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Technique SL29:Using Silverlight "List" Controls to Define Blocks that can be Bypassed

Obsolete

Microsoft ended support for Silverlight in 2021, and authors are encouraged to use HTML for accessible web content.

About this Technique

This technique is not referenced from any Understanding document.

This technique applies to content implemented in Microsoft Silverlight.

Description

The objective of this technique is to use some of the basic user interface objects in Silveright to produce blocks of content that are identified as a "List" to accessibility frameworks and to Silverlight's own tab sequence navigation behavior.

Using the "List" technique results in a tab sequence behavior whereby that element is treated as a single tab stop, even if that element has consituent elements (the list items) that would otherwise be considered additional tab stops in the main tab sequence. In the default key handling, when the user presses the TAB key while focus is on a List, the focus goes to the next element after the List. To focus the items of the list, the user would press the Arrow keys rather than TAB. In the Silverlight programming model for controls, this tab sequence behavior is expressed by the TabNavigation property holding the value of Once. The Silverlight ListBox is a control that reports itself as "List" role, and that has a default TabNavigation.Once value. Thus ListBox as per this technique is a lightweight technique for producing blocks that can be bypassed. It is specifically a lightweight technique because it can be accomplished by writing simple application-level XAML and using only the Silverlight core libraries.

Silverlight also supports more full-featured techniques for producing bypass blocks that are based on common user interface features such as menus or toolbars. However, using toolbars in Silverlight is inherently not as lightweight because the Silverlight core libraries themselves do not include a ready-made toolbar. Silverlight provides a ContextMenu as part of the Silverlight Toolkit extensions, but the behavior of this particular menu does not easily address the bypass block scenario. Silverlight includes all the infrastructure and necessary base classes for defining a toolbar or a menu that could address the bypass block scenario. Many third-party control implementations of menus and toolbars exist, either as part of control packages that are sold by control vendors, or through community mechanisms such as CodePlex or third-party sites that provide free source code. For some examples, see the following:

If application authors use a built-in control such as ListBox where the accessibility framework reported role is not traditionally associated with a navigation role, it is a best practice to set AutomationProperties.Name such that the name informs the user of the purpose of the list control. Otherwise, the role alone leaves this ambiguous. For example, an author might name the list control "Navigation control".

Often the List control itself is focusable. So long as the List control has a visual focus indicator, that behavior might be acceptable. However, it might provide a better user experience to deliberately have the List itself non-focusable, and instead have focus fall to the first List item when focus reaches that region. Otherwise, the List might be perceived as an "extra" tab stop to some users. To enable that behavior, set IsTabStop to false on the List control. The List itself still provides the intended tab navigation behavior, and is still reported and identified to accessibility frameworks and assistive technologies, even when the List container is not focusable. This is shown in Example 1, by setting IsTabStop as part of a Style.

When an accessibility framework presents a List, assistive technologies are generally expected to continue to support use of the same key behavior as the default behavior, and to report to users that the item is a List when it is focused. If assistive technologies use the accessibility framework APIs for navigation, the items in the list are considered child elements. Navigating either by spatial direction (e.g. NAVDIR_RIGHT in MSAA) or sequential direction (e.g. NAVDIR_NEXT in MSAA) skips the list items and goes to the spatial/next peer.

Examples

Example 1: Customize the behavior and appearance of a ListBox to construct a navigation control that can be bypassed

In this example, several properties that influence the items presentation behavior of the Silverlight core control ListBox are adjusted to make it suitable for a navigation control. The behavior of this control is that when the tab sequence reaches the control, "next" or spatial navigation continues on to other controls, rather than through the child controls of the list's items/options. This is enabled and properly reported because ListBox reports its accessibility framework role as "List", uses TabNavigation = Once as default (because it is the default, TabNavigation does not have to be set explicitly in the markup). ListBox has default key handling for the arrow keys (to enable traversing the choices in the menu by keyboard-only). The control could also be visually a menu or perhaps other user interface control metaphors, depending on how it is visually templated and composited. Regardless of appearance, the accessibility framework and any assistive technologies based on that framework will treat the control as a "List". This example is templated as a horizontally oriented toolbar-type control. The items in this example are Button controls, but could be templated to not appear quite as button-like, or could instead use another focusable control for the items such as a read-only TextBox. 

<UserControl x:Class="TabNavigation.MainPage"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
   <StackPanel x:Name="LayoutRoot" Background="White">
       <ListBox AutomationProperties.Name="Navigation Control">
           <ListBox.ItemsPanel>
               <ItemsPanelTemplate>
                   <StackPanel Orientation="Horizontal"/>
               </ItemsPanelTemplate>
           </ListBox.ItemsPanel>
           <ListBox.ItemContainerStyle>
               <Style TargetType="Control">
                   <Setter Property="IsTabStop" Value="False"/>
               </Style>
           </ListBox.ItemContainerStyle>
           <Button>Home</Button>
           <Button>Search</Button>
           <Button>Tools</Button>
           <Button>Help</Button>
       </ListBox>
   </StackPanel>
   <Button>Button here to show a focusable peer control beyond the list</Button>
</UserControl>

The following is an illustration of what such a control might look like:

Screen shot of a focusable control beyond a list of buttons

This example is shown in operation in the working example of Tab Navigation.

Related Resources

No endorsement implied.

Tests

Procedure

  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 traverse typical tab sequence within the Silverlight application. Verify that common areas in the user interface composition ("blocks") that are reporting the List role per this technique can be bypassed without having to tab through each constituent part (the "items/children" of the List).
  3. Verify that the list children are still accessible by keyboard, by using ARROW keys rather than TAB.
  4. Engage an accessibility framework test tool such as UIAVerify. Examine roles in the automation tree, and verify that the List used for bypass behavior reports a combination of name+role that is consistent with the behavior.
  5. Use a screen reader to verify that name and role are reported properly.

Expected Results

#2 and #3 are true, and either #4 OR #5 are true.

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