WPF中Polygon.Points绑定替代方案：基于线段坐标生成顶点

方案一：正确使用MultiValueConverter绑定Points #

你之前多绑定失败，多半是转换器实现或者绑定写法不对，下面是完整的正确操作步骤：

1. 编写MultiValueConverter
   实现IMultiValueConverter接口，将X1、Y1、X2、Y2四个值转换为多边形顶点集合，示例以线段中点的三角形为例，你可以根据需求修改顶点计算逻辑：

public class LineToPolygonPointsConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values.Length != 4 || values.Any(v => v == DependencyProperty.UnsetValue))
            return new PointCollection();

        double x1 = (double)values[0];
        double y1 = (double)values[1];
        double x2 = (double)values[2];
        double y2 = (double)values[3];

        // 计算线段中点
        var midX = (x1 + x2) / 2;
        var midY = (y1 + y2) / 2;
        // 生成线段垂直方向的向量，用于确定多边形顶点位置
        var dx = x2 - x1;
        var dy = y2 - y1;
        var perpDx = -dy;
        var perpDy = dx;
        // 调整多边形大小，10为顶点偏移量，可按需修改
        var length = Math.Sqrt(perpDx * perpDx + perpDy * perpDy);
        if (length == 0) return new PointCollection();
        var scale = 10 / length;
        perpDx *= scale;
        perpDy *= scale;

        return new PointCollection
        {
            new Point(midX, midY),
            new Point(midX + perpDx, midY + perpDy),
            new Point(midX - perpDx, midY - perpDy)
        };
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException(); // 反向转换无需实现
    }
}

2. 在XAML中配置绑定
   先在资源中声明转换器，再给Polygon的Points属性设置多绑定：

<Window.Resources>
    <!-- local为转换器所在命名空间，记得替换成你的实际命名空间 -->
    <local:LineToPolygonPointsConverter x:Key="LineToPolygonConverter"/>
</Window.Resources>

<Polygon Fill="Black">
    <Polygon.Points>
        <MultiBinding Converter="{StaticResource LineToPolygonConverter}">
            <Binding Path="X1"/>
            <Binding Path="Y1"/>
            <Binding Path="X2"/>
            <Binding Path="Y2"/>
        </MultiBinding>
    </Polygon.Points>
</Polygon>

这样X1/Y1/X2/Y2任一值变化时，转换器都会自动重新计算多边形顶点。

方案二：ViewModel中添加计算属性（无需转换器） #

如果不想使用转换器，可直接在ViewModel中添加只读的PolygonPoints属性，依赖X1/Y1/X2/Y2的变化自动更新：

public PointCollection PolygonPoints
{
    get
    {
        var midX = (X1 + X2) / 2;
        var midY = (Y1 + Y2) / 2;
        var dx = X2 - X1;
        var dy = Y2 - Y1;
        var perpDx = -dy;
        var perpDy = dx;
        var length = Math.Sqrt(perpDx * perpDx + perpDy * perpDy);
        if (length == 0) return new PointCollection();
        var scale = 10 / length;
        perpDx *= scale;
        perpDy *= scale;

        return new PointCollection
        {
            new Point(midX, midY),
            new Point(midX + perpDx, midY + perpDy),
            new Point(midX - perpDx, midY - perpDy)
        };
    }
}

然后在每个坐标属性的Set方法中，触发PolygonPoints的属性变更通知：

private double _X1;
public double X1
{
    get => _X1;
    set
    {
        SetProperty(ref _X1, value, () => X1);
        OnPropertyChanged(() => PolygonPoints);
    }
}

// Y1、X2、Y2的Set方法需同样添加OnPropertyChanged(() => PolygonPoints);

最后在XAML中直接绑定该属性：

<Polygon Fill="Black" Points="{Binding PolygonPoints}"/>

这种方式将逻辑集中在ViewModel中，适合顶点计算逻辑复杂的场景。

方案三：使用附加属性（视图层封装） #

如果希望将逻辑封装在视图层，不修改ViewModel，可以编写附加属性：

public static class PolygonHelper
{
    public static readonly DependencyProperty LineEndPointsProperty =
        DependencyProperty.RegisterAttached(
            "LineEndPoints", 
            typeof(double[]), 
            typeof(PolygonHelper),
            new PropertyMetadata(null, OnLineEndPointsChanged));

    public static double[] GetLineEndPoints(DependencyObject obj)
    {
        return (double[])obj.GetValue(LineEndPointsProperty);
    }

    public static void SetLineEndPoints(DependencyObject obj, double[] value)
    {
        obj.SetValue(LineEndPointsProperty, value);
    }

    private static void OnLineEndPointsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not Polygon polygon || e.NewValue is not double[] points || points.Length != 4)
            return;

        double x1 = points[0];
        double y1 = points[1];
        double x2 = points[2];
        double y2 = points[3];

        // 复用顶点计算逻辑
        var midX = (x1 + x2) / 2;
        var midY = (y1 + y2) / 2;
        var dx = x2 - x1;
        var dy = y2 - y1;
        var perpDx = -dy;
        var perpDy = dx;
        var length = Math.Sqrt(perpDx * perpDx + perpDy * perpDy);
        if (length == 0)
        {
            polygon.Points = new PointCollection();
            return;
        }
        var scale = 10 / length;
        perpDx *= scale;
        perpDy *= scale;

        polygon.Points = new PointCollection
        {
            new Point(midX, midY),
            new Point(midX + perpDx, midY + perpDy),
            new Point(midX - perpDx, midY - perpDy)
        };
    }
}

在XAML中通过多绑定给附加属性传递坐标值：

<Polygon Fill="Black">
    <local:PolygonHelper.LineEndPoints>
        <MultiBinding>
            <Binding Path="X1"/>
            <Binding Path="Y1"/>
            <Binding Path="X2"/>
            <Binding Path="Y2"/>
        </MultiBinding>
    </local:PolygonHelper.LineEndPoints>
</Polygon>

这种方式适合需要在多个视图中复用该逻辑的场景，无需重复编写转换器或ViewModel属性。

内容的提问来源于stack exchange，提问作者Danial