WPF TextBlock animation not firing when source is updated - wpf

Question

I am simply trying to have some text flash red momentarily when the source for this TextBlock is updated. The text that the TextBlock is bound to works just fine, but for some reason the animation won't fire. I'm at a bit of a loss.
Any ideas?
<Border BorderBrush="{StaticResource Button.BackgroundBrush}"
Background="{StaticResource Screener.Background}"
BorderThickness="0,1,0,0"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch">
<TextBlock Text="{Binding AlertBoxMessage, Mode=OneWay, NotifyOnSourceUpdated=True}"
Name="AlertBox"
MinHeight="55"
FontWeight="Normal"
FontSize="16"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Foreground="Black">
<TextBlock.Style>
<Style TargetType="{x:Type TextBlock}">
<Style.Triggers>
<EventTrigger RoutedEvent="Binding.SourceUpdated" >
<BeginStoryboard>
<Storyboard Storyboard.TargetProperty="Foreground">
<ColorAnimation From="Black"
To="Red"
AutoReverse="True"
RepeatBehavior="3"
Duration="0:0:2"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Style.Triggers>
</Style>
</TextBlock.Style>
</TextBlock>
</Border>

Answer 1

First of all I think you misinterpreted the Binding.SourceUpdated event.
Each binding consists of two endpoints - a source and a target. The target is the object on which the binding is set, target property is the DependencyProperty whose value will be bound, the source is the object against which Binding.Path will be resolved, and source property is the property to which the target property is bound. In your case the target is the TextBlock, target property is Text, source is the (inherited) data context (a view-model I presume), and source property is AlertBoxMessage.
As you probably know bindings can work in several modes. TwoWay will allow updating in both source-to-target and target-to-source directions, whereas OneWay will only do source-to-target updates.
Key information here is that the Binding.SourceUpdated will be raised whenever a target-to-source transfer occurs (and Binding.TargetUpdated for source-to-target transfer, see MSDN). In your case however the value is always updated in source-to-target direction, because you set the binding mode to OneWay, so Binding.SourceUpdated is never raised. To achieve your goal you should use Binding.TargetUpdated event instead (with Binding.NotifyOnTargetUpdated set to true).
There are however several other issues with your code that will prevent it from working, so let's go over them:
You're trying to animate the TextBlock.Foreground property (of type Brush) with a ColorAnimation, which can only be used to animate a property of type Color, and you'll get an InvalidOperationException. You should set StoryBoard.TargetProperty="Foreground.Color"
It may seem a bit counter-intuitive but setting RepeatBehavior="3" will make the animation repeat for 3 days, and not 3 times (see this question). You should use RepeatBehavior="3x"
To sum things up, here's the code that should do what you expect:
<TextBlock Text="{Binding AlertBoxMessage, Mode=OneWay, NotifyOnTargetUpdated=True}"
Name="AlertBox"
MinHeight="55"
FontWeight="Normal"
FontSize="16"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Foreground="Black">
<TextBlock.Style>
<Style TargetType="{x:Type TextBlock}">
<Style.Triggers>
<EventTrigger RoutedEvent="Binding.TargetUpdated">
<BeginStoryboard>
<Storyboard Storyboard.TargetProperty="Foreground.Color">
<ColorAnimation From="Black"
To="Red"
AutoReverse="True"
RepeatBehavior="3x"
Duration="0:0:2"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Style.Triggers>
</Style>
</TextBlock.Style>
</TextBlock>