Question

NOTE I have asked the related question (with an accepted answer): How to combine DataTrigger and Trigger?

I think I need to combine an EventTrigger and a DataTrigger to achieve what I m after:

when an item appears in my ListBox, it should flash for a few moments
if the item is Critical then it should remain highlighted

Currently I have a DataTemplate that looks like this:

<DataTemplate DataType="{x:Type Notifications:NotificationViewModel}">
    <Grid HorizontalAlignment="Stretch">
        <Border Name="Background" CornerRadius="8" Background="#80c0c0c0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />
        <Border Name="Highlight"  CornerRadius="8" Background="Red"       HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />
        <!-- snip actual visual stuff -->
        <Grid.Triggers>
            <EventTrigger RoutedEvent="Grid.Loaded">
                <EventTrigger.Actions>
                    <BeginStoryboard>
                        <Storyboard>
                            <DoubleAnimation x:Name="LoadedAnimation" 
                                             Storyboard.TargetName="Highlight" 
                                             Storyboard.TargetProperty="Opacity" 
                                             From="0" To="1" 
                                             RepeatBehavior="5x" 
                                             Duration="0:00:0.2" 
                                             AutoReverse="True" />
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger.Actions>
            </EventTrigger>
        </Grid.Triggers>
    </Grid>
    <DataTemplate.Triggers>
        <DataTrigger Binding="{Binding Path=IsCritical}" Value="True">
            <Setter TargetName="LoadedAnimation" Property="RepeatBehavior" Value="5.5x" />
        </DataTrigger>
    </DataTemplate.Triggers>
</DataTemplate>

The idea is that an EventTrigger animates the Highlight border s opacity between 0 and 1 and back again repeatedly when the item is first loaded, drawing the user s attention to it. The DataTrigger determines the number of times to animate. If the view model reports that the item IsCritical then the animation occurs 5.5 times (such that it ends at opacity 1), otherwise it occurs 5 times (ending at opacity 0.)

However the above XAML doesn t work because the DataTrigger s setter fails with:

Child with Name LoadedAnimation not found in VisualTree.

Fair enough. So, shy of using a custom value converter or putting the animation count on the view model and binding to it, what are my options?

Answer 1

I know you said you weren t keen on the idea of a converter, but it looks like the Blend solutions require a library to be installed. The converter isn t much work and signals intent that the rate is directly dependent on the IsCritical property:

public class CriticalAnimationRateConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // Error handling omitted for brevity.
        if ((bool)value)
            return new System.Windows.Media.Animation.RepeatBehavior(5.5);
        else
            return new System.Windows.Media.Animation.RepeatBehavior(5.0);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

And then update your animation:

<DoubleAnimation Storyboard.TargetName="Highlight"
                 Storyboard.TargetProperty="Opacity"
                 From="0"
                 To="1"
                 RepeatBehavior="{Binding IsCritical, Converter={StaticResource CriticalAnimationRateConverter}}"
                 Duration="0:00:0.2"
                 AutoReverse="True" />

The DataTrigger can then be removed.

Answer 2

I would use a behavior instead of triggers in this case. You can write a behavior that attaches an event handler to the associated object s load event and then applies the animation. The behavior may expose some properties, I would expose an AnimationCount (int) property that tells the behavior how many time to repeat the animation on the element that it is associated with. You can then bind this property to the IsCritical property in the view model and use a value converter to convert false to 5 and true to 5.5

Hope this helps

Answer 3

If you have access to the Blend SDK (you should if you re using VS2012+), you should be able to accomplish this entirely in XAML, with something like this (disclaimer: untested):

<Grid HorizontalAlignment="Stretch">
    <VisualStateManager.VisualStateGroups>
        <VisualStateGroup x:Name="NotificationStates">
            <VisualState x:Name="Flashing">
                <Storyboard>
                    <DoubleAnimation x:Name="LoadedAnimation" 
                                     Storyboard.TargetName="Highlight" 
                                     Storyboard.TargetProperty="Opacity" 
                                     From="0" To="1" 
                                     RepeatBehavior="5x" 
                                     Duration="0:00:0.2" 
                                     AutoReverse="True" />
                </Storyboard>
            </VisualState>
            <VisualState x:Name="Normal" />
        </VisualStateGroup>
    </VisualStateManager.VisualStateGroups>
    <Border Name="Background" CornerRadius="8" Background="#80c0c0c0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />
    <Border Name="Highlight"  CornerRadius="8" Background="Red"       HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />
    <!-- snip actual visual stuff -->
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Loaded">
            <ic:GoToStateAction StateName="Flashing"/>
        </i:EventTrigger>
        <ie:DataTrigger Binding="{Binding Path=IsCritical}" Value="True">
            <ic:GoToStateAction StateName="Flashing"/>
        </ie:DataTrigger>
    </i:Interaction.Triggers>
</Grid>

Extract your Storyboard to a VisualState, then use the expression library to switch states within the XAML. You ll need Microsoft.Expression.Interactions library, see also WPF/Silverlight States - Activate from XAML?

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