Add ContrastBrushConverter

src/Avalonia.Controls/ColorPicker/Converters/ContrastBrushConverter.cs

@@ -0,0 +1,88 @@
+using System;
+using System.Globalization;
+using Avalonia;
+using Avalonia.Controls.Primitives;
+using Avalonia.Data.Converters;
+using Avalonia.Media;
+
+namespace FinancialManager.UWP
+{
+    /// <summary>
+    /// Gets a color, either black or white, depending on the perceived brightness of the supplied color.
+    /// </summary>
+    public class ContrastBrushConverter : IValueConverter
+    {
+        /// <summary>
+        /// Gets or sets the alpha channel threshold below which a default color is used instead of black/white.
+        /// </summary>
+        public byte AlphaThreshold { get; set; } = 128;
+
+        /// <inheritdoc/>
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        {
+            Color comparisonColor;
+            Color? defaultColor = null;
+
+            // Get the changing color to compare against
+            if (value is Color valueColor)
+            {
+                comparisonColor = valueColor;
+            }
+            else if (value is HsvColor valueHsvColor)
+            {
+                comparisonColor = valueHsvColor.ToRgb();
+            }
+            else if (value is SolidColorBrush valueBrush)
+            {
+                comparisonColor = valueBrush.Color;
+            }
+            else
+            {
+                // Invalid color value provided
+                return AvaloniaProperty.UnsetValue;
+            }
+
+            // Get the default color when transparency is high
+            if (parameter is Color parameterColor)
+            {
+                defaultColor = parameterColor;
+            }
+            else if (parameter is HsvColor parameterHsvColor)
+            {
+                defaultColor = parameterHsvColor.ToRgb();
+            }
+            else if (parameter is SolidColorBrush parameterBrush)
+            {
+                defaultColor = parameterBrush.Color;
+            }
+
+            if (comparisonColor.A < AlphaThreshold &&
+                defaultColor.HasValue)
+            {
+                // If the transparency is less than the threshold just use the default brush
+                // This can commonly be something like the TextControlForeground brush
+                return new SolidColorBrush(defaultColor.Value);
+            }
+            else
+            {
+                // Chose a white/black brush based on contrast to the base color
+                if (ColorHelpers.GetRelativeLuminance(comparisonColor) > 0.5)
+                {
+                    // Bright color, use a dark for contrast
+                    return new SolidColorBrush(Colors.Black);
+                }
+                else
+                {
+                    // Dark color, use a light for contrast
+                    return new SolidColorBrush(Colors.White);
+                }
+            }
+        }
+
+        /// <inheritdoc/>
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}