Using WinRTTriggers within data templates

Kaki104 recently commented that they were having a problem using triggers in a certain situation. Helpfully a simple repro was provided – a simple grid view, with an attempt to start a storyboard when an item is clicked:


The trigger had been placed within the data template of the item:

<DataTemplate x:Key="DataTemplate1">
    <Grid HorizontalAlignment="Left" Width="250" Height="250">
            <Storyboard x:Key="storyboard1">
                <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Border.Background).(SolidColorBrush.Color)" Storyboard.TargetName="border">
                    <EasingColorKeyFrame KeyTime="0" Value="#FFB8B5B5"/>
                    <EasingColorKeyFrame KeyTime="0:0:1" Value="#FF3C3B3B"/>
                <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Panel.Background).(SolidColorBrush.Color)" Storyboard.TargetName="stackPanel">
                    <EasingColorKeyFrame KeyTime="0" Value="#A69C5151"/>
                    <EasingColorKeyFrame KeyTime="0:0:1" Value="#A6F9F3F3"/>

            <t:PropertyChangedTrigger Binding="{Binding Title}">
                <t:ControlStoryboardAction Action="Start" Storyboard="{StaticResource storyboard1}"/>

        <Border x:Name="border" Background="#FF3D3D3D"/>
        <StackPanel x:Name="stackPanel" VerticalAlignment="Bottom" Background="#A6000000">
            <TextBlock Text="{Binding Title}" Foreground="{StaticResource ListViewItemOverlayForegroundThemeBrush}" Style="{StaticResource TitleTextStyle}" Height="60" Margin="15,0,15,0" LayoutUpdated="TextBlock_LayoutUpdated_1" SelectionChanged="TextBlock_SelectionChanged_1"/>
            <TextBlock Text="{Binding Subtitle}" Foreground="{StaticResource ListViewItemOverlaySecondaryForegroundThemeBrush}" Style="{StaticResource CaptionTextStyle}" TextWrapping="NoWrap" Margin="15,0,15,10"/>

Interestingly the designer complains that there is an “illegal qualified name character” in the XAML, but when you run the code, everything works as expected.


Now, I’ve dug around a little and haven’t been able to work out why this is – debugging the host Visual Studio instance doesn’t reveal any exceptions being thrown, so I can only assume the parser is misreporting a problem, though in my experience it’s usually something I’m doing that’s wrong. If anyone can reveal what that is, I’d be very grateful!

A simple workaround for this is to just move the contents of the grid into a separate UserControl and reference that from within the DataTemplate instead:

<DataTemplate x:Key="DataTemplate1">
    <local:ItemUserControl />

This way you don’t get designer errors and everything works at run time.

I’ve uploaded the sample code for this here, if you’re interested.

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