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HTML5: Article

Using HTML5 Application Cache to Create Offline Web Applications

Provides an easy way to prefetch some or all of your web app’s assets

Application Cache Events
The window.applicationCache object fires several events related to the state of the cache. window.applicationCache.status is a numerical property that tells you the state of the cache:

0-UNCACHED
1-IDLE
2-CHECKING
3-DOWNLOADING
4-UPDATEREADY
5-OBSOLETE

The following table shows the window.applicationCache callback attributes you can use in your applications:

Callback Attribute

Event

onchecking

CHECKING

ondownloading

DOWNLOADING

onupdateready

UPDATEREADY

onobsolete

OBSOLETE

oncached

CACHED

onerror

ERROR

onnoupdateready

NOUPDATE

onprogress

PROGRESS

The following code snippet shows how you can use these event callback attributes in your code.

Note: you can programmatically call window.applicationCache.update() to check for updates and window.applicationCache.swapCache() to connect the browser to use the latest version of the updated cache. The browsers will do this under the covers when you refresh the page.

window.applicationCache.onchecking = function(e) {
log("Checking for application update");
}
window.applicationCache.onnoupdate = function(e) {
log("No application update found");
}
window.applicationCache.onupdateready = function(e) {
log("Application update ready");
//Now connect the browser to use the new cache
window.applicationCache.swapCache();
}
window.applicationCache.onobsolete = function(e) {
log("Application obsolete");
}
window.applicationCache.ondownloading = function(e) {
log("Downloading application update");
}
window.applicationCache.oncached = function(e) {
log("Application cached");
}
window.applicationCache.onerror = function(e) {
log("Application cache error");
}
window.applicationCache.onprogress = function(e) {
log("Application Cache progress");
}

You can use the application cache events to code up some cool user notification functionality. For example, while you're downloading an application (receiving progress events), you can show the progress of the download. And when the onupdateready event fires, you can swap the browser's active cache to the new cache and instruct the user that (1) a new update has been downloaded, and (2) they must refresh the page to see the changes (since the cache is now downloaded and ready for use, but the page was already loaded from the application cache before the download started and will not be automatically refreshed.

See Also: Check out Ben Nadel's blog post about handling application cache events: http://www.bennadel.com/blog/2029-Using-HTML5-Offline-Application-Cache-Events-In-Javascript.htm

Detecting Online and Offline Status
HTML5 also allows you to detect whether you're online or offline. The following code shows how you can add event listeners for online and offline events.

You can use this to stop communicating with a server and to instead store things in HTML5 Web Storage (window.localStorage or window.sessionStorage) or in an HTML5 Web SQL Database while you're offline.

window.addEventListener("online", function(e) {
log("Application is now online");
// Send app data to server
}, true);
window.addEventListener("offline", function(e) {
log("Application is now offline");
window.localStorage.myLocalKey = ‘Some Data';
}, true);

Since these events do not fire when the page loads, you can also detect the initial status of a page by calling window.applicationCache.status.

Accessing Application Cache Content
You might be wondering where all of those files are stored. The application cache files are actually somewhere on your device's hard disk. For example, if you open the page about:cache in Firefox it will show you the exact location. Internally, the files may be stored in a SQLite database, but how the files are stored is an implementation detail that is left up to the browsers to figure out.


The
about:cache page in Firefox

Security
Since it is possible to access other people's files from an application cache on a shared computer (by navigating to the same site), it is critical that you do not store personal or sensitive data in an application cache. It is also important to keep in mind that you cannot always write to the application cache even if your browser supports the feature. This is because most private browsing modes (For example, Safari's Private Browsing mode, shown in the following image, or Chrome's Incognito Mode) prevent you from writing to an application cache for security reasons. It is therefore important to check for errors and to never assume you can access the cache.


Safari's Private Browsing Mode

Disk Quota
If you try to store a lot of data in an application cache, you may run into quota errors. Firefox and Opera provide a handy way to increase storage size for specific offline web applications while Chrome and Safari support this only through the use of special startup parameters. In your applications you should listen for errors such as the following:

Application Cache Error event: Failed to commit new cache to storage as it would exceed the quota.

In the future, browsers will hopefully have graceful, on-the-fly quota upgrade mechanisms for Application caching, like those of Opera's Web Storage, which prompts you as you are about to exceed your quota.

Clearing the Cache
In order to debug and test web apps that use application cache, it is a good idea to start with a clean slate to avoid false positives. To do this you must first blow away any existing application caches. Typically you do this by clearing the cache. Ensure that you are not doing this while a page from an offline web application is still open in the browser; that may cause problems with clearing the cache for that site.

Note: An application cache is created using the manifest's complete URL. You can have multiple manifest files in a site, which then allows you to split up loading of files. Each of these manifests will create a separate application cache. Here is how you can clear the cache and application cache in the various browsers:

Browser

Steps to Clear a Cache

Chrome

Settings Menu > Tools >
Clear Browsing Data

Firefox

Tools > Clear Recent History

&
Tools > Options (Preferences on Mac OS X) > Advanced > Network > Select specific application cache > Remove

 

Safari

Settings Menu > Reset Safari

Opera

Tools > Preferences > Storage

(+ Tools > Clear Private Data)

 

Internet Explorer

N/A

Best Practices
To recap, here are some of the best practices when it comes to using HTML5 offline web applications:

  • Manifest errors are often fatal, so it is best to use a predeployment script that programmatically creates a list of files to cache so you avoid typos and missing files.
  • The manifest file must be updated to trigger any changed files to be downloaded. Use a version Comment in the manifest file to track the changes and force application cache busting.
  • If you're testing on a web server on your local machine, you can avoid having to clear the cache non-stop by hosting your site on different domain names. You can accomplish this by hacking your hosts file, which is located at \WINDOWS\system32\drivers\etc\hosts on Windows and at /etc/hosts on UNIX. For example, the following entries allow a locally running web server to host files on both offline0.example.com and offline1.example.com, thus creating two separate application caches:

127.0.0.1 localhost
127.0.0.1 offline0.example.com
127.0.0.1 offline1.example.com

  • Check to see if files are really requested from the server or pulled from the application cache by monitoring the web server logs, as shown in the following screen-shot

Checking which files are requested and served up in Python's SimpleHTTPServer server log

Summary
In this article you learnt about the ins and outs of HTML5 Application Cache, a new feature that can be used to create offline web applications. This article explained how application caching worked and added some clarification about common misconceptions while providing a few best practices along the way.

More Stories By Peter Lubbers

Peter Lubbers is the Director of Documentation and Training at Kaazing where he oversees all aspects of documentation and training. He is the co-author of the Apress book Pro HTML5 Programming and teaches HTML5 training courses. An HTML5 and WebSocket enthusiast, Peter frequently speaks at international events.

Prior to joining Kaazing, Peter worked as an information architect at Oracle, where he wrote many books. He also develops documentation automation solutions and two of his inventions are patented.

A native of the Netherlands, Peter served as a Special Forces commando in the Royal Dutch Green Berets. In his spare time (ha!) Peter likes to run ultra-marathons. He is the 2007 and 2009 ultrarunner.net series champion and three-time winner of the Tahoe Super Triple marathon. Peter lives on the edge of the Tahoe National Forest and loves to run in the Sierra Nevada foothills and around Lake Tahoe (preferably in one go!).

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