WCAG 2.0 達成方法集

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SL5: Defining a Focusable Image Class for Silverlight


これらの達成方法(参考)の使用法及び、それらがWCAG 2.0 達成基準(規定)とどのように関係するかに関する重要な情報については、WCAG 達成基準の達成方法を理解するを参照のこと。適用(対象)セクションは、その達成方法の範囲について説明しており、特定の技術に関する達成方法があるからといって、WCAG 2.0 を満たすコンテンツを作成する際に、常にその技術が使用可能であるわけではない。




SL5 に関するユーザエージェントサポートノート (英語)を参照のこと。Silverlight Technology Notesも参照。


The objective of this technique is to wrap the Silverlight Image class inside a UI container class that is focusable. If the image is focusable, users who use the TAB sequence to navigate content while the assistive technology is active, and/or assistive technologies that construct navigation structures that are based on the TAB sequence, can both detect the image in navigation. The assistive technology can then associate alternative text for that image within the navigation structure, and report the information to the user.

Many existing assistive technologies do not construct initial navigation views that are derived from UI Automation information if it is coming from a non-focusable element in a Silverlight user interface. This is particularly true if the assistive technology is in a navigation mode that is specifically intended to help users enter information into a form or similar interactive interface element; an example of this situation is the Forms Mode of the JAWS screen reader.

Image is an example of a Silverlight element that is not focusable. This technique and the example therein are intended to circumvent the possible omission of a nonfocusable Silverlight Image element from certain navigation views in existing assistive technology implementations. The Silverlight Image is wrapped with a display/viewer control class that is focusable. This image-wrapping control is initially presented in assistive technology representations of a Silverlight user interface that use only focusable elements when constructing the assistive technology's representation of the application.

The image wrapper class uses the AutomationProperties.Name property to provide a short text alternative for the contained Image, so that the alternative text can be read or otherwise presented by assistive technologies. The Silverlight API AutomationProperties.Name directly sets Name in the UI Automation tree. The properties in the UI Automation tree are reported to assistive technologies, when the assistive technology implements behavior that acts as a UI Automation client. Name is one of the accessibility framework properties that most assistive technologies present in some way, for purposes of both name and value information, and setting Name is the common technique for exposing text alternatives for any other Control class (for example, for a button with an image, as shown in the technique SL18: Providing Text Equivalent for Nontext Silverlight Controls With AutomationProperties.Name).

This technique is intended for cases where application authors deliberately do not want a visible image caption for the image to be part of the user interface, and the image is a part of a larger interactive user interface control or page. Otherwise, if there is a visible caption, authors can use SL26: Using LabeledBy to Associate Labels and Targets in Silverlight.


The two examples are intended to be used together, if an application is both defining and consuming the focusable image control.

事例 1: Defining the FocusableImage XAML template and C# code behavior

Silverlight supports a control development model whereby the visual appearance of a control is largely defined in XAML, and the behavior of a control (such as its event handling and hookups to services) are implemented in a managed code language such as C#. The following is the XAML template, which includes a visual state that shows visually when the control is focused in UI.

 <Style TargetType="local:FocusableImage">
   <Setter Property="Template">
      <ControlTemplate TargetType="local:FocusableImage">
            <VisualStateGroup x:Name="FocusStates">
              <VisualState x:Name="Focused">
 Duration="0" To="Blue"/>
              <VisualState x:Name="Unfocused"/>
             Margin="2" Opacity="10"
             Source="{TemplateBinding Source}"/>

The following is the C# class definition and logic. The logic includes invoking a default automation peer on creation, and loading the template as defined in the previous XAML example through the Silverlight "generic.xaml" resource convention for custom controls.

namespace ImageEquivalent
   public class FocusableImage : Control
       protected override System.Windows.Automation.Peers.AutomationPeer OnCreateAutomationPeer()
           return new FrameworkElementAutomationPeer(this);
       public FocusableImage()
           this.DefaultStyleKey = typeof(FocusableImage);
       public ImageSource Source
           get { return (ImageSource)this.GetValue(SourceProperty); }
           set { this.SetValue(SourceProperty,value); }
       public static DependencyProperty SourceProperty = DependencyProperty.Register(
       Boolean _Focused;
       void ChangeState()
           if (_Focused)
       protected override void OnGotFocus(RoutedEventArgs e)
           this._Focused = true;
       protected override void OnLostFocus(RoutedEventArgs e)
           this._Focused = false;

This example is shown in operation in the working example of Focusable Image.

事例 2: Using the FocusableImage class in UI and applying AutomationProperties.Name

Now that the image is wrapped by a focusable control, you can instantiate an instance of the wrapper UI inside a Silverlight layout container, specify AutomationProperties.Name at the level of the wrapper control’s tag, and have that text serve as the alternative text for the referenced source image file.

     Height="300" Width="400
     AutomationProperties.Name="Diagram of secret lair"
     Source="/diagram_lair.png" />




Automation Tree


  1. Open the test HTML page in a Silverlight-supported useragent host; to use UI Automation, use Microsoft Windows as platform.

  2. Use the tab sequence inside the Silverlight content area to focus the control.

  3. Using an accessibility framework verification tool, check that the string content is promoted as the default Name applied to the control.

注記: Accessibility framework verification tools typically show the entirety of an automation tree for a given application, and in fact will show the tree for all applications running on the Windows client machine. Focusing the control as in #2 is thus not strictly speaking necessary. However, manually focusing using the application interface is often a faster way to step into the automation tree as opposed to having to open an extensive series of nested nodes starting from the browser host application root. Whether this functionality exists depends on which accessibility framework verification tool is being used for testing.


#3 is true.



Screen Reader


  1. Using a browser that supports Silverlight, open an HTML page that references a Silverlight application through an object tag. To use UI Automation, use Microsoft Windows as platform.

  2. Engage the screen reader. Move focus to the control (for example, use the tab sequence).

  3. Check that the Name applied to the image is read by the screen reader.


#3 is true.