Я создал простую анимацию со смесью: L-образный контур и анимация DoubleKeyFrame, которая изменяет смещения GradientStop, так что L выглядит так, как будто он заполнен.
С некоторыми дополнительными усилиями вы также можете повернуть кисть так, чтобы ее путь следовал за изгибом, и вы получите более хороший эффект заполнения. Анимация запускается в рамках Loaded Event и повторяется вечно по демонстрационным причинам.
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Class="WpfApplication.MainWindow"
x:Name="Window"
Title="MainWindow"
Width="640" Height="480">
<Window.Resources>
<Storyboard x:Key="Storyboard1">
<PointAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(LinearGradientBrush.StartPoint)" Storyboard.TargetName="ColorFill">
<EasingPointKeyFrame KeyTime="0" Value="0.025,0.954"/>
</PointAnimationUsingKeyFrames>
<PointAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(LinearGradientBrush.EndPoint)" Storyboard.TargetName="ColorFill">
<EasingPointKeyFrame KeyTime="0" Value="0.97,0.004"/>
</PointAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Color)" Storyboard.TargetName="ColorFill">
<EasingColorKeyFrame KeyTime="0" Value="White"/>
</ColorAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames RepeatBehavior="Forever" Storyboard.TargetProperty="(Shape.Fill).(GradientBrush.GradientStops)[1].(GradientStop.Offset)" Storyboard.TargetName="ColorFill">
<EasingDoubleKeyFrame KeyTime="0" Value="0"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.1" Value="0.046"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.2" Value="0.11"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.3" Value="0.184"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.4" Value="0.251"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.5" Value="0.343"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.6" Value="0.445"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.7" Value="0.542"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.8" Value="0.65"/>
<EasingDoubleKeyFrame KeyTime="0:0:0.9" Value="0.798"/>
<EasingDoubleKeyFrame KeyTime="0:0:1" Value="0.897"/>
<EasingDoubleKeyFrame KeyTime="0:0:1.1" Value="0.969"/>
<EasingDoubleKeyFrame KeyTime="0:0:1.2" Value="1"/>
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</Window.Resources>
<Window.Triggers>
<EventTrigger RoutedEvent="FrameworkElement.Loaded">
<BeginStoryboard Storyboard="{StaticResource Storyboard1}"/>
</EventTrigger>
</Window.Triggers>
<Grid x:Name="LayoutRoot" Margin="0,0,-195.6,-162">
<Path Data="M105.8,91" Fill="#FFF4F4F5" HorizontalAlignment="Left" Height="1" Margin="105.8,91,0,0" Stretch="Fill" Stroke="Black" VerticalAlignment="Top" Width="1"/>
<Path x:Name="ColorFill" Data="M187.2,0.5 L234.4,0.5 C242.07676,0.5 248.3,6.7232418 248.3,14.399999 L248.3,187.2 248.3,234.39999 C248.3,234.87979 248.2757,235.35391 248.22824,235.82118 248.18079,236.28847 248.11018,236.7489 248.0176,237.20132 247.92503,237.65376 247.81046,238.09818 247.67509,238.53343 247.53971,238.96867 247.38351,239.39476 247.20767,239.8105 247.03183,240.22624 246.83633,240.63165 246.62235,241.02556 246.40836,241.41946 246.1759,241.80188 245.9261,242.17162 245.67631,242.54137 245.40919,242.89844 245.12592,243.24168 244.84264,243.58493 244.54321,243.91435
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234.8798,248.3 234.4,248.3 L187.2,248.3 14.400009,248.3 C6.7232513,248.3 0.5,242.07675 0.5,234.39999 L0.5,187.2 C0.5,179.52324 6.7232513,173.3 14.400009,173.3 L173.3,173.3 173.3,14.399999 C173.3,6.7232418 179.52324,0.5 187.2,0.5 z" HorizontalAlignment="Left" Margin="-5.4,267.4,0,89.8" RenderTransformOrigin="0.847266882902349,0.499999993867046" Stretch="Fill" Stroke="Black" Width="248.8">
<Path.Fill>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="Green" Offset="0"/>
<GradientStop Color="White" Offset="1"/>
</LinearGradientBrush>
</Path.Fill>
<Path.RenderTransform>
<TransformGroup>
<ScaleTransform/>
<SkewTransform/>
<RotateTransform Angle="90"/>
<TranslateTransform/>
</TransformGroup>
</Path.RenderTransform>
</Path>
</Grid>
</Window>