user's
inbox, tweets, images, or article
contents.

Push
鈼� The Push API is built on top of SerivceWorker.
鈼� Allows the ServiceWorker to be awoken in response to
a message from the OS's messaging service.
鈼� Happens even when the user doesn't have a tab open
to your site, only the ServiceWorker is woken up.
鈼� You request permission to do this from a page & the
user will be prompted.

Background Sync
鈼� Built on top of ServiceWorker.
鈼� It allows you to request background data
synchronisation as a one-off, or on an (extremely
heuristic) interval.
鈼� This happens even when the user doesn't have a tab
open to your site, only the ServiceWorker is woken up.
鈼� You request permission to do this from a page & the
user will be prompted.

鈼� If installation fails, the browser is not good at
telling you about it
鈼� fetch() is only available in service workers
鈼� No credentials by default (cookies)
鈼� Non-CORS fail by default (opaque response)
鈼� Handling responsive images

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