WPF Animated Animation - Why Can't By By Have Negative Value?

Question

I am animating in WPF and I'm confused. For RectAnimation, By cannot be negative in height and width.

So, if you use the keyword “By”, can the rectangles only grow (instead of “To”)?

Here is a sample code:

<Page  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >
  <StackPanel Orientation="Vertical" HorizontalAlignment="Left">
    <Path Stroke="Black" StrokeThickness="1" Fill="LemonChiffon">
      <Path.Data>
        <RectangleGeometry x:Name="myRectangleGeometry" Rect="0,200,100,100" />

      </Path.Data>
      <Path.Triggers>
        <EventTrigger RoutedEvent="Path.Loaded">
          <BeginStoryboard>
            <Storyboard> 
              <!-- Animate the Rect property of the RectangleGeometry
                   which causes the rectangle to animate postion and size. -->
              <RectAnimation
              Storyboard.TargetName="myRectangleGeometry"
              Storyboard.TargetProperty ="Rect"
              Duration="0:0:2" FillBehavior="HoldEnd" 
              From="0,0,100,100"
              By="600,50,200,-50" />
                               ^
            </Storyboard>      |
          </BeginStoryboard>   |
        </EventTrigger>        |
      </Path.Triggers>         |
    </Path>                    |
  </StackPanel>                |
</Page>                        |
                               |
This returns an error, but only if it is negative.

Answer 1

In fact, you define a rectangle in the property Byof the RectAnimation object. Unable to create a rectangle with negative dimensions.

By="x,y,Width,Height"

What you probably want to do can be done using the property To:

To="600,50,300,50"