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Windows Presentation Foundation in .NET 4 : Introducing WPF - The Architecture of WPF

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WPF uses a multilayered architecture. At the top, your application interacts with a high-level set of services that are completely written in managed C# code. The actual work of translating .NET objects into Direct3D textures and triangles happens behind the scenes, using a lower-level unmanaged component called milcore.dll. milcore.dll is implemented in unmanaged code because it needs tight integration with Direct3D and because it's extremely performance-sensitive.

Figure 1 shows the layers at work in a WPF application.

Figure 1. The architecture of WPF

Figure 1-2 includes these key components:

  • PresentationFramework.dll. This holds the top-level WPF types, including those that represent windows, panels, and other types of controls. It also implements higher-level programming abstractions such as styles. Most of the classes you'll use directly come from this assembly.

  • PresentationCore.dll. This holds base types, such as UIElement and Visual, from which all shapes and controls derive. If you don't need the full window and control abstraction layer, you can drop down to this level and still take advantage of WPF's rendering engine.

  • WindowsBase.dll. This holds even more basic ingredients that have the potential to be reused outside of WPF, such as DispatcherObject and DependencyObject, which introduces the plumbing for dependency properties .

  • milcore.dll. This is the core of the WPF rendering system and the foundation of the Media Integration Layer (MIL). Its composition engine translates visual elements into the triangle and textures that Direct3D expects. Although milcore.dll is considered part of WPF, it's also an essential system component for Windows Vista and Windows 7. In fact, the Desktop Window Manager (DWM) uses milcore.dll to render the desktop.

    NOTE

    milcore.dll is sometimes referred to as the engine for "managed graphics." Much as the common language runtime (CLR) manages the lifetime of a .NET application, milcore.dll manages the display state. And just as the CLR saves you from worrying about releasing objects and reclaiming memory, milcore.dll saves you from thinking about invalidating and repainting a window. You simply create the objects with the content you want to show, and milcore.dll paints the appropriate portions of the window as it is dragged around, covered and uncovered, minimized and restored, and so on.

  • WindowsCodecs.dll. This is a low-level API that provides imaging support (for example, processing, displaying, and scaling bitmaps and JPEGs).

  • Direct3D. This is the low-level API through which all the graphics in a WPF application are rendered.

  • User32. This is used to determine what program gets what real estate. As a result, it's still involved in WPF, but it plays no part in rendering common controls.

The most important fact that you should realize is Direct3D renders all the drawing in WPF. It doesn't matter whether you have a modest video card or a much more powerful one, whether you're using basic controls or drawing more complex content, or whether you're running your application on Windows XP, Windows Vista, or Windows 7. Even two-dimensional shapes and ordinary text are transformed into triangles and passed through the 3-D pipeline. There is no fallback to GDI+ or User32.

1. The Class Hierarchy

Figure 2 shows a basic overview with some of the key branches of the class hierarchy.

Figure 2. The fundamental classes of WPF

The following sections describe the core classes in this diagram. Many of these classes lead to whole branches of elements (such as shapes, panels, and controls).

NOTE

The core WPF namespaces begin with System.Windows (for example, System.Windows, System.Windows.Controls, and System.Windows.Media). The sole exception is namespaces that begin with System.Windows.Forms, which are part of the Windows Forms toolkit.

1.1. System.Threading.DispatcherObject

WPF applications use the familiar single-thread affinity (STA) model, which means the entire user interface is owned by a single thread. It's not safe to interact with user interface elements from another thread. To facilitate this model, each WPF application is governed by a dispatcher that coordinates messages (which result from keyboard input, mouse movements, and framework processes such as layout). By deriving from DispatcherObject, every element in your user interface can verify whether code is running on the correct thread and access the dispatcher to marshal code to the user interface thread.

1.2. System.Windows.DependencyObject

In WPF, the central way of interacting with onscreen elements is through properties. Early on in the design cycle, the WPF architects decided to create a more powerful property model that baked in features such as change notification, inherited default values, and more economical property storage. The ultimate result is the dependency property feature. By deriving from DependencyObject, WPF classes get support for dependency properties.

1.3. System.Windows.Media.Visual

Every element that appears in a WPF window is, at heart, a Visual. You can think of the Visual class as a single drawing object that encapsulates drawing instructions, additional details about how the drawing should be performed (such as clipping, opacity, and transformation settings), and basic functionality (such as hit testing). The Visual class also provides the link between the managed WPF libraries and the milcore.dll that renders your display. Any class that derives from Visual has the ability to be displayed on a window. If you prefer to create your user interface using a lightweight API that doesn't have the higher-level framework features of WPF, you can program directly with Visual objects.

1.4. System.Windows.UIElement

UIElement adds support for WPF essentials such as layout, input, focus, and events (which the WPF team refers to by the acronym LIFE). For example, it's here that the two-step measure and arrange layout process is defined,. It's also here that raw mouse clicks and key presses are transformed to more useful events such as MouseEnter. As with properties, WPF implements an enhanced event-passing system called routed events.

1.5. System.Windows.FrameworkElement

FrameworkElement is the final stop in the core WPF inheritance tree. It implements some of the members that are merely defined by UIElement. For example, UIElement sets the foundation for the WPF layout system, but FrameworkElement includes the key properties (such as HorizontalAlignment and Margin) that support it. UIElement also adds support for data binding, animation, and styles, all of which are core features.

1.6. System.Windows.Shapes.Shape

Basic shapes classes, such as Rectangle, Polygon, Ellipse, Line, and Path, derive from this class. These shapes can be used alongside more traditional Windows widgets such as buttons and text boxes.

1.7. System.Windows.Controls.Control

A control is an element that can interact with the user. It obviously includes classes such as TextBox, Button, and ListBox. The Control class adds additional properties for setting the font and the foreground and background colors. But the most interesting detail it provides is template support, which allows you to replace the standard appearance of a control with your own stylish drawing.

NOTE

In Windows Forms programming, every visual item in a form is referred to as a control. In WPF, this isn't the case. Visual items are called elements, and only some elements are actually controls (those that can receive focus and interact with the user). To make this system even more confusing, many elements are defined in the System.Windows.Controls namespace, even though they don't derive from System.Windows.Controls.Control and aren't considered controls. One example is the Panel class.

1.8. System.Windows.Controls.ContentControl

This is the base class for all controls that have a single piece of content. This includes everything from the humble Label to the Window. The most impressive part of this model  is the fact that this single piece of content can be anything from an ordinary string to a layout panel with a combination of other shapes and controls.

1.9. System.Windows.Controls.ItemsControl

This is the base class for all controls that show a collection of items, such as the ListBox and TreeView. List controls are remarkably flexible—for example, using the features that are built into the ItemsControl class, you can transform the lowly ListBox into a list of radio buttons, a list of check boxes, a tiled display of images, or a combination of completely different elements that you've chosen. In fact, in WPF, menus, toolbars, and status bars are actually specialized lists, and the classes that implement them all derive from ItemsControl.

1.10. System.Windows.Controls.Panel
This is the base class for all layout containers—elements that can contain one or more children and arrange them according to specific layout rules. These containers are the foundation of the WPF layout system, and using them is the key to arranging your content in the most attractive, flexible way possible.
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