Windows Presentation Foundation
The Windows Presentation Foundation (or WPF), formerly code-named Avalon, is a graphical subsystem in .NET Framework 3.0 (formerly called WinFX), which uses a markup language, known as XAML, for rich user interface development. WPF is included with Windows Vista and Windows Server 2008, and is also available for Windows XP Service Pack 2 or later, and Windows Server 2003. It provides a consistent programming model for building applications and provides a clear separation between the user interface and the business logic. A WPF application can be deployed on the desktop or hosted in a web browser. It also enables rich control, design, and development of the visual aspects of Windows programs. It aims to unify a number of application services: user interface, 2D and 3D drawing, fixed and adaptive documents, advanced typography, vector graphics, raster graphics, animation, data binding, audio and video. Although WinForms will continue to be widely used, and Microsoft has created only a few WPF applications, the company promotes WPF for line-of-business applications. Future Microsoft software will be built on WPF, for example, the code editor in Visual Studio 2010 has been rebuilt on WPF.
Microsoft Silverlight is a web-based subset of WPF that enables Flash-like web and mobile applications with the same programming model as .NET applications. 3D features are not supported, but XPS and vector-based drawing are included.
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The architecture of Windows Presentation Foundation spans across both managed code and native code components. However, the public API exposed is only available via managed code. While the majority of WPF is in managed code, the composition engine which renders the WPF applications is a native component. It is named Media Integration Layer (MIL) and resides in milcore.dll. It interfaces directly with DirectX and provides basic support for 2D and 3D surfaces, timer-controlled manipulation of contents of a surface with a view to exposing animation constructs at a higher level, and compositing the individual elements of a WPF application into a final 3D "scene" that represents the UI of the application and renders it to the screen. The media codecs are also implemented in unmanaged code, and are shipped as windowscodecs.dll. In the managed world, PresentationCore (presentationcore.dll) provides a managed wrapper for MIL and implements the core services for WPF,including a property system that is aware of the dependencies between the setters and consumers of the property, a message dispatching system by means of a Dispatcher object to implement a specialized event system and services which can implement a layout system such as measurement for UI elements. PresentationFramework (presentationframework.dll) implements the end-user presentational features, including layouts, time-dependent, story-board based animations, and data binding.
WPF exposes a property system for objects which inherit from DependencyObject, that is aware of the dependencies between the consumers of the property, and can trigger actions based on changes in properties. Properties can be either hard coded values or expressions, which are specific expressions that evaluate to a result. In the initial release, however, the set of expressions supported is closed. The value of the properties can be inherited from parent objects as well. WPF properties support change notifications, which invoke bound behaviors whenever some property of some element is changed. Custom behaviors can be used to propagate a property change notification across a set of WPF objects. This is used by the layout system to trigger a recalculation of the layout on property-changes, thus exposing a declarative programming style for WPF, whereby almost everything, from setting colors and positions to animating elements can be achieved by setting properties. This allows WPF applications to be written in XAML, which is a declarative mark-up language, by binding the keywords and attributes directly to WPF classes and properties.
The UI elements of an WPF application are maintained as a class of Visual objects. Visual objects provide a managed interface to a composition tree which is maintained by Media Integration Layer (MIL). Each element of WPF creates and adds one or more composition nodes to the tree. The composition nodes contain rendering instructions, such as clipping and transformation instructions, along with other visual attributes. Thus the entire application is represented as a collection of composition nodes, which are stored in a buffer in the system memory. Periodically, MIL walks the tree and executes the rendering instructions in each node, thus compositing each element on to a DirectX surface, which is then rendered on screen. MIL uses the painter's algorithm, where all the components are rendered from back of the screen to the front, which allows complex effects like transparencies to be easily achieved. This rendering process is hardware accelerated using the GPU. The composition tree is cached by MIL, creating a retained mode graphics, so that any changes to the composition tree needs only to be incrementally communicated to MIL. This also frees the applications of managing repainting the screen, MIL can do that itself as it has all the information necessary. Animations can be implemented as time-triggered changes to the composition tree. On the user visible side, animations are specified declaratively, by setting some animation effect to some element via a property and specifying the duration. The code-behind updates the specific nodes of the tree, via Visual objects, to represent both the intermediate states at specified time intervals as well as the final state of the element. MIL will render the changes to the element automatically.
All WPF applications start with two threads: one for managing the UI and another background thread for handling rendering and repainting. Rendering and repainting is managed by WPF itself, without any developer intervention. The UI thread houses the Dispatcher (via an instance of DispatcherObject), which maintains a queue of UI operations that need to be performed (as a tree of Visual objects), sorted by priority. UI events, including changing a property that affects the layout, and user interaction events raised are queued up in the dispatcher, which invokes the handlers for the events. Microsoft recommends that the event handlers only update the properties to reflect new content for application responsiveness; the new content be generated or retrieved in a background thread. The render thread picks up a copy of the visual tree and walks the tree calculating which components will be visible and renders them to Direct3D surfaces. The render thread also caches the visual tree, so only changes to the tree need to be communicated, which will result in updating only the changed pixels. WPF supports an extensible layout model. Layout is divided into two phases: Measure and Arrange. The Measure phase recursively calls all elements and determine the size they will take. In the Arrange phase, the child elements are recursively arranged by their parents, invoking the layout algorithm of the layout module in use.
Following the success of markup languages for web development, WPF introduces a new language known as eXtensible Application Markup Language (XAML) (pronounced as "Zammel"), which is based on XML. XAML is designed as a more efficient method of developing application user interfaces.
The specific advantage that XAML brings to WPF is that XAML is a completely declarative language. In a declarative programming language, the developer (or designer) describes the behavior and integration of components without the use of procedural programming. This allows someone with little or no traditional programming experience to create an entire working application with no programming. Although it is rare that an entire application will be built completely in XAML, the introduction of XAML allows application designers to more effectively contribute to the application development cycle. Using XAML to develop user interfaces also allows for separation of model and view; which is considered a good architectural principle. In XAML, elements and attributes map to classes and properties in the underlying APIs.
Windows Presentation Foundation (hereafter referred to as WPF) is a new API for creating graphical user interfaces for the Windows platform. It is an alternative to WinForms that further empowers developers by providing an API capable of taking full advantage of the multimedia facilities of the modern PC. Unlike WinForms, it does not wrap Win32 but is completely new, built from the ground up using .NET. The fundamental power of WPF comes from the fact that it is vector based, hardware accelerated and resolution independent.
Reminiscent of WinForms, you’ll find many of the controls you are used to when building applications: Button, ComboBox, ListBox, etc. However, in WPF, your UI design is represented in a completely different fashion. Instead of using a designer generated code file or a resource file as the source of a UI definition, WPF uses XML. Specifically, it uses XAML. If you want to work with WPF (part of .NET 3.0) there are a few things you are going to need. First off, .NET 3.0 was designed to work on Vista, XP (SP2) and Windows Server 2003 (SP1). Any one of these OS’s and the .NET 3.0 runtime will allow you to run applications built with WPF.
