Callbacks are getting momentum, not only in ASP.NET

The recent launch of the Google Suggest beta attracted a lot of attention to XmlHttp callbacks. Such features where client-side script asks the server for very focused updates to the page without reposting it entirely have been possible ever since frames and javascript exist (I've done a web-based chat application and treeview based on hidden frame posting more than 4 years ago), but the XmlHttp APIs first found in IE 5 and then in Mozilla, Safari and Opera (without the need to use an ActiveX, which is a great improvement) have made it a lot easier and a lot less hacky. There's even an ongoing W3C attempt at standardizing such APIs (which unfortunately adds a third syntax to the IE and Mozilla syntaxes).

So everyone is now realizing what potential it holds to make better web applications. Here's an interesting blog entry about the Google implementation.

But even this very nice Google feature has been available for ASP.NET developers under an easy to reuse form for years: check out this excellent WebControl.

This being said, to my knowledge, ASP.NET 2.0 remains the only server-side technology to natively support callbacks with ready-to-use server and client-side event-driven APIs.