Using ObservableCollection firing Binding.TargetUpdated only on single item - wpf

Question

I have three Textblocks with the Text Property bound to the items of an ObservableCollection:
<TextBlock Style="{StaticResource FadeInTextBlock}" Text="{Binding Path=CurrentAnswers[0], NotifyOnTargetUpdated=True}" />
<TextBlock Style="{StaticResource FadeInTextBlock}" Text="{Binding Path=CurrentAnswers[1], NotifyOnTargetUpdated=True}" />
<TextBlock Style="{StaticResource FadeInTextBlock}" Text="{Binding Path=CurrentAnswers[2], NotifyOnTargetUpdated=True}" />
The Property with INotifyPropertyChanged implemented:
public ObservableCollection<Answer> CurrentAnswers
{
get { return currentAnswers; }
set { currentAnswers = value; RaisePropertyChanged("CurrentAnswers"); }
}
Each Textblock uses the same style containing a trigger for the Binding.TargetUpdated event which fades in the actual text:
<Style x:Key="FadeInTextBlockTwo" TargetType="TextBlock">
<Style.Triggers>
<EventTrigger RoutedEvent="Binding.TargetUpdated">
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="Opacity" Duration="0:0:0" To="0.0"/>
<DoubleAnimation Storyboard.TargetProperty="Opacity" Duration="0:0:1" From="0.0" To="1.0" BeginTime="0:0:0"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Style.Triggers>
</Style>
When I change ONE item in the ObservableCollection ALL Textblocks are firing the event and do the fade in:
CurrentAnswer[1] = "New Text";
// Textblock 1 - 3 do the fade in animation,
// even if only Textblock 2 has been updated
How can I limit the animation to only the Textblock whose bound value has been updated?

Answer 1

When you're using indexes you bind to indexer property of your collection and UI does not know which index has changed it is just notified that indexer property has changed without specifying which index so, in your case, it refreshes all 3 TextBlocks raising TargetUpdated event. What happens is ObservableCollection raises PropertyChanged event with Binding.IndexerName as property name. To solve your problem instead of using 3 TextBlocks you can use ItemsControl
<ItemsControl ItemsSource="{Binding CurrentAnswers}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Style="{StaticResource FadeInTextBlock}" Text="{Binding NotifyOnTargetUpdated=True}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
this will repeat TextBlock as many time as many items you have in CurrentAnswers