I don't know if what you are saying is exactly true. In the XAML below, the TextBlock will display the Width of the Rectangle. When you click on the Rectangle, the Width property is animated from 50 to 300. Along every increment, the TextBlock changes in value. Am I not understanding your question?
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<TextBlock Text="{Binding ElementName=Rect,Path=Width}" Grid.Row="0" />
<Rectangle Grid.Row="1"
Name="Rect"
Height="30"
Width="50"
Fill="Blue"
HorizontalAlignment="Left">
<Rectangle.Triggers>
<EventTrigger RoutedEvent="Rectangle.MouseLeftButtonDown">
<EventTrigger.Actions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="Width"
From="50"
To="300"
Duration="0:0:10"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger.Actions>
</EventTrigger>
</Rectangle.Triggers>
</Rectangle>
</Grid>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…