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Windows Presentation Foundation

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This subsystem is a part of .NET Framework 3.0

The Windows Presentation Foundation (or WPF), formerly code named Avalon, is the graphical subsystem feature of the .NET Framework 3.0 (formerly called WinFX)[1] and is directly related to XAML.[2] It is pre-installed in Windows Vista,[3] the latest version of the Microsoft Windows operating system. WPF is also available for installation on Windows XP SP2 and Windows Server 2003. It provides a consistent programming model for building applications and provides a clear separation between the UI and the business logic. A WPF application can be deployed on the desktop or hosted in a web browser. It also enables richer control, design, and development of the visual aspects of Windows programs. It aims to unify a host 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.

Microsoft Silverlight is a web-based subset of WPF. During development it was named WPF/E, which stood for "Windows Presentation Foundation Everywhere". Silverlight is based on XAML and JScript. The Silverlight subset enables Flash-like web and mobile applications with the same code as Windows .NET applications. 3D features are not supported, but XPS, vector-based drawing and hardware acceleration are included.

Architecture

The WPF architecture. Blue elements are Windows components; brown ones are WPF components.

The architecture of Windows Presentation Foundation spans across both managed code as well as native code components; however, the public API exposed is only available via managed code. While majority of WPF is in managed code, the composition engine which renders the WPF applications is a native component. It is named as Media Integration Layer (MIL) and resides as 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 rendering it to the screen.[4][5] The media codecs are also implemented in unmanaged code, and are shipped as windowscodecs.dll.[4] In the managed world, PresentationCore (presentationcore.dll) provides a managed wrapper for MIL as well as implements the core services for WPF,[4] 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.[5] PresentationFramework (presentationframework.dll) implements the end-user presentational features, including layouts, time-dependent, story-board based animations, and data binding.[5]

WPF exposes a property system for WPF 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.[5] 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.[5] 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 is maintained as a class of Visual objects. Visual objects provide a managed interface to a composition tree which is maintaned 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.[5] 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.[6] 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, 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.[6] Once the properties are set, WPF will invoke the rendering thread to render the changes. 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.[5][7]

Features

Part of a series of articles on
Windows Vista
New features
Siblings

The following is a partial list of WPF features.

Graphical Services

  • All graphics (including desktop items like windows) are Direct3D applications.
    • This aims to provide a unified avenue for displaying graphics, as well as more advanced graphical features.
    • Routing the graphics through Direct3D allows Windows to offload graphics tasks to the Graphics Processing Unit found on the computer's graphics card. This reduces the workload on the computer's Central processing unit.
    • Supports vector-based graphics, which allow lossless scaling.
    • Supports 3D model rendering and interaction in 2D applications.
    • Interactive 2D content can be overlaid on 3D surfaces, natively.[8]

Interoperability

  • WPF provides interoperability with Win32: Via hosting, one can use Windows Presentation Foundation inside existing Win32 code, or one can use existing Win32 code inside Windows Presentation Foundation.[9]
  • Interoperability with Windows Forms is also possible through the use of the ElementHost and WindowsFormsHost classes.

Media Services

  • WPF provides shape primitives for 2D graphics along with a built-in set of brushes, pens, geometries, and transforms.
  • The 3D capabilities in WPF are a subset of the full featureset provided by Direct3D. However, WPF provides tighter integration with other features like user interface (UI), documents, and media. This makes it possible to have 3D UI, 3D documents, and 3D media.
  • There is support for most common image formats.
  • WPF supports the video formats WMV, MPEG and some AVI files.
  • WPF supports time-based animations, in contrast to the frame-based approach. This delinks the speed of the animation from how the system is performing.
  • Text rendering is supported using ClearType. This provides for sub-pixel positioning, natural advance widths, and Y-direction anti-aliasing. WPF also supports OpenType font features.

Animations

  • Time-based animation. Scene redraws are time triggered.[10]
  • Presentation timers are initialized and managed by WPF.[10]
  • Scene changes co-ordinated by using a storyboard.[10]
  • Animations can be triggered by other external events, including user action.[10]
  • Animation effects can be defined on a per-object basis, which can be accessed directly from XAML markup.[10]
  • Ships with a set of predefined effects, such as fade out for all controls.[10]

Data binding

  • WPF has a built-in set of data services to enable application developers to bind and manipulate data within applications. There exists support for three types of data binding:
    • one time: where the client ignores updates on the server.
    • one way: where the client has read-only access to data.
    • two way: where client can read from and write data to the server.
  • LINQ queries, specifically LINQ to XML, can also act as data sources for data binding.[8]
  • Binding of data has no bearing on its presentation. WPF provides data templates to control presentation of data.

User interface

  • A set of built-in controls is provided as part of WPF, containing items such as button, menu, and list box. Lacks a DataGrid control but 3rd-party vendors are already offering some.
  • A powerful concept in the WPF is the logical separation of a control from its appearance.
    • A control's template can be overridden to completely change its visual appearance.
    • A control can contain any other control or layout, allowing for unprecedented control over composition.
  • Features retained mode graphics, so that applications do not have to be bothered with repainting the display.

Annotations

  • Annotations can be applied on a per-object basis, for objects in a Document or FlowDocument.
  • WPF only provides the capability for creating, storing and managing annotations; each application must expose the UI on their own.

Imaging

  • WPF can natively access Windows Imaging Component (WIC) codecs and APIs allowing developers to write image codecs for their specific image file formats.

Effects

  • WPF provides for bitmap effects, however, they are rendered in software. GPU features like pixel shaders are not used for bitmap effects.
  • Special effects such as dropshadows and blurring are built in.
  • Other effects such as reflections can be easily implemented.

Documents

  • WPF natively supports paginated documents. It provides the DocumentReader class, which is for reading fixed layout documents. The FlowDocumentReader class offers different view modes such as per-page or scrollable and also reflows text if the viewing area is resized.
  • Natively supports XML Paper Specification documents.
  • Supports reading and writing paginated documents using Open Packaging Convention.

Text

WPF includes a number of extremely rich typographic and text rendering features that were not available in GDI. This is the first Windows programming interface to expose OpenType features to software developers, supporting both OpenType, TrueType, and OpenType CFF (Compact Font Format) fonts.

Support for OpenType typographic features includes:

The WPF text engine also supports built-in spell checking. It also supports such features as automatic line spacing, enhanced international text, language-guided line breaking, hyphenation, and justification, bitmap effects, transforms, and text effects such as shadows, blur, glow, rotation etc. Animated text is also supported; this refers to animated glyphs, as well as real-time changes in position, size, color, and opacity of the text.

WPF text rendering takes advantage of advances in ClearType technology, such as sub-pixel positioning, natural advance widths, Y-direction anti-aliasing, hardware-accelerated text rendering, as well as aggressive caching of pre-rendered text in video memory.[11] However, due to the resolution-independent architecture of WPF, ClearType cannot be turned off in WPF applications.[12]

The extent to which glyphs are cached is dependent on the video card. DirectX 10 cards are able to cache the font glyphs in video memory, then perform the composition (assembling of character glyphs in the correct order, with the correct spacing), alpha-blending (application of anti-aliasing), and RGB blending (ClearType's sub-pixel color calculations), entirely in hardware. This means that only the original glyphs need to be stored in video memory once per font (Microsoft estimates that this would require 2 MB of video memory per font), and other operations such as the display of anti-aliased text on top of other graphics – including video – can also be done with no computation effort on the part of the CPU. DirectX 9 cards are only able to cache the alpha-blended glyphs in memory, thus requiring the CPU to handle glyph composition and alpha-blending before passing this to the video card. Caching these partially-rendered glyphs requires significantly more memory (Microsoft estimates 5 MB per process). Cards that don't support DirectX 9 have no hardware-accelerated text rendering capabilities.

Alternative input

WPF supports digital ink-related functionality.

Accessibility

WPF supports Microsoft UI Automation to allow developers to create accessible interfaces.

XAML

Main article: Extensible Application Markup Language

Following the success of markup languages for web development, WPF introduces a new language known as eXtensible Application Markup Language (XAML) (pronounced "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 API's.

As in web development, both layouts and specific themes are well suited to markup, but XAML is not required for either. Indeed, all elements of WPF may be coded in a .NET language (C#, VB.net). The XAML code can ultimately be compiled into a managed assembly in the same way all .NET languages are, which means that the use of XAML for development does not incur a performance cost. XAML can also be compiled and run "on demand" similar to an HTML web-page.

Although XAML has been introduced as an integral part of WPF, the XAML standard itself is not specific to WPF (or even .NET). XAML can also be used to develop applications using any programming API and is in itself language independent. Nevertheless, special care has been taken in developing the WPF APIs to maximize interoperability with the declarative model that XAML introduces.

It is likely that many applications, such as Microsoft PowerPoint and Word, will support exporting their content to XAML.

There are several subset (or profiles) of XAML, such as:

  • XAML Presentation (XAML-P) - incorporates all items encompassing WPF v1.0
  • XML Paper Specification (XPS) - a subset of XAML-P for representing fixed format documents and used as the spool format for the printing subsystem in Windows Vista

There are also profiles specific to Workflow Foundation, and other domain specific subsets will likely emerge in the future.

Tools

There are a number of development tools available for developing Windows Presentation Foundation applications, most currently of beta quality, some commercially released.

  • Microsoft Cider is the XAML designer in the form of an add-in for the Visual Studio 2005 IDE for building Windows Presentation Foundation applications. It is available in CTP form for Visual Studio 2005, there are no plans for an RTM version for Visual Studio 2005. Cider is integrated into Visual Studio 2008. [13]
  • Microsoft Expression Blend is a designer-oriented tool that provides a canvas for the creation of WPF applications with 2D and 3D graphics, text and forms content. It generates XAML that may be exported into other tools.
  • Microsoft Expression Design is a bitmap and 2D-vector graphics tool that allows export to XAML.

Deployment

  • WPF isn't just for building traditional standalone applications. Its deployment model offers both standalone and XAML Browser Applications (XBAP) flavors. The programming model for building either flavor of application is similar.
    • Standalone applications are those that have been locally installed on the computer using software such as ClickOnce or Windows Installer (MSI) and which run on the desktop. Standalone applications are considered full trust and have full access to a computer's resources.
    • XAML Browser Applications (XBAPs) are programs that are hosted inside a web browser such as Firefox or Internet Explorer. Hosted applications run in a partial trust sandbox environment, and are not given full access to the computer's resources and not all WPF functionality is available. The hosted environment is intended to protect the computer from malicious applications. Starting an XBAP (pronounced "ex-bap") from an HTML page or vice versa is seamless (there is no security or installation prompt). Although one gets the perception of the application running in the browser, it actually runs in an out-of-process executable different from the browser. As of the release of .NET Framework 3.0, XBAPs only run in Internet Explorer. With the release of .NET Framework 3.5, they will also run in Mozilla Firefox.

Microsoft Silverlight

Main article: Microsoft Silverlight

Silverlight is a cross-platform, cross-browser XAML-based WPF technology[14] that provides features such as video, vector graphics, and animations to multiple operating systems including Windows Vista, Windows XP, and Mac OS X. Specifically, it is currently provided as an add-on for Mozilla Firefox, Internet Explorer 6 and above, and Apple Safari. Silverlight and WPF only share the XAML presentation layer.

These extensions allow browsers to use Silverlight's graphics capabilities. The browser extensions are similar to Adobe Flash, a highly popular graphic plug-in available for most browsers. Internet Explorer has additional native support for WPF through the use of XBAP, available in Windows Vista and Windows XP with the .NET Framework version 3.0 installed. Silverlight is supported in older versions of the browser.

Silverlight works in concert with XAML and is scriptable with JScript. Version 1.1 will also contain a version of the Common Language Runtime so it can execute VB.Net and C# code in a sandbox with limited local file access.

Silverlight is seen as a direct competitor of Adobe Flash Player, created by Macromedia which is now a part of Adobe Systems.

The first CTP of Silverlight, then named by its development code-name of WPF/E, was released in December 2006. Additional CTPs have been released in January and February. SilverLight version 1.0 was released on September 05, 2007.

WPF Applications

WPF is intended to be the next-generation graphics API for Windows applications on the desktop. The following are some of Microsoft's own product or other third-party mainstream applications written in WPF:

Main article: List of WPF applications

.NET Micro Framework

The .NET Micro Framework includes a GUI object model loosely based on WPF, although without support for XAML.

References

  1. ^ MSDN .NET Framework: Windows Presentation Foundation
  2. ^ MSDN .NET Development: WPF : XAML Overview
  3. ^ Windows Vista: Windows Presentation Foundation (Avalon) FAQ
  4. ^ a b c Chris Anderson. "MIL information". Retrieved 2007-10-14.
  5. ^ a b c d e f g "WPF Architecture". MSDN. Retrieved 2007-10-14.
  6. ^ a b "Threading Model". MSDN. Retrieved 2007-10-14.
  7. ^ "The Layout System". MSDN. Retrieved 2007-10-14.
  8. ^ a b "What's New in WPF 3.5? Here's Fifteen Cool Features..." Retrieved 2007-10-14.
  9. ^ WPF and Win32 Interoperation Overview
  10. ^ a b c d e f "Aimation overview". MSDN. Retrieved 2007-10-14.
  11. ^ MSDN Library : .NET Development : WPF : ClearType Overview
  12. ^ WPF ClearType anti-aliasing cannot be turned off
  13. ^ Visual Studio 2005 extensions for .NET Framework 3.0 (WCF & WPF), November 2006 CTP
  14. ^ ASP.NET : WPF/E - December 2006 Community Technology Preview

See also
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