Using the Silverlight Default Tab Sequence and Altering Tab Sequences With Properties

Important Information about Techniques

See Understanding Techniques for WCAG Success Criteria for important information about the usage of these informative techniques and how they relate to the normative WCAG 2.1 success criteria. The Applicability section explains the scope of the technique, and the presence of techniques for a specific technology does not imply that the technology can be used in all situations to create content that meets WCAG 2.1.

Applicability

Note

Microsoft has stopped updating and distributing Silverlight, and authors are encouraged to use HTML for accessible web content.

This technique is not referenced from any Understanding document.

Description

The objective of this technique is to use the default Silverlight tab sequence, or alternatively to apply the options that Silverlight application authors can use for altering the tab sequence. Application authors might alter the tab sequence in cases where the default tab sequence is not desirable for some reason, and those reasons might vary per application or application scenario. The tab sequence can be altered in order to create a meaningful sequence in the tab order, so that assistive technologies that rely on traversal of focusable elements can use and determine the meaningful sequence.

Silverlight uses structured definitions for defining its user interface presentations, where the declaration order is significant because it becomes the structure of the run-time visual tree. The structured definitions also define the layout and presentation structure in most cases. The structured definition concept is described in more detail in .

The Silverlight development platform attempts to create an overall system where the logical order of how elements are defined in XAML and code, and then presented in a user interface, will also match a logical tab sequence and logical reading order when presented to the user. In many cases, a Silverlight application author can write an application without necessarily worrying about the tab sequence, can test the tab sequence during a verification and testing phase of development, and will not need to set any specific properties to adjust the tab sequence. As a broad generalization, a Silverlight tab sequence will be constructed so that it traverses elements left to right, and top to bottom, and will behave similarly to how HTML would behave if the HTML analogs of Silverlight elements were constructed and presented in the same way. However, there are specific Silverlight controls that deliberately alter the tab sequence, or whose elements are made keyboard-accessible through a keyboard navigation technique other than TAB. For more information, see Focus Overview on MSDN.

How Silverlight implements tab sequence concepts

The Silverlight programming model defines a Control class that is a base class of many of the practical controls that produce a Silverlight application user interface. One of the behaviors of the Control class is that only a Control can receive keyboard focus as a discrete element within the Silverlight content area.

When a Silverlight application user interface is constructed from the visual tree, a default tab sequence for all Silverlight content is also constructed, using the same principles of order that were used by the visible layout. This default tab sequence is usually adequate as a tab sequence that supports users that press the TAB key to traverse the UI. The same TAB sequence and/or the focusable state of controls is also used by many assistive technologies or modes of assistive technologies to construct the representation of the interface for the Silverlight content.

For cases where developers decide that the default tab sequence is not adequate, the developer can take one of two approaches for changing the tab sequence:

  • Change other properties of the control where a change to the tab sequence happens as a secondary effect.
  • Reorder the tab sequence directly.

Changing control properties

  • Setting the Visibility property of a control to Collapsed causes the control to no longer render in the UI. As a secondary effect, that control is removed from the tab sequence.
  • Setting the IsEnabled property of a control to false causes the control to no longer be focusable by keyboard or clickable by the mouse. In many cases, the visual appearance of the control changes also, through a theme style. For example, the control may appear as gray rather than black. As a secondary effect, that control is removed from the tab sequence.

Changing specific tab properties

  • Setting the IsTabStop property of a control to false causes the control to no longer be focusable by keyboard or programmatic focus, and that control is removed from the tab sequence.
  • Setting the TabIndex property of a control to a specific index causes the control to be inserted at that position in the tab sequence. The default value of TabIndex is Single.MaxValue, therefore any non-default value promotes that control to be first in an otherwise default tab sequence. More typically, authors would specify a TabIndex for any controls that are involved in a deliberate segment of tab order re-ordering.

Tab order and language

Left-to-right is the default only for languages that use left-to-right reading order. For languages that use right-to-left reading order, right-to-left is also the default tab order as implemented by Silverlight runtime behavior. That language preference is declared by the acting CultureInfo. For more information on CultureInfo, see .

Examples

Example 1: Default tab order, based on ordering in the StackPanel

In this example, a StackPanel has a natural layout order of top-to-bottom, and that will also be the tab order of each StackPanel child element (FirstName, then LastName).

   <StackPanel x:Name="LayoutRoot" Background="White">
       <StackPanel Orientation="Horizontal">
           <TextBlock Name="lbl_FirstName">First name</TextBlock>
           <TextBox AutomationProperties.LabeledBy="{Binding ElementName=lbl_FirstName}" Name="tbFirstName" Width="100"/>
       </StackPanel>
       <StackPanel Orientation="Horizontal">
           <TextBlock Name="lbl_LastName">First name</TextBlock>
           <TextBox AutomationProperties.LabeledBy="{Binding ElementName=lbl_LastName}" Name="tbLastName" Width="100"/>
       </StackPanel>
   </StackPanel>

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

Example 2: Tab order, modified by TabIndex

A form is marked up using a data table that includes the fields of the groom in the first column and the fields of the bride in the second column. The order in the content is row by row but the author feels it is more logical for users to navigate the form column by column. This way, all the groom's criteria can be filled in before moving on to the bride's criteria. The TabIndex attributes of the Silverlight elements are used to specify a tab order that navigates column by column. This example specifically illustrates how changing tab order can change the meaningful sequence.

 <UserControl x:Class="TabSequence.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">
       <TextBlock>he first column contains the search criteria 
 of the groom, the second column the search criteria of 
 of the bride</TextBlock>
       <Grid>
       <Grid.RowDefinitions>
         <RowDefinition/>
         <RowDefinition/>
         <RowDefinition/>
         <RowDefinition/>
       </Grid.RowDefinitions>
       <Grid.ColumnDefinitions>
         <ColumnDefinition/>
         <ColumnDefinition/>
         <ColumnDefinition/>
       </Grid.ColumnDefinitions>
       <TextBlock>Search criteria</TextBlock>
       <TextBlock Grid.Column="1">Groom</TextBlock>
       <TextBlock Grid.Column="2">Bride</TextBlock>
       <TextBlock Grid.Row="1">First name</TextBlock>
       <TextBox Grid.Row="1" Grid.Column="1" TabIndex="1"/>
       <TextBox Grid.Row="1" Grid.Column="2" TabIndex="4"/>
       <TextBlock Grid.Row="2">Last name</TextBlock>
       <TextBox Grid.Row="2" Grid.Column="1" TabIndex="2"/>
       <TextBox Grid.Row="2" Grid.Column="2" TabIndex="5"/>
       <TextBlock Grid.Row="3" >Place of birth</TextBlock>
       <TextBox Grid.Row="3" Grid.Column="1" TabIndex="3"/>
       <TextBox Grid.Row="3" Grid.Column="2" TabIndex="6"/>
       </Grid>
   </StackPanel>
 </UserControl>
 

This example is shown in operation in the working example of Tab Sequence TabIndex.

Example 3: Tab order, modified by changing runtime Control properties

In this example, a radio button choice in a form controls whether certain other fields in the form are relevant or not relevant. The current radio button selection toggles the IsEnabled property in such fields to enable or disable them based on how the user selected the preceding logical element, which also affects whether the fields appear in the further tab sequence. The following is UI definition in XAML.

<UserControl x:Class="TabSequence.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">
       <TextBlock>Registration</TextBlock>
       <Grid>
       <Grid.RowDefinitions>
         <RowDefinition/>
         <RowDefinition/>
         <RowDefinition/>
         <RowDefinition/>
       </Grid.RowDefinitions>
       <Grid.ColumnDefinitions>
         <ColumnDefinition/>
         <ColumnDefinition/>
         <ColumnDefinition/>
       </Grid.ColumnDefinitions>
           <StackPanel Orientation="Horizontal">
               <RadioButton GroupName="Registration" Checked="RadioButton_CheckedG">Guest</RadioButton>
               <RadioButton GroupName="Registration" Checked="RadioButton_CheckedC">Custom</RadioButton>
           </StackPanel>
               <TextBlock Grid.Row="1">First name</TextBlock>
           <TextBox x:Name="tb_fn" IsEnabled="false" Grid.Row="1" Grid.Column="1" />
           <TextBlock Grid.Row="2">Last name</TextBlock>
           <TextBox  x:Name="tb_ln" IsEnabled="false" Grid.Row="2" Grid.Column="1" />
       </Grid>
   </StackPanel>
</UserControl>

The following is event handler code.

       private void RadioButton_CheckedC(object sender, RoutedEventArgs e)
       {
           tb_fn.IsEnabled = true;
           tb_ln.IsEnabled = true;
       }
       private void RadioButton_CheckedG(object sender, RoutedEventArgs e)
       {
           tb_fn.IsEnabled = false;
           tb_ln.IsEnabled = false;
       }
       

This example is shown in operation in the working example of Tab Sequence Enabled.

Resources

Resources are for information purposes only, 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. Engage the screen reader. Press the TAB key to traverse the sequence of elements inside the Silverlight content area.
  3. Verify that the order in which elements are traversed in a tab sequence is also the expected order of the elements as they are presented visually, particularly in such cases where the order of each element is significant per SC 1.3.2.

Expected Results

#3 is true.