I am working on a countdown control for a WPF application. The following xaml demonstrates what I am after:
<Canvas>
<Canvas>
<Ellipse Width="110" Height="110" Fill="Black" Canvas.Left="95" Canvas.Top="95" />
<Ellipse Width="100" Height="100" Fill="Red" Canvas.Left="100" Canvas.Top="100" />
<TextBlock VerticalAlignment="Center" HorizontalAlignment="Center" FontSize="72" Canvas.Left="131" Canvas.Top="100">3</TextBlock>
</Canvas>
<Canvas>
<Path Stroke="Blue" Fill="White">
<Path.Data>
<PathGeometry>
<PathFigure StartPoint="150,150">
<LineSegment Point="200,150" />
<ArcSegment x:Name="arc1" Point="200,150" Size="50,50" SweepDirection="Clockwise" />
<ArcSegment x:Name="arc2" Point="200,150" Size="50,50" SweepDirection="Clockwise" />
<LineSegment Point="150,150" />
</PathFigure>
</PathGeometry>
</Path.Data>
<Path.Triggers>
<EventTrigger RoutedEvent="Path.Loaded">
<BeginStoryboard>
<Storyboard Duration="0:0:1" Storyboard.TargetProperty="Point" RepeatBehavior="Forever">
<PointAnimationUsingPath Duration="0:0:0.5" Storyboard.TargetName="arc1">
<PointAnimationUsingPath.PathGeometry>
<PathGeometry>
<PathFigure StartPoint="200,150">
<ArcSegment Size="50,50" Point="100,150" SweepDirection="Clockwise" />
</PathFigure>
</PathGeometry>
</PointAnimationUsingPath.PathGeometry>
</PointAnimationUsingPath>
<PointAnimationUsingPath Duration="0:0:0.5" Storyboard.TargetName="arc2">
<PointAnimationUsingPath.PathGeometry>
<PathGeometry>
<PathFigure StartPoint="200,150">
<ArcSegment Size="50,50" Point="100,150" SweepDirection="Clockwise" />
</PathFigure>
</PathGeometry>
</PointAnimationUsingPath.PathGeometry>
</PointAnimationUsingPath>
<PointAnimationUsingPath BeginTime="0:0:0.5" Duration="0:0:0.5" Storyboard.TargetName="arc2">
<PointAnimationUsingPath.PathGeometry>
<PathGeometry>
<PathFigure StartPoint="100,150">
<ArcSegment Size="50,50" Point="200,150" SweepDirection="Clockwise" />
</PathFigure>
</PathGeometry>
</PointAnimationUsingPath.PathGeometry>
</PointAnimationUsingPath>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Path.Triggers>
</Path>
</Canvas>
</Canvas>
Now, instead of making the whole thing white, I would like to “reveal” the next number in the countdown. I thought using the OpacityMask would work, but when I tried that things got weird. Here is the xaml I am working on:
<Canvas>
<Canvas>
<Ellipse Width="110" Height="110" Fill="Black" Canvas.Left="95" Canvas.Top="95" />
<Ellipse Width="100" Height="100" Fill="Red" Canvas.Left="100" Canvas.Top="100" />
<TextBlock VerticalAlignment="Center" HorizontalAlignment="Center" FontSize="72" Canvas.Left="131" Canvas.Top="100">3</TextBlock>
</Canvas>
<Canvas>
<Ellipse Width="110" Height="110" Fill="Black" Canvas.Left="95" Canvas.Top="95" />
<Ellipse Width="100" Height="100" Fill="Yellow" Canvas.Left="100" Canvas.Top="100" />
<TextBlock VerticalAlignment="Center" HorizontalAlignment="Center" FontSize="72" Canvas.Left="131" Canvas.Top="100">2</TextBlock>
<Canvas.OpacityMask>
<VisualBrush>
<VisualBrush.Visual>
<Path Stroke="Blue" Fill="White">
<Path.Data>
<PathGeometry>
<PathFigure StartPoint="150,150">
<LineSegment Point="200,150" />
<ArcSegment x:Name="arc1" Point="200,150" Size="50,50" SweepDirection="Clockwise" />
<ArcSegment x:Name="arc2" Point="200,150" Size="50,50" SweepDirection="Clockwise" />
<LineSegment Point="150,150" />
</PathFigure>
</PathGeometry>
</Path.Data>
<Path.Triggers>
<EventTrigger RoutedEvent="Path.Loaded">
<BeginStoryboard>
<Storyboard Duration="0:0:10" Storyboard.TargetProperty="Point" RepeatBehavior="Forever">
<PointAnimationUsingPath Duration="0:0:5" Storyboard.TargetName="arc1">
<PointAnimationUsingPath.PathGeometry>
<PathGeometry>
<PathFigure StartPoint="200,150">
<ArcSegment Size="50,50" Point="100,150" SweepDirection="Clockwise" />
</PathFigure>
</PathGeometry>
</PointAnimationUsingPath.PathGeometry>
</PointAnimationUsingPath>
<PointAnimationUsingPath Duration="0:0:5" Storyboard.TargetName="arc2">
<PointAnimationUsingPath.PathGeometry>
<PathGeometry>
<PathFigure StartPoint="200,150">
<ArcSegment Size="50,50" Point="100,150" SweepDirection="Clockwise" />
</PathFigure>
</PathGeometry>
</PointAnimationUsingPath.PathGeometry>
</PointAnimationUsingPath>
<PointAnimationUsingPath BeginTime="0:0:5" Duration="0:0:5" Storyboard.TargetName="arc2">
<PointAnimationUsingPath.PathGeometry>
<PathGeometry>
<PathFigure StartPoint="100,150">
<ArcSegment Size="50,50" Point="200,150" SweepDirection="Clockwise" />
</PathFigure>
</PathGeometry>
</PointAnimationUsingPath.PathGeometry>
</PointAnimationUsingPath>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Path.Triggers>
</Path>
</VisualBrush.Visual>
</VisualBrush>
</Canvas.OpacityMask>
</Canvas>
</Canvas>
I am not sure how to describe what is happening, or why it is behaving in this way, but it is not what I would like nor what I expected. Is there something simple I am missing? Or can anyone offer an alternative approach that will accomplish what I want?
I found the issue. It was related to the viewport/viewbox of the visual brush I was using to create the opacity mask. Here is the relevant change:
I am not an expert in WPF by any means, but my understanding of what was going wrong is as follows. The default viewport of a brush starts at the top-left of the bounding container. In my case the bounding container was the path I was animating. During the animation the top-left of the container was changing. The different segments making up the path were defined relative to this changing top-left point. Basically, they were being drawn relative to a moving target. The viewbox settings are there to indicate that we want to work with the entire original visual with the same coordinate system it uses.
Anyone with a better understanding of what was going on who can give a clearer explanation, feel free to edit this answer or add a comment.