(GH-4050) Add CornerRadiusFilterConverter

Filters a CornerRadius by the given Filter property. Result can be a new CornerRadius or a value of it's 4 corners.

src/MahApps.Metro/Converters/CornerRadiusFilterConverter.cs

@@ -0,0 +1,63 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace MahApps.Metro.Converters
+{
+    /// <summary>
+    /// Filters a CornerRadius by the given Filter property. Result can be a new CornerRadius or a value of it's 4 corners.
+    /// </summary>
+    public class CornerRadiusFilterConverter : IValueConverter
+    {
+        public RadiusType Filter { get; set; } = RadiusType.None;
+
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (value is CornerRadius cornerRadius)
+            {
+                var filter = this.Filter;
+
+                // yes, we can override it with the parameter value
+                if (parameter is RadiusType radiusType)
+                {
+                    filter = radiusType;
+                }
+
+                switch (filter)
+                {
+                    default:
+                        return cornerRadius;
+                    case RadiusType.Left:
+                        return new CornerRadius(cornerRadius.TopLeft, 0, 0, cornerRadius.BottomLeft);
+                    case RadiusType.Top:
+                        return new CornerRadius(cornerRadius.TopLeft, cornerRadius.TopRight, 0, 0);
+                    case RadiusType.Right:
+                        return new CornerRadius(0, cornerRadius.TopRight, cornerRadius.BottomRight, 0);
+                    case RadiusType.Bottom:
+                        return new CornerRadius(0, 0, cornerRadius.BottomRight, cornerRadius.BottomLeft);
+                    case RadiusType.TopLeft:
+                        return cornerRadius.TopLeft;
+                    case RadiusType.TopRight:
+                        return cornerRadius.TopRight;
+                    case RadiusType.BottomRight:
+                        return cornerRadius.BottomRight;
+                    case RadiusType.BottomLeft:
+                        return cornerRadius.BottomLeft;
+                }
+            }
+
+            return Binding.DoNothing;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            // for now no back converting
+            return DependencyProperty.UnsetValue;
+        }
+    }
+}