Adorners Overview

Adorners are a special type of FrameworkElement, used to provide visual cues to a user. Among other uses, Adorners can be used to add functional handles to elements or provide state information about a control.

This topic contains the following sections.

• About Adorners
• Implementing a Custom Adorner
• Rendering Behavior for Adorners
• Events and Hit Testing
• Adorning a Single UIElement
• Adorning the Children of a Panel
• Related Topics

About Adorners
An Adorner is a custom FrameworkElement that is bound to a UIElement. Adorners are rendered in an AdornerLayer, which is a rendering surface that is always on top of the adorned element or a collection of adorned elements. Rendering of an adorner is independent from rendering of the UIElement that the adorner is bound to. An adorner is typically positioned relative to the element to which it is bound, using the standard 2-D coordinate origin located at the upper-left of the adorned element.

Common applications for adorners include:

• Adding functional handles to a UIElement that enable a user to manipulate the element in some way (resize, rotate, reposition, etc.).
• Provide visual feedback to indicate various states, or in response to various events.
• Overlay visual decorations on a UIElement.
• Visually mask or override part or all of a UIElement.

Windows Presentation Foundation (WPF) provides a basic framework for adorning visual elements. The following table lists the primary types used when adorning objects, and their purpose. Several usage examples follow.

The adorners framework provided by Windows Presentation Foundation (WPF) is intended primarily to support the creation of custom adorners. A custom adorner is created by implementing a class that inherits from the abstract Adorner class.

Note:  The parent of an Adorner is the AdornerLayer that renders the Adorner, not the element being adorned.

The following example shows a class that implements a simple adorner. The example adorner simply adorns the corners of a UIElement with circles.

// Adorners must subclass the abstract base class Adorner.
public class SimpleCircleAdorner : Adorner
{
 // Be sure to call the base class constructor.
 public SimpleCircleAdorner(UIElement adornedElement)
   : base(adornedElement)
 {
 }

 // A common way to implement an adorner's rendering behavior is to override the OnRender
 // method, which is called by the layout system as part of a rendering pass.
 protected override void OnRender(DrawingContext drawingContext)
 {
   Rect adornedElementRect = new Rect(this.AdornedElement.DesiredSize);

   // Some arbitrary drawing implements.
   SolidColorBrush renderBrush = new SolidColorBrush(Colors.Green);
   renderBrush.Opacity = 0.2;
   Pen renderPen = new Pen(new SolidColorBrush(Colors.Navy), 1.5);
   double renderRadius = 5.0;

   // Draw a circle at each corner.
   drawingContext.DrawEllipse(renderBrush, renderPen, adornedElementRect.TopLeft, renderRadius, renderRadius);

The following image shows the SimpleCircleAdorner applied to a TextBox.

Read more: MSDN

Posted via email from Jasper-net