XAML-Map-Control/MapControl/MapPath.WPF.cs

// XAML Map Control - https://github.com/ClemensFischer/XAML-Map-Control
// © 2017 Clemens Fischer
// Licensed under the Microsoft Public License (Ms-PL)

using System.Windows;
using System.Windows.Media;
using System.Windows.Shapes;

namespace MapControl
{
    public partial class MapPath : Shape
    {
        public static readonly DependencyProperty DataProperty = DependencyProperty.Register(
            nameof(Data), typeof(Geometry), typeof(MapPath), new FrameworkPropertyMetadata(
                null, FrameworkPropertyMetadataOptions.AffectsRender, DataPropertyChanged, CoerceDataProperty));

        static MapPath()
        {
            StretchProperty.OverrideMetadata(typeof(MapPath),
                new FrameworkPropertyMetadata { CoerceValueCallback = (o, v) => Stretch.None });
        }

        public Geometry Data
        {
            get { return (Geometry)GetValue(DataProperty); }
            set { SetValue(DataProperty, value); }
        }

        protected override Geometry DefiningGeometry
        {
            get { return Data; }
        }

        protected override Size MeasureOverride(Size constraint)
        {
            // Shape.MeasureOverride sometimes returns an empty Size.
            return new Size(1, 1);
        }

        private static void DataPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            if (!ReferenceEquals(e.OldValue, e.NewValue))
            {
                var mapPath = (MapPath)obj;

                if (e.OldValue != null)
                {
                    ((Geometry)e.OldValue).ClearValue(Geometry.TransformProperty);
                }

                if (e.NewValue != null)
                {
                    ((Geometry)e.NewValue).Transform = mapPath.viewportTransform;
                }
            }
        }

        private static object CoerceDataProperty(DependencyObject obj, object value)
        {
            var data = (Geometry)value;

            return (data != null && data.IsFrozen) ? data.CloneCurrentValue() : data;
        }
    }
}