如何在CustomControl的一部分上获取鼠标位置

在制作customcontrol时，我试图获得鼠标在Canvas上的位置，该Canvas是我的自定义控件内的容器。问题是，当我将customcontrol放在测试应用程序中时，仅是窗口中的网格，我得到的总是鼠标在窗口本身上的位置，而不是鼠标在customcontrol上的位置。

public class HueWheel : Control
{
    static HueWheel()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(HueWheel), new FrameworkPropertyMetadata(typeof(HueWheel)));
    }
    private bool _isPressed = false;
    //private Canvas _templateCanvas = null;
    private Canvas _PART_FirstCanvas;
    private Canvas _PART_SecondCanvas;
    private Slider _PART_Slider;

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        _PART_FirstCanvas = (Canvas)GetTemplateChild("PART_FirstCanvas");
        _PART_SecondCanvas = (Canvas)GetTemplateChild("PART_SecondCanvas");
        _PART_Slider = (Slider)GetTemplateChild("PART_Slider");
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        if (_isPressed)
        {
            const double RADIUS = 150;
            Point newPos = e.GetPosition(_PART_SecondCanvas);
            double angle = MyHelper.GetAngleR(newPos, RADIUS);
            _PART_Slider.Value = (_PART_Slider.Maximum - _PART_Slider.Minimum) * angle  / (2 * Math.PI);
        }
    }

    protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
    {
        _isPressed = true;
    }

    protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
    {
        _isPressed = false;
    }
}

这是Generic.xaml中包含的xaml

<Style TargetType="{x:Type local:HueWheel}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:HueWheel}">
                <Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
                    <Slider x:Name="PART_Slider">
                        <Slider.Template>
                            <ControlTemplate>
                                <Canvas x:Name="PART_FirstCanvas" Width="300" Height="300">
                                        <Image Stretch="Fill" Source="Assets/HueCircle.PNG" Focusable="False" Height="300" Width="300" RenderTransformOrigin="0.5,0.5">
                                            <Image.RenderTransform>
                                                <TransformGroup>
                                                    <ScaleTransform/>
                                                    <SkewTransform/>
                                                    <RotateTransform Angle="270"/>
                                                    <TranslateTransform/>
                                                </TransformGroup>
                                            </Image.RenderTransform>
                                        </Image>
                                        <Ellipse Fill="Transparent" Width="300" Height="300" Canvas.Left="0" Canvas.Top="0"/>
                                        <Canvas x:Name="PART_SecondCanvas">
                                            <Line Stroke="Transparent" StrokeThickness="5" X1="150" Y1="150" X2="150" Y2="0"/>
                                            <Ellipse Fill="Black" Width="20" Height="20" Canvas.Left="140" Canvas.Top="30"/>
                                            <Canvas.RenderTransform>
                                                <RotateTransform CenterX="150" CenterY="150">
                                                    <RotateTransform.Angle>
                                                        <MultiBinding Converter="{StaticResource ValueAngleConverter}">
                                                            <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Value"/>
                                                            <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Minimum"/>
                                                            <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Maximum"/>
                                                        </MultiBinding>
                                                    </RotateTransform.Angle>
                                                </RotateTransform>
                                            </Canvas.RenderTransform>
                                        </Canvas>
                                    </Canvas>
                            </ControlTemplate>
                        </Slider.Template>
                    </Slider>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

另外，仅当鼠标在画布上时才输出鼠标位置，因此在这里工作的一半左右……有点混乱。

这是我用于角度计算的两个函数：

public static double GetAngle(double value, double maximum, double minimum)
{
    double current = (value / (maximum - minimum)) * 360;
    if (current == 360)
        current = 359.999;

    return current;
}


public static double GetAngleR(Point pos, double radius)
{
    Point center = new Point(radius, radius);
    double xDiff = center.X - pos.X;
    double yDiff = center.Y - pos.Y;
    double r = Math.Sqrt(xDiff * xDiff + yDiff * yDiff);

    double angle = Math.Acos((center.Y - pos.Y) / r);

    if (pos.X < radius)
        angle = 2 * Math.PI - angle;

    if (Double.IsNaN(angle))
        return 0.0;
    else
        return angle;
}

有什么提示吗？

谢谢