[WPF] 圆形等待效果

自己做着玩儿的，留着以后用，效果类似下面的 GIF 动画。

<Grid Width="35" Height="35">
    <Grid.Resources>
        <DrawingBrush x:Key="brush" Stretch="None" AlignmentX="Center" AlignmentY="Top">
            <DrawingBrush.Drawing>
                <GeometryDrawing Brush="Black">
                    <GeometryDrawing.Geometry>
                        <EllipseGeometry RadiusX="2" RadiusY="5"/>
                    </GeometryDrawing.Geometry>
                </GeometryDrawing>
            </DrawingBrush.Drawing>
        </DrawingBrush>
    </Grid.Resources>

    <Rectangle x:Name="r01" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="0"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r02" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="30"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r03" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="60"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r04" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="90"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r05" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="120"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r06" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="150"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r07" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="180"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r08" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="210"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r09" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="240"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r10" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="270"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r11" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="300"/>
        </Rectangle.RenderTransform>
    </Rectangle>
    <Rectangle x:Name="r12" Fill="{StaticResource brush}" Opacity="0.2" RenderTransformOrigin="0.5,0.5">
        <Rectangle.RenderTransform>
            <RotateTransform Angle="330"/>
        </Rectangle.RenderTransform>
    </Rectangle>
                
    <Grid.Triggers>
        <EventTrigger RoutedEvent="Grid.Loaded">
            <BeginStoryboard>
                <Storyboard RepeatBehavior="Forever">
                    <DoubleAnimation Storyboard.TargetName="r01" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.00000" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r02" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.08333" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r03" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.16666" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r04" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.24999" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r05" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.33332" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r06" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.41665" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r07" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.49998" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r08" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.58331" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r09" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.66664" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r10" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.74997" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r11" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.83330" To="0"/>
                    <DoubleAnimation Storyboard.TargetName="r12" Storyboard.TargetProperty="Opacity" AutoReverse="True" Duration="0:0:0.08333" BeginTime="0:0:0.91663" To="0"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </Grid.Triggers>
</Grid>