Comment Articles

Longhorn: The nuts and bolts

John Carroll ZDNet.com

Published: 04 Nov 2003 15:55 GMT

Avalon is a complete upgrade to the process of writing Windows applications. In a way, Avalon turns Windows development into a more advanced and feature-rich version of Web-style development. This constitutes more than just a conceptual similarity. One of the means by which Longhorn applications can be consumed is by accessing them from a Web server using a browser (obviously, Internet Explorer), causing them to run within a "sandbox" managed by the .Net runtime. These downloadable applications act like more functional Web pages, as they have access to the full set of rich user interface controls offered as part of the Longhorn operating system. Longhorn also makes it easy to integrate common Web-paradigm concepts into desktop applications, such as page forward / page back logic, and "page history" functionality.

Windows applications still follow the same model seen in .Net WinForms and even Java's JFC, wherein controls are constructed within a class and laid out on screen either manually or through some programmatic layout functionality. However, the goal of Avalon is to make it possible for many user interfaces, even complex ones with a custom look and feel for individual controls or dynamic background content, to be defined in eXtensible Application Markup Language (XAML). XAML (pronounced "zamel") is an XML grammar oriented around painting user interfaces and laying out controls that maps directly to the Longhorn application object model. XAML isn't just an updated version of a Windows resource file, however. XAML enables the creation of complex user interfaces. You can define the drawing of polygons on the screen, and alter the rendering behaviour of those polygons by applying transformations to them. You can specify video that will be used as background to a particular control, and alter the display of that background video, again, using transformations. You can specify layout rules for controls within a particular container. In short, XAML is a complete XML grammar for user interface rendering that goes far beyond anything seen in old-fashioned WIN32 resource files.

The XAML approach to user interfaces creates a more complete separation of look and feel from code that makes the user interface functional (business logic, etc.) than was possible with WIN32. User interfaces defined in XAML can be generated using design tools, such as a future version of Adobe AfterEffects demoed at the opening keynote. The output of these tools can be passed to programmers responsible for hooking into events generated by the Avalon object model in order to make the user interface dynamic.