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1.

An offline web application is a
list of URLs — HTML, CSS, JavaScript, images, or any other kind of resource.
The home page of the offline web application points to this list, called a
manifest file, which is just a text file located elsewhere on the web server. A
web browser that implements HTML5 offline applications will read the list of
URLs from the manifest file, download the resources, cache them locally, and
automatically keep the local copies up to date as they change. When the time
comes that you try to access the web application without a network connection,
your web browser will automatically switch over to the local copies instead.

2.

There’s a flag in the DOM that
will tell you whether you’re online or offline. There are events that fire when
your offline status changes (one minute you’re offline and the next minute
you’re online, or vice-versa).

3.

A cache manifest file is a list
of all of the resources that your web application might need to access while
it’s disconnected from the network. In order to bootstrap the process of
downloading and caching these resources, you need to point to the manifest
file, using a manifest

attribute on your <html>

element:

<!DOCTYPE HTML>

<html manifest="/cache.manifest">

<body>

...

</body>

</html>

Your cache
manifest file must be served with the content type text/cache-manifest

.

4.

Every page of your web
application needs a manifest

attribute that points to the cache manifest for
the entire application.

5.

The first line of every cache
manifest file is:

CACHE MANIFEST

After that,
all manifest files are divided into three parts: the “explicit” section, the
“fallback” section, and the “online whitelist” section. Each section has a
header, on its own line. If the manifest file doesn’t have any section headers,
all the listed resources are implicitly in the “explicit” section.

6.

The line marked CACHE:

is the
beginning of the “explicit” section. Resources in the “explicit” section will
get downloaded and cached locally, and will be used in place of their online
counterparts whenever you are disconnected from the network.

7.

When you navigate to an HTML
page with a

manifest

attribute, the page itself is assumed to be part of the web
application, so you don’t need to list it in the manifest file itself. However,
if your web application spans multiple pages, you should list all of the HTML
pages in the manifest file, otherwise the browser would not know that there are
other HTML pages that need to be downloaded and cached.

8.

The line marked NETWORK:

is the
beginning of the “online whitelist” section. Resources in this section are
never cached and are not available offline. (Attempting to load them while
offline will result in an error.)

9.

The line marked FALLBACK:

is
the beginning of the “fallback” section. In a fallback section, you can define
substitutions for online resources that, for whatever reason, can’t be cached
or weren’t cached successfully:

CACHE MANIFEST

FALLBACK:

/ /offline.html

NETWORK:

*

The first
part of the line (before the space) is URL pattern. The single character (/)
will match any page on your site. When you try to visit a page while you’re
offline, if your browser doesn’t find the page in the appcache, instead of
displaying an error message, it will display the page /offline.html

, as
specified in the second half of that line in the fallback section.

10.

When you visit any page that
points to a cache manifest, your browser says “hey, this page is part of an
offline web application, is it one I know about?” If your browser hasn’t ever downloaded
this particular cache manifest file, it will set up a new offline “appcache”
(short for “application cache”), download all the resources listed in the cache
manifest, and then add the current page to the appcache. If your browser does
know about this cache manifest, it will simply add the current page to the
existing appcache. It means that you can have an offline web application that
“lazily” adds pages as you visit them. You don’t need to list every single one
of your HTML pages in your cache manifest.

11.

A single start character (

) has
special meaning in a network

section. It’s called the “online whitelist
wildcard flag.” That’s a fancy way of saying that anything that isn’t in the
appcache can still be downloaded from the original web address, as long as you
have an internet connection. This is important for an “open-ended” offline web
application. Without this wildcard flag, the browser wouldn’t load any
externally-hosted images or videos.

12.

When your browser visits a page
that points to a cache manifest, it fires off a series of events on the window.applicationCache

object:

a)

As soon as it notices a manifest

attribute on the <html>

element, your browser fires a checking

event. The
checking

event is always fired, regardless of whether you have previously
visited this page or any other page that points to the same cache manifest.

b)

If your browser has never seen
this cache manifest before:

        §

It will fire a downloading

event, then start to download the
resources listed in the cache manifest.

        §

While it’s downloading, your browser will periodically fire progress

events, which contain information on how many files have been downloaded
already and how many files are still queued to be downloaded.

        §

After all resources listed in the cache manifest have been
downloaded successfully, the browser fires one final event, cached

.

c)

On the other hand, if you have
previously visited this page or any other page that points to the same cache
manifest, your browser will check whether the cache manifest changed since the
last time your browser checked it:

        §

If the answer is no, your browser will immediately fire a noupdate

event.

        §

If the answer is yes, your browser will fire a downloading

event and
start re-downloading every single resource listed in the cache manifest. While
it’s downloading, your browser will periodically fire progress

events, which
contain information on how many files have been downloaded already and how many
files are still queued to be downloaded. After all resources listed in the
cache manifest have been re-downloaded successfully, the browser fires one
final event, updateready

.

        §

The new version is not yet in use.

To
“hot-swap” to the new version without forcing the user to reload the page, you
can manually call the window.applicationCache.swapCache()

function.

13.

If, at any point in the above
process, something goes horribly wrong, your browser will fire an error

event
and stop. Here is a hopelessly abbreviated list of things that could go wrong:

a)

The cache manifest returned an
HTTP error 404 (Page Not Found) or 410 (Permanently Gone).

b)

The cache manifest was found
and hadn’t changed, but the HTML page that pointed to the manifest failed to
download properly.

c)

The cache manifest changed
while the update was being run.

d)

The cache manifest was found
and had changed, but the browser failed to download one of the resources listed
in the cache manifest.

14.

Your browser checks whether a
cache manifest file has change as below:

a)

Via normal HTTP semantics, Some
of these HTTP headers (Expires and Cache-Control) tell your browser how it is
allowed to cache the file without ever asking the server whether it has
changed.

b)

If the cache manifest has
expired (according to its HTTP headers), then your browser will ask the server
whether there is a new version, and if so, the browser will download it.

c)

Once it’s downloaded the new
cache manifest file, your browser will check the contents against the copy it
downloaded last time. If the contents of the cache manifest file are the same
as they were last time, your browser won’t re-download any of the resources
listed in the manifest.

15.

If your cache manifest file
hasn’t changed, the browser will never notice that one of the previously cached
resources has changed. The easiest way is to include a comment line with a
revision number in cache manifest. Change the revision number in the comment
when some of the resources listed in cache manifest has been changed. Then the
web server will return the newly changed cache manifest file, your browser will
notice that the contents of the file have changed, and it will kick off the process
to re-download all the resources listed in the manifest.

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