Implement and manipulate document structures and objects - Programming in HTML5 with JavaScript and CSS3 - page 80

When the update method is called, an update to the cache is prepared. When that's ready
to download, the status of the application cache changes to UpdateReady . When this is set, a
call to the swapCache method tells the application to switch to the most recent cache.

EXAM TIP
The call to the update method is asynchronous. Therefore, you must handle the
onupdateready event to determine when the update has completed the download process.

In addition to the properties and methods, the AppCache object can raise a series of
events that you can handle. The application cache typically operates in the background, and
you won't need these events. However, in some cases handling some of the events and forc-
ing an update can be useful. Table 1-16 lists the available events.

TABLE 1-16 Events available from the applicationCache object

Event Description

The browser is checking for an update to the application manifest, or the
application is being cached for the first time.

onchecking

The application manifest has no update available.

onnoupdate

The browser is downloading what it has been told to do per the manifest file. ondownloading

Files are being downloaded to the offline cache. This event fires periodically to
report progress.

onprogress

The download of the cache has completed. oncached

The resources listed in the manifest have been newly redownloaded, and the
swapCache method might be called.

onupdateready

A manifest file is no longer available.

onobsolete

An error has occurred. This could result from many things. Appropriate logging is
necessary to get the information and resolve.

onerror

Most of these events might not be used often, if at all. The most common scenario is to
handle the onupdateready method and then make a call to the swapCache method, as in this
example:

window.onload = function () {
var appCache = window.applicationCache;
appCache.oncached = function (e) { alert("cache successfully downloaded."); };
appCache.onupdateready = function (e) { appCache.swapCache(); };
}

Using the application cache is more about configuration than about coding. However, it's
important that you're aware the API is available for advanced scenarios where you need more
control over the process, or when you need to receive timely information about the process,
such as by handling the onprogress event.