WPF - Confusing DataTrigger/DoubleAnimation behaviour - wpf

Question

I'm trying to animate the ScaleY property of a LayoutTransform based on a DataTrigger bound to a boolean on my ViewModel class. The animation happens when the value is first seen to be false by the DataTrigger (when the application first starts) and when i first change it to true in a checkbox's checked event but not when i set it to false in the same checkbox's unchecked event.
A simplified version of what i'm doing is listed below.
The ViewModel class is very simple, containing a single boolean DependencyProperty called Selected.
public class VM : DependencyObject
{
public bool Selected
{
get { return (bool)GetValue(SelectedProperty); }
set { SetValue(SelectedProperty, value); }
}
// Using a DependencyProperty as the backing store for Selected. This enables animation, styling, binding, etc...
public static readonly DependencyProperty SelectedProperty =
DependencyProperty.Register("Selected", typeof(bool), typeof(VM), new UIPropertyMetadata(false));
}
The Window.xaml contains a button and a checkbox. When the checkbox is checked, i set the ViewModel's 'Selected' property to true and false when it is unchecked. Here's the code for both the xaml and it's code-behind.
<Window x:Class="DataTriggers.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:y="clr-namespace:DataTriggers"
Title="Window1" Height="300" Width="300">
<Window.Resources>
<y:VM x:Key="VM"/>
<Style TargetType="Button" x:Key="but">
<Style.Triggers>
<DataTrigger Binding="{Binding Selected}" Value="False">
<DataTrigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Duration="0:0:1"
To="0"
Storyboard.TargetProperty="(LayoutTransform).(ScaleTransform.ScaleY)"/>
</Storyboard>
</BeginStoryboard>
</DataTrigger.EnterActions>
</DataTrigger>
<DataTrigger Binding="{Binding Selected}" Value="True">
<DataTrigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Duration="0:0:1"
To="1"
Storyboard.TargetProperty="(LayoutTransform).(ScaleTransform.ScaleY)"/>
</Storyboard>
</BeginStoryboard>
</DataTrigger.EnterActions>
</DataTrigger>
</Style.Triggers>
</Style>
</Window.Resources>
<StackPanel>
<Button Style="{StaticResource but}" DataContext="{StaticResource VM}">
<Button.LayoutTransform>
<ScaleTransform></ScaleTransform>
</Button.LayoutTransform>
me
</Button>
<CheckBox Checked="CheckBox_Checked" Unchecked="CheckBox_Unchecked"/>
</StackPanel>
public partial class Window1 : Window
{
public Window1()
{
InitializeComponent();
}
private void CheckBox_Checked(object sender, RoutedEventArgs e)
{
VM vm = this.FindResource("VM") as VM;
vm.Selected = true;
}
private void CheckBox_Unchecked(object sender, RoutedEventArgs e)
{
VM vm = this.FindResource("VM") as VM;
vm.Selected = false;
}
}
I know that the DataTrigger fires when the property is false because if i change the DoubleAnimation to a simple Setter operating on the Opacity property then i see the correct results. So it would seem to be a problem with how I'm using the DoubleAnimation.
Any help would be appriciated.

Answer 1

This is ODD behavior but i decided to refactor the 'False' case into the DataTrigger's ExitActions like this -
<DataTrigger Binding="{Binding Selected}" Value="True">
<DataTrigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Duration="0:0:1"
To="1"
Storyboard.TargetProperty="(RenderTransform).(ScaleTransform.ScaleY)"/>
</Storyboard>
</BeginStoryboard>
</DataTrigger.EnterActions>
<DataTrigger.ExitActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Duration="0:0:1"
To="0"
Storyboard.TargetProperty="(RenderTransform).(ScaleTransform.ScaleY)"/>
</Storyboard>
</BeginStoryboard>
</DataTrigger.ExitActions>
</DataTrigger>
That works as intended. I don't know what the difference is between the two cases but at least it's an answer.