public String CloudThoughts{ get; set;}

Update 2/22/2010 See this article for a better way of using Ajax with Visualforce/Apex.

JQuery and JSON have become my tools of choice for designing and developing Single Page Applications (or SPA). Why?

1. The user experience is better if the entire page is not refreshed when executing a single action
2. There's a vast library of JQuery plug-ins and a Developer community to tap into.
3. Using JSON as the standard object model and separating the UI from the database allows me to design a UI only once and port the application to other cloud platforms (for example, the VF code below can run on i-Dialogue, Google Apps Engine, or Azure by only changing a few lines of code).

Credit goes to Ron Hess for showing me this pattern.I've made very little changes to his Apex code example, except to add JQuery and handling actionFunction rerender callbacks slightly differently.

The basic pattern I'm using for developing Single Page Applications for Visualforce is to call actionFunctions from JQuery, which in turn call Apex controller methods that construct JSON strings. The web page then handles the JSON formatted response in re-rendered outputPanels. The code below demonstrates a simple auto complete function that searches Salesforce Accounts that match a user entered input on each keypress. Clicking on an Account drills down to Account details by calling another actionFunction that retrieves specific Account information.

Some Caveats

• The outputPanel scripts will execute the first time the page is loaded, so a check for null is required in the callback to prevent first-time execution
• The dynamic re-rendering of script within an outputPanel makes it difficult to do true functional programming and create closures that elegantly handle the callbacks. More complex applications may have to utilize global state variables, increasing the mutability of the application (and potential for bugs)
• The mutability of Apex controller properties requires a one-to-one relationship between actionFunction handlers and their corresponding response strings.
• actionFunctions send the page ViewState in the AJAX XmlHttpRequest and return a blob of XML (apparently using the Sarissa open source library) in the response, which has a slightly slower performance than what you'd get using JQuery's native AJAX utility methods.

Source Code