ApplicationCache object

Provides a way to cache web resources (for example, html, css, js, images, audio, video, and so on) to improve performance, reduce server loads, and enable access to content when there is no network connectivity.

Syntax


 cache = window.applicationCache

Members

The ApplicationCache object has these types of members:

Events
Methods
Properties

Events

The ApplicationCache object has these events.

Event	Description
oncached	Event handler provided to see if the resources have been loaded into the cache.
onchecking	Event handler provided to see if the browser is in the process of checking for an update, or is attempting to download the manifest for the first time.
ondownloading	Event handler provided to determine if the browser has found an update and is downloading it, or is downloading the resources for the first time.
onerror	Event handler provided if an error has taken place.
onnoupdate	Event handler provided to indicate that the manifest has not changed.
onobsolete	Event handler provided to indicate that the application cache cannot be found.
onprogress	Event handler provided to indicate that the resources for the cache are still being downloaded.
onupdateready	Event handler provided to indicate that previously cached resources have been updated.

Methods

The ApplicationCache object has these methods.

Method	Description
abort	Aborts the application cache download process.
addEventListener	Registers an event handler for the specified event type.
dispatchEvent	Sends an event to the current element.
removeEventListener	Removes an event handler that the addEventListener method registered.
swapCache	Swaps an old cache for a newer one.
update	Triggers the update of the existing cache. If no updates are available, an update will not be forced.

Properties

The ApplicationCache object has these properties.

Property	Access type	Description
status	Read-only	Indicates the current status of the cache.

Remarks

In order to cache files, you must do the following:

Create a manifest file that lists the files and other web resources you want to cache.
Reference the manifest file in the header of every page you want cached.

The manifest file defines which web resources are cached when a user browses to your site. The manifest file typically has the extension .appcache and must be served as a text/cache-manifest mime type. Each webpage must add a manifest attribute to the HTML element similar to this:


<html manifest="clock.appcache">

You can use an absolute or relative URL. The cache manifest file lists the files that will be cached, using the format:


CACHE MANIFEST
#Version 1

CACHE:
script/scriptfilename1.js
css/cssfilename.css
images/imagename1.png
images/imagename2.jpg
images/imagename3.png

FALLBACK:
#This will enable imagename4.png to be as a replacement for 
#all resources under the images dir when there is no network connectivity.
images/ images/imagename4.png

NETWORK:
#This will prevent other network resources from being accessed.
images/imagename5.png

The first line of the manifest must read CACHE MANIFEST and the lines that follow it list the web resources you want to cache. You can use the # symbol to make comments.

Alternatively, web resources that need to be cached can be specified by adding a “CACHE:” header section before the resources (as shown previously).

In addition, fallback resources can be defined to replace general or specific web resources when there is no network connectivity. This is done by adding a “FALLBACK:” header section before the resources. These resources can be wildcard to specify a catch all mechanism (as shown previously). Be aware that there is a space after the first "images/" in the FALLBACK: section. This indicates that any files contained under the "images/" directory will fall back to a specific web resource (for example, images/imagename4.png) if they cannot be accessed from the network.

Also, web resources can be specified to only be loaded from the network. This is done by adding a "NETWORK: " header section before the resources. This functionality can be used to scope down the number of resources that can be accessed from the network, thus, creating an allowed only list (as shown previously).

The ApplicationCache object has the following properties:

The ApplicationCache object supports the following events:

The ApplicationCache object has the following methods:

ApplicationCache functionality is independent of HTTP caching headers.

The manifest file implicitly includes itself as a page to be cached. It also needs to have the same domain of origin as the page that contains it.