HTML and CSS Reference
In-Depth Information
Offline Application Events
In the preceding sections, you created an offline application by creating a cache manifest. The browser did
all the work of downloading and storing the application files in a local cache. When the browser is busy
downloading and caching an application, it raises several
applicationCache
object events.
applicationCache
is an object whose properties, methods, and events can be handled using JavaScript
code. Using
applicationCache
events, you can track various stages of an offline application's life cycle. Let's
see what kind of events the
applicationCache
object fires:
Whenever you request a web page whose
manifest
attribute points to a cache
manifest file, the
checking
event of the
applicationCache
object is raised. The
checking
event is raised regardless of whether the application is already cached.
1.
2.
If the browser has never cached this application before, it downloads the
application's cache manifest as indicated in the
manifest
attribute and begins
downloading files as listed in the cache manifest. At the same time, the
downloading
event of the
applicationCache
object is raised.
3.
The cache manifest may contain many files, and the downloading operation
may be lengthy. That is why periodically the
applicationCache
object raises a
progress
event notifying you about the progress of the download operation.
4.
After all the files listed in the cache manifest are downloaded, the
applicationCache
object raises the
cached
event. This event is an indication that
the entire application is now available offline.
5.
If you're accessing an application that is already cached, the browser checks
whether the application's cache manifest file has changed since the last
download.
6.
If the cache manifest file is unchanged, there is no need to update the offline
cache, and the
noupdate
event of the
applicationCache
object is raised.
7.
If the cache manifest has changed since the last download, the browser begins
downloading the application files and raises the
downloading
and
progress
events of the
applicationCache
object as before.
8.
When all the application files have been successfully downloaded, the
updateready
event of the
applicationCache
object is raised.
9. It's also possible that an application that was cached earlier is no longer an
offline application—that is, its cache manifest has been removed from the
server. In this case, the browser raises the
obsolete
event of the
applicationCache
object and removes the existing files from the cache. The
application is now treated as a
connected
application and needs a live network
connection with the server in order to function properly.
10. If there is an error in any of the previous steps, the
error
event of the
applicationCache
object is raised.
The
applicationCache
object events just discussed are listed in Table 8-2 for your convenience.
