The session on AJAX patterns was very cool. In one demo application (a photo album application), six specific patterns were addressed and a little on how to solve it was also shown.
Pattern - Script to Enable Interactivity
Sort of a no-brainer, but using script to enable interactive elements is sort of the basis of a rich application. In this particular pattern, it was more about making it easy to script what you're looking to do. ASP.NET AJAX
offers a lot of shortcuts to help you do that scripting.
This pattern also addressed the notion of separating script from behavior. ASP.NET AJAX introduces the notion of "extender controls" that allow you to use server controls to modify the behavior of controls in the page. An example was shown where some existing markup got modified by adding an extender - a server control registering script to modify HTML on the client side. It's a great way to do the separation.
Pattern - Logical Navigation
AJAX applications have typically lost the ability to use the back/forward buttons and the ability to bookmark a page. ASP.NET Futures
contains a "History" control that allows you to enable your AJAX elements to support state, sort of like ViewState, but on the URL. Modifying the page contents modifies the browser URL and, thus, enables logical navigation and bookmarking. As long as your scripts store enough history state to be able to recreate a logical view, this looks like a great way to overcome some shortcomings in AJAX.
Pattern - Update Indicators
Notifying a user of what changed when an AJAX request finishes is helpful so they can see the results of an action. The UpdateAnimation control in ASP.NET AJAX is one way to do that - it performs AJAX updates in an animated fashion so movement is the key for the user. There is a prototype UpdateIndicator control that scrolls the page to the location of the change and does a highlight animation on the change; this isn't in ASP.NET AJAX now but will hopefully be in the future.
Pattern - Smart Data Access
Possibly a poorly-named pattern, but the idea is that you should use HTML properly such that external services like search engine crawlers or programmatic site map generators can correctly access/index the content you post. Use tags in the correct semantic sense (e.g., if it's not a header, don't put it in <h1 /> tags). Also, keep in mind the way you display pages in non-scripted environments, such as in a search engine crawler or when the user has script disabled. Your content should look good either way.
Pattern - Mashups (Using External Services)
There's a lot of data out there, and a lot of services providing added value. Make use of them where you can. The example shown was a call to Flickr to get images and data.
What was interesting about the discussion of this pattern was less the "what" than the "how." Browsers don't allow cross-site scripting, so you have one of two options to get third-party data into your application.
You can use a server-side proxy where you create a proxy on your site that requests the third-party data. Your application then talks to your proxy to get the data. This is a good general-purpose solution and allows you to take advantage of things like caching calls on your site and gives you the ability to manipulate the data before passing it to the client (possibly optimizing it). The downside is that it does use up your server's bandwidth.
The other option is JSONP, which is a way you can add a script reference to your page that requests data in JSON format from a third-party service and when that data gets returned, it gets passed to a callback that you specify. ASP.NET AJAX supports this by allowing you to specify your own executor in an AJAX call, so the result of the call gets passed to your callback.
Yahoo! Design Pattern Library