XAML

    <Grid>

        <!--  滚动条背景  -->
        <Rectangle
            x:Name="BackgroundRect"
            Width="400" Height="14"
            Fill="LightGray" />

        <!--  滑轨的滑块， 使用 2 个 path 相交得到一个滑块， 然后移动第二个滑块的位置，得到一个运行的滑块动画  -->
        <Path
            x:Name="SlideRailPath"
            Width="{Binding ElementName=BackgroundRect, Path=ActualWidth}"
            Height="{Binding ElementName=BackgroundRect, Path=ActualHeight}"
            Fill="CornflowerBlue">
            <Path.Triggers>
                <EventTrigger RoutedEvent="Loaded">
                    <BeginStoryboard>
                        <Storyboard>
                            <!--  动画调整滑块的 x 值  -->
                            <DoubleAnimation
                                RepeatBehavior="Forever"
                                Storyboard.TargetName="SliderTransform" Storyboard.TargetProperty="X" From="-50"
                                To="{Binding ElementName=SlideRailPath, Path=ActualWidth}"
                                Duration="0:0:5" />
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </Path.Triggers>

            <Path.Data>

                <!--  两个相交的矩形得到一个方块， Intersect 指定相交有效  -->
                <CombinedGeometry GeometryCombineMode="Intersect">

                    <CombinedGeometry.Geometry1>
                        <RectangleGeometry>
                            <RectangleGeometry.Rect>
                                <!--  自动计算方块的大小， 与轨道一致大小  -->
                                <MultiBinding Converter="{StaticResource RectWithParentSizeValueConverter}">
                                    <Binding ElementName="SlideRailPath" Path="ActualWidth" />
                                    <Binding ElementName="SlideRailPath" Path="ActualHeight" />
                                </MultiBinding>
                            </RectangleGeometry.Rect>
                        </RectangleGeometry>
                    </CombinedGeometry.Geometry1>

                    <CombinedGeometry.Geometry2>
                        <!--  滑块的大小， 设置 x 值为 Transform 值，可以使用动画调整  -->
                        <RectangleGeometry Rect="0 0 50 14">
                            <RectangleGeometry.Transform>
                                <TranslateTransform x:Name="SliderTransform" X="50" />
                            </RectangleGeometry.Transform>
                        </RectangleGeometry>
                    </CombinedGeometry.Geometry2>

                </CombinedGeometry>
            </Path.Data>

        </Path>

    </Grid>