Techniques for WCAG 2.0

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SL14: Providing Custom Control Key Handling for Keyboard Functionality in Silverlight


This technique relates to:

User Agent and Assistive Technology Support Notes

See User Agents Supported for general information on user agent support.


The objective of this technique is to implement built-in handling of key events in a custom control. If a custom control is correctly implemented, then any Silverlight applications that include the control can rely on the built-in key handling for some or all of the desired keyboard equivalence of a control's functionality.

Defining a custom control requires that the control author write a default template for the control and also the initialization logic, including the default implementations for built-in keyboard equivalence. Typically, control authors provide keyboard equivalence for any actions that can be activated by a mouse click on the control surface, and that are not already providing a keyboard equivalence through the implementation of a composite part.

All input events report a specific source that is communicated to handler code as an event parameter, so that the application author can identify which element in their Silverlight UI was being interacted with, and the application can perform an action that is relevant to that user input. In the case of mouse events, the event source is the element that the mouse pointer is over at the time. In the case of key events, the event source is the element that has focus. The element that has focus is visually indicated so that the user knows which element they are interacting with (see SL2: Changing The Visual Focus Indicator in Silverlight). Assistive technologies often have parallel conventions whereby the user is made aware of which element is visually focused and is the current input scope presented by the assistive technology.

Browser hosts and keyboard events

Silverlight is hosted as a plug-in inside a browser host. The Silverlight runtime only receives the input events that the browser host forwards to hosted plug-ins through a browser-specific program access layer. Occasionally the browser host receives input that the browser host itself handles in some way, and does not forward the keyboard event. An example is that a Silverlight application hosted by an Internet Explorer browser host on Windows operating system cannot detect a press of the ALT key, because Internet Explorer processes this input and performs the action of bringing keyboard focus to the Internet Explorer menu bar. Silverlight authors might need to be aware of browser-specific input handling models and not rely on key events for keys that are essentially reserved for use by a browser host. For more information, see Keyboard Support.

Application authors should choose keys that avoid browser conflicts, but still are a natural choice for an accelerator. Using the CTRL key as a modifier is a convention that is frequently used in existing Silverlight applications.

Informing users of which keys to use for keyboard equivalence

If a control supports user interaction, which key to use to engage the keyboard equivalent behavior is not always obvious. One way to inform users of the possible key options that a control supports is to author an AutomationProperties.HelpText value in the application UI that gives instructions such as "Press the plus key to increment value". This is up to the application author to do; the control definitions do not provide a means to set HelpText by default, because any display technique for end user help is potentially too application-specific to be encapsulated in control definitions. Application authors might also consider using tooltips, providing a menu framework that visually indicates the key associations (perhaps with the Windows key-underlined convention), providing a generalized application Help, or displaying plain text in the user interface.

The On* method pattern in Silverlight

Silverlight classes often have methods that follow the naming pattern On* where the star is a string that also identifies an event. These On* methods are prewired event handlers, defined as virtual methods so that subclasses can override them. A consumer of a control class can change or augment the default behavior associated with that event by overriding the method, and typically also calls the base implementation so that the base functionality is preserved. This principle is illustrated in Example 1 by the overrides of OnGotFocus and OnLostFocus. Controls that introduce new events should consider also exposing a virtual On* method that pairs with the event, so that consumers of the custom control can use the same pattern.


Example 1: KeyNumericUpDown Control That Handles Arrow Key Equivalence for + and - Buttons

This example implements a custom Silverlight control that displays an integer value, and can increment or decrement the integer value based on user actions. When a user interacts with the control, the user can click the "+" and "-" buttons that are component parts of the control. The "+" and "-" button parts are deliberately not in the Silverlight tab sequence, because this is intended to be a complete control, where only the control itself (and not its constituent parts) are focusable and are reported as an element to the accessibility framework. To provide keyboard equivalence, the control defines a KeyUp handler. The design of the control treats an Up Arrow key press as equivalent to activating the "+" button, and the Down Arrow key as equivalent to activating the "-" button. The control implementation reinforces this behavior by having the button Click event handlers and the cases of the KeyUp handler call the same underlying helper functions (Increment() and Decrement()).

Handling the + and - keys as alternate or additional keyboard equivalents for the actions is also possible (if that is desired, handler would check for Key.Add or Key.Subtract values).

The following is the XAML-defined control template for this control.

   <Style TargetType="local:KeysNumericUpDown">
       <Setter Property="BorderThickness" Value="1"/>
       <Setter Property="Height" Value="22"/>
       <Setter Property="BorderBrush">
               <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                   <GradientStop Color="#FFA3AEB9" Offset="0"/>
                   <GradientStop Color="#FF8399A9" Offset="0.375"/>
                   <GradientStop Color="#FF718597" Offset="0.375"/>
                   <GradientStop Color="#FF617584" Offset="1"/>
       <Setter Property="Template">
               <ControlTemplate TargetType="local:KeysNumericUpDown">
                   <Grid x:Name="CompositionRoot">
                       <TextBox x:Name="Text" IsTabStop="False" AcceptsReturn="False"
 BorderThickness="0" Foreground="{TemplateBinding Foreground}" FontWeight="{TemplateBinding FontWeight}"
 FontStyle="{TemplateBinding FontStyle}" FontStretch="{TemplateBinding FontStretch}"
 FontSize="{TemplateBinding FontSize}" FontFamily="{TemplateBinding FontFamily}" MinWidth="20"
 TextAlignment="Right" VerticalAlignment="Center"  TextWrapping="NoWrap" Text="{TemplateBinding Value}">
                                   <Style TargetType="TextBox">
                                       <Setter Property="Template">
                                               <ControlTemplate TargetType="TextBox">
                                                   <ScrollViewer x:Name="ContentElement" BorderThickness="0" Padding="0"/>
                       <StackPanel Orientation="Vertical" Grid.Column="1">
                       <Button Width="18" Height="18" IsTabStop="False" x:Name="plusButton">+</Button>
                       <Button Width="18" Height="18" IsTabStop="False" x:Name="minusButton">-</Button>
                       <Border x:Name="FocusVisualElement" BorderBrush="#FF45D6FA" BorderThickness="{TemplateBinding BorderThickness}" 
                       CornerRadius="1,1,1,1" IsHitTestVisible="False" Opacity="0"/>

The following is the implementation of the control class. Overrides of the base class are omitted for clarity, as is automation support. Note the event wiring in OnApplyTemplate; this is a common pattern for custom control definitions.

When this control is included in application UI, the usage is very simple. Note that there are no key handlers on this instance; the necessary key handling to wire up the increment/decrement logic is already built-in to all instances of the control.

This example is shown in operation in the working example of Numeric Up / Down control.


Resources are for information purposes only, no endorsement implied.



  1. Using a browser that supports Silverlight, open an HTML page that references a Silverlight application through an object tag.

  2. Press TAB key to move keyboard focus to various element parts of the user interface, and in particular to areas that are known to be custom control implementations.

  3. Check that custom key commands exist for all these user interface actions and that these key commands are made known to the user.

Expected Results

#3 is true.

If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.

Techniques are Informative

Techniques are informative—that means they are not required. The basis for determining conformance to WCAG 2.0 is the success criteria from the WCAG 2.0 standard—not the techniques. For important information about techniques, please see the Understanding Techniques for WCAG Success Criteria section of Understanding WCAG 2.0.