HTML5 Offline

HTML5 provides two robust offline capabilities already implemented in popular mobile devices, such as the iPhone and Android, and on modern desktop browsers based on the Webkit and Gecko rendering engines.

Usage

The easiest way to use Rack::Offline is by using Rails::Offline in a Rails application.
In your router:

match "/application.manifest" => Rails::Offline

This will automatically cache all JavaScript, CSS, and HTML in your public
directory, and will cause the cache to be updated each request in development
mode.

You can fine-tune the behavior of Rack::Offline by using it directly:

offline = Rack::Offline.configure do
  cache "images/masthead.png"

  public_path = Rails.public_path
  Dir[public_path.join("javascripts/*.js")].each do |file|
    cache file.relative_path_from(public_path)
  end

  network "/"
end

You can pass an options Hash into #configure in Rack::Offline:

name	purpose	value in Rails::Offline
:cache	false means that the browser should download the assets on each request if a connection to the server can be made	the same as config.cache_classes
:logger	a logger to send messages to	Rails.logger
:root	The location of files listed in the manifest	Rails.public_path

Application Cache

The App Cache allows you to specify that the browser should cache certain files, and ensure that the user can access them even if the device is offline.

You specify an application’s cache with a new manifest attribute on the html element, which must point at a location on the web that serves the manifest. A manifest looks something like this:

CACHE MANIFEST

javascripts/application.js
javascripts/jquery.js
images/masthead.png

NETWORK:
/

This specifies that the browser should cache the three files immediately following CACHE MANIFEST, and require a network connection for all other URLs.

Unlike HTTP caches, the browser treats the files listed in the manifest as an atomic unit: either it can serve all of them out of the manifest or it needs to update all of them. It will not flush the cache unless the user specifically asks the browser to clear the cache or for security reasons.

Additionally, the HTML file that supplies the manifest attribute is implicitly in the manifest. This means that the browser can load the HTML file and all its cached assets as a unit, even if the device is offline.

In short, the App Cache is a much stickier, atomic cache. After storing an App Cache, the browser takes the following (simplified) steps in subsequent requests:

1. Immediately serve the HTML file and its assets from the App Cache. This happens
   whether or not the device is online
2. If the device is offline, treat any resources not specified in the App Cache
   as 404s. This means that images will appear broken, for instance, unless you
   make sure to include them in the App Cache.
3. Asynchronously try to download the file specified in the manifest attribute
4. If it successfully downloads the file, compare the manifest byte-for-byte with
   the stored manifest.
   • If it is identical, do nothing.
   • If it is not identical, download (again, asynchronously), all assets specified
     in the manifest
5. Along the way, fire a number of JavaScript events. For instance, if the browser
   updates the cache, fire an updateready event. You can use this event to
   display a notice to the user that the version of the HTML they are using is
   out of date

App Cache Considerations

The first browser hit after you change the HTML will always serve up stale HTML
and JavaScript. You can mitigate this in two obvious ways:

1. Treat your mobile web app as an API consumer and make sure that your app
   can support a “client” that’s one version older than the current version
   of the API.
2. Force the user to reload the HTML to see newer data. You can detect this
   situation by listening for the updateready event

A good recommendation is to have your server support clients at most one
version old, but force older clients to reload the page to get newer data.

Regular users of your application will receive updates through normal usage,
and will never be forced to update. Irregular users may be forced to update
if they pick up the application months after they last used in. In all, a
pretty good trade-off.

While this may seem cumbersome at first, it makes it possible for your users
to browse around your application more naturally when they have flaky
connections, because the process of updating assets (including HTML)
always happens in the background.

Updating the App Cache

You will need to make sure that you update the cache manifest when any of
the underlying assets change.

Rack::Offline handles this using two strategies:

1. In development, it generates a SHA hash based on the timestamp for each
   request. This means that the browser will always interpret the cache
   manifest as stale. Note that, as discussed in the previous section,
   you will need to reload the page twice to get updated assets.
2. In production, it generates a SHA hash once based on the contents of
   all the assets in the manifest. This means that the cache manifest will
   not be considered stale unless the underlying assets change.

Rails::Offline caches all JavaScript, CSS, images and HTML
files in public and uses config.cache_classes to determine which of
the above modes to use. In Rails, you can get more fine-grained control
over the process by using Rack::Offline directly.

Local Storage

Browsers that support the App Cache also support Local Storage, from the
HTML5 Web Storage Spec. IE8 and above also support Local
Storage.

Local Storage is a JavaScript API to an extremely simple key-value store.

It works the same as accessing an Object in JavaScript, but persists the
value across sessions.

localStorage.title = "Welcome!"
localStorage.title //=> "Welcome!"

delete localStorage.title
localStorage.title //=> undefined

Browsers can offer different amounts of storage using this API. The
iPhone, for instance, offers 5MB of storage, after which it asks the
user for permission to store an additional 10MB.

You can reclaim storage from a key by deleteing it or
by overwriting its value. You can also enumerate over all keys in
the localStorage using the normal JavaScript for/in
API.

In combination with the App Cache, you can use Local Storge to store
data on the device, making it possible to show stale data to your
users even if no connection is available (or in flaky connection
scenarios).

Basic JavaScript Strategy

You can implement a simple offline application using only a few
lines of JavaScript. For simplicity, I will use jQuery, but you
can easily implement this in pure JavaScript as well. The
example is heavily commented, but the total number of lines of
actual JavaScript is quite small.

jQuery(function($) {
  // Declare a function that can take a JS object and
  // populate our HTML. Because we used the App Cache
  // the HTML will be present regardless of online status
  var updateArticles = function(object) {
    template = $("#articles")
    localStorage.articles = JSON.stringify(object);
    $("#article-list").html(template.render(object));
  }

  // Create a flag so we don't poll the server twice
  // at once
  var updating = false;

  // Create a function that will ask the server for
  // updates to the article list
  var remoteUpdate = function() {
    // Don't ping the server again if we're in the
    // process of updating
    if(updating) return;

    updating = true;

    $("#loading").show();
    $.getJSON("/article_list.json", function(json) {
      updateArticles(json);
      $("#loading").hide();
      updating = false;
    });
  }

  // If we have "articles" in the localStorage object,
  // update the HTML with the stale articles. Even if
  // the user never gets online, they will at least
  // see the stale content
  if(localStorage.articles) updateArticles(JSON.parse(localStorage.articles));

  // If the user was offline, and goes online, ask
  // the server for updates
  $(window).bind("online", remoteUpdate);

  // If the user is online, ask for updates now
  if(window.navigator.onLine) remoteUpdate();
})