meagar/jsoffline.md

## jsoffline.md

      
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Note2: See https://github.com/meagar/taking-javascript-offline for code examples; any time a string like 2-basic-caching appears, that's a branch which supports that slide
Taking JavaScript Offline
Who am I?
@meagar
Matthew Eagar / Tech Lead / 500px
Case Study
Data Capture

Survey consumers on the street
Take photos, capture signatures
On Windows, IOS, Android devices
Sync to central data store (Rails)
With no wifi or cell network

Solution?
Native App
Three Native Apps
PhoneGap
HTML5!
HTML5

Appcache
IndexedDB
Location
WebSockets
FileReader

HTML5
Store site structure in appcache
RESTful API for data
Temporary storage in IndexedDB
It worked!
CODE BREAK

Sample Backbone app
Basic RESTful API
JS + Coffee + EJS + Jade?

1-no-cache
Appcache
A mechanism for storing/accessing static assets offline


Driven by manifests


Define paths to cache

JS/CSS/images/HTML


Full of gotchas


5mb limit


Appcache isn't...
A douche bag :(

Basic caching
manifests...

Contain sections, paths, comments
included via <html manifest="...">
Served with Content-type: text/appcache *
Have a .appcache extension *
start with CACHE MANIFEST

Basic caching
CACHE MANIFEST

CACHE:
# Static assets
/js/jquery.js
/js/app.js
/css/bootstrap.css
/css/styles.css

# Single-page backbone apps
/posts
/users

Basic caching
<!doctype html>
<html manifest="/my_cache.appcache">
<head>
  <script src="/js/jquery.js"></script>
  <!-- ... -->

More code!
Add a basic manifest
Hooray for Chrome!
2-basic-caching
Gotcha #1
Once appcache'd, content only comes from appcache
Gotcha #2
On cached pages, all requests go through Appcache

Gotcha #2
On cached pages, all requests go through Appcache

Gotcha #3
Appcache only updates if the manifest changes
Gotcha #4
Request that triggers an update still comes from appcache
Changes require two page reloads to become apparent
So...
On cached pages, all requests go through Appcache

So...
On cached pages, all requests go through Appcache

NETWORK sections
Appcache allows NETWORK sections to define content that shouldn't be cached
NETWORK sections
CACHE MANIFEST

CACHE:

# Single-page backbone apps
/posts
/users

NETWORK:
/api/posts

2-basic-cache
Tiny Gotcha #...5?
Implicit Caching
You can't NETWORK a page which has a manifest; it's cached implicitly
Going offline
Some content can't/shouldn't be cached
FALLBACK lets us provide defaults for use while offline
FALLBACK:
# Don't show user profiles while offline
/users/* /offline

Aside: Going offline
Our SPA can detect the presence of a network connection and render accordingly
navigator.onLine // true/false
window.addEventListener('online', fn);
window.addEventListener('offline', fn);

3-going-offline
Dealing with realistic network conditions
Networks are slow
The appcache doesn't like slow networks
4-waiting-on-appcache
Gotcha #6
Manifest downloads are brittle

Entire manifest is redownloaded
Upgrade is interruptable
Users unaware that updates are happening
Errors cancel entire update

Appcache events
Appache emits several useful events
Can tie into them to provide a GUI for updates
Appcache events
First load:

checking
downloading
progress(, progress, ...)
cached

Appcache events
Subsequent loads

checking
noupdate
downloading
progress(, progress, ...)
cached

Building a dedicated updater!
More code!

Detect cache downloading
Show progress

4-waiting-on-appcache
Appcache Obsoletion
Allows programatic expiration of the entire cache
Still subject to the double-reload requirement
Forces browser into uncached state
5-obsoletion
Gotcha #lots
Obsolete caches!
Why obsoletion instead of updating?
Why obsoletion?


/login - online only, set is_authorized


/index - offline

check is_authorized

bounce to index


/login - change to auth_token


What should I appcache?
Static sites for mobile!
Web apps that server structure first, data later
Anything!
That's the appcache!
IndexedDB
IndexedDB
Very quickly...

A key/value store
larger than localStorage
Size varies from browser-to-browser
Prompts the user for more storage

IndexedDB
Very quickly...

Doesn't warn you when it's full *
Supports transactions, migrations, indices
Faster than you might expect, slower than you  might hope

IndexedDB
Behavies like REST:
Backbone.sync
IndexedDB
backbone-indexeddb.js
6-indexeddb