Pausing or Stopping A Decorative Silverlight Animation

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 relates to Success Criterion 2.2.2: Pause, Stop, Hide (Sufficient).

Description

The objective of this technique is to associate a "Pause" or "Stop" action for a Silverlight animation with a user interface control. This enables a user to pause or stop an animation in Silverlight content.

The Silverlight animation system is generalized such that nearly any Silverlight property of type Double, Point or Color can be animated, or a property can cycle through discrete object values. Thus the possibilities for which properties in the user interface can be animated are quite broad. The general technique shown can be used to pause or stop any Silverlight animation, including those that are purely decorative.

Pause versus Stop

Silverlight has two discrete methods for animation control: a Pause method and a Stop method. The difference in behavior is that Pause uses whatever the last value was while the animation was still running, and holds that value permanenently (unless the animation is restarted). Stop sets the value to be whatever value existed before the animation was started. However, calling Stop on an animation often results in a behavior that looks like a "reset" to the user; this is particularly true if the animation is animating an element's position on screen. In many cases, what might be a conceptual "stop" for the user is better accomplished by a "permanent Pause" in the Silverlight animation API. Whether to call Pause or Stop is an aesthetic decision and application authors can experiment to see which behavior has the best appearance. If application authors choose to use Stop, authors can simply replace the call to .Pause() with a call to .Stop() for any code that is based on this technique's example.

Examples

Example 1: Pausing a decorative animation

The following is the XAML UI. The animated object and the animation behavior are both described in XAML, as is the control that users can activate to pause the animation. 

<UserControl x:Class="PauseBouncyBall.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
 <UserControl.Resources>
   <Storyboard x:Key="anim" RepeatBehavior="Forever" >
       <DoubleAnimationUsingKeyFrames Storyboard.TargetName="Ball"
          Storyboard.TargetProperty="(Canvas.Top)"
        FillBehavior="HoldEnd" AutoReverse="True">
               <EasingDoubleKeyFrame Value="100" KeyTime="00:00:01">
                   <EasingDoubleKeyFrame.EasingFunction>
                       <BounceEase Bounces="-1" EasingMode="EaseIn"/>
                   </EasingDoubleKeyFrame.EasingFunction>
               </EasingDoubleKeyFrame>
           </DoubleAnimationUsingKeyFrames>
   </Storyboard>
 </UserControl.Resources>
 <Canvas x:Name="LayoutRoot" Background="White" Height="600" Width="800">
   <Ellipse Name="Ball" Fill="Red" Width="20" Height="20" Canvas.Top="200">
       <Ellipse.RenderTransform>
           <TransformGroup>
               <TranslateTransform/>
           </TransformGroup>
       </Ellipse.RenderTransform>
   </Ellipse>
   <Button HorizontalAlignment="Left" Width="200" Click="Button_Click">Stop the bouncy ball please!</Button>
 </Canvas>
</UserControl>

The following is the C# logic. One function is the "page" constructor, which is what starts and loops the animation. The other function is the event handler for the UI control (a button). The event handler retrieves the animation definition from the page resources, and calls the Pause method on the animation. 

       public MainPage()
       {
           InitializeComponent();
           (this.Resources["anim"] as Storyboard).Begin();
       }
       private void Button_Click(object sender, RoutedEventArgs e)
       {
           (this.Resources["anim"] as Storyboard).Pause();
       }

This example is shown in operation in the working example of Pause Bouncy Ball.

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. For Silverlight content with moving, blinking, scrolling or auto-updating content that is the result of a running Silverlight animation:
  2. Check for a mechanism to stop the movement, blinking, scrolling or auto-updating.
  3. Check that the movement, blinking, scrolling or auto-updating stops when the mechanism is activated and does not restart by itself.
  4. For pause, check that the animation can be restarted using a start mechanism.

Expected Results

#3 is true.