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Avalon: The Longhorn Presentation Layer

Posted December 18, 2003

Avalon is the code name for a core portion of Longhorn's presentation layer (see Figure 1). The phrase is used frequently to refer to the entire presentation layer, but strictly speaking, Avalon doesn't include Windows Forms or ASP.NET. Avalon provides an expanded view of the user interface layer with specific models for (1) documents and (2) media. Many other features, including controls and even the (3) Personalization and Profiling Services of ASP.NET, are defined as part of the Longhorn operating system as well.

Avalon interacts with the base operating system. For example, the "Longhorn: The Base Operating System" diagram includes a box for GDI/GDI+ that shows the graphics and input portions of the base operating system. Some of these items reside in the kernel; others do not. These core items aren't repeated in this view of Avalon, but Avalon depends on them to accomplish and display tasks.

Avalon offers a fundamental change in how you interact with your computer, and is probably the most significant UI change since Windows 1.0. It promises fundamental changes in technology, such as the way graphics are buffered internally. These features will include more efficient integration with sound and video, complex graphics displays, and sophisticated use of transparency. Microsoft believes the real advances here will come when experts in human computer interactions are free from the limitations of Win32 and can develop new controls and techniques that offer more of an immersive experience. An immersive experience decreases the effort you spend interacting with the computer and increases your ability to focus on the task at hand. A good computer game is an immersive experience—allowing you to concentrate on your goal, not the way you control your character.

It isn't in the diagram, but another aspect of Avalon is the declarative graphics language, Extensible Application Markup Language (XAML). XAML is an XML-based language for describing graphics. You'll be able to declare the new Avalon controls using XAML or in the source code of your language of choice. You'll also be able to use WinForm controls and XAML controls side by side as you migrate your applications.

Details are few at this time, but the models Microsoft presented at PDC also include slots for a (4) desktop composition engine and a (5) presentation object manager. —Kathleen Dollard