public override object GetCurrentValue(object defaultOriginValue, object defaultDestinationValue, AnimationClock animationClock) {
			GridLength from = (GridLength) GetValue(GridLengthAnimation.FromProperty);
			GridLength to = (GridLength) GetValue(GridLengthAnimation.ToProperty);
			double fromValue = from.Value;
			double toValue = to.Value;
			if (fromValue < toValue){
				return new GridLength((animationClock.CurrentProgress.Value) * (toValue - fromValue) + fromValue, from.GridUnitType);
			}else{
				return new GridLength((1 - animationClock.CurrentProgress.Value) * (fromValue - toValue) + toValue, to.GridUnitType);
			}
		}

<Grid.RowDefinitions>
	<RowDefinition Name="rowResults" Height="2*"/>
	<RowDefinition Name="rowFavorites" Height="*"/>
	<RowDefinition Height="auto"/>
</Grid.RowDefinitions>

		<Storyboard x:Key="CollapseResults">
			<animations:GridLengthAnimation Storyboard.TargetName="rowResults" Storyboard.TargetProperty="Height" From="2*" To="0*" Duration="0:0:0.5"/>
		</Storyboard>
		<Storyboard x:Key="ShowResults">
			<animations:GridLengthAnimation Storyboard.TargetName="rowResults" Storyboard.TargetProperty="Height" From="{Binding Height, ElementName=rowResults}" To="2*" Duration="0:0:0.5"/>
		</Storyboard>
		<Storyboard x:Key="CollapseFavorites">
			<animations:GridLengthAnimation Storyboard.TargetName="rowFavorites" Storyboard.TargetProperty="Height" From="*" To="0*" Duration="0:0:0.5"/>
		</Storyboard>
		<Storyboard x:Key="ShowFavorites">
			<animations:GridLengthAnimation Storyboard.TargetName="rowFavorites" Storyboard.TargetProperty="Height" From="{Binding Height, ElementName=rowFavorites}" To="*" Duration="0:0:0.5"/>
		</Storyboard>