feat: Add Badge and Badge demo.

src/Ursa/Common/CornerPosition.cs

@@ -0,0 +1,9 @@
+namespace Ursa.Common;
+
+public enum CornerPosition
+{
+    TopLeft,
+    TopRight,
+    BottomLeft,
+    BottomRight,
+}

src/Ursa/Controls/Badge.cs

@@ -0,0 +1,105 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Primitives;
+using Avalonia.Media;
+using Avalonia.Styling;
+using Ursa.Common;
+
+namespace Ursa.Controls;
+
+[TemplatePart(PART_ContentPresenter, typeof(ContentPresenter))]
+[TemplatePart(PART_BadgeContainer, typeof(Border))]
+[TemplatePart(PART_BadgeContentPresenter, typeof(ContentPresenter))]
+public class Badge: ContentControl
+{
+    public const string PART_ContentPresenter = "PART_ContentPresenter";
+    public const string PART_BadgeContainer = "PART_BadgeContainer";
+    public const string PART_BadgeContentPresenter = "PART_BadgeContentPresenter";
+    
+    private ContentPresenter? _content;
+    private Border? _badgeContainer;
+    private ContentPresenter? _badgeContent;
+
+    public static readonly StyledProperty<ControlTheme> BadgeThemeProperty = AvaloniaProperty.Register<Badge, ControlTheme>(
+        nameof(BadgeTheme));
+    public ControlTheme BadgeTheme
+    {
+        get => GetValue(BadgeThemeProperty);
+        set => SetValue(BadgeThemeProperty, value);
+    }
+
+    public static readonly StyledProperty<bool> DotProperty = AvaloniaProperty.Register<Badge, bool>(
+        nameof(Dot));
+    public bool Dot
+    {
+        get => GetValue(DotProperty);
+        set => SetValue(DotProperty, value);
+    }
+
+    public static readonly StyledProperty<object?> BadgeContentProperty = AvaloniaProperty.Register<Badge, object?>(
+        nameof(BadgeContent));
+    public object? BadgeContent
+    {
+        get => GetValue(BadgeContentProperty);
+        set => SetValue(BadgeContentProperty, value);
+    }
+
+    public static readonly StyledProperty<CornerPosition> CornerPositionProperty = AvaloniaProperty.Register<Badge, CornerPosition>(
+        nameof(CornerPosition));
+    public CornerPosition CornerPosition
+    {
+        get => GetValue(CornerPositionProperty);
+        set => SetValue(CornerPositionProperty, value);
+    }
+
+    public static readonly StyledProperty<int> OverflowCountProperty = AvaloniaProperty.Register<Badge, int>(
+        nameof(OverflowCount));
+    public int OverflowCount
+    {
+        get => GetValue(OverflowCountProperty);
+        set => SetValue(OverflowCountProperty, value);
+    }
+
+    static Badge()
+    {
+        BadgeContentProperty.Changed.AddClassHandler<Badge>((badge, args) => badge.UpdateBadgePosition());
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        _content = e.NameScope.Find<ContentPresenter>(PART_ContentPresenter);
+        _badgeContainer = e.NameScope.Find<Border>(PART_BadgeContainer);
+        _badgeContent = e.NameScope.Find<ContentPresenter>(PART_BadgeContentPresenter);
+    }
+
+    protected override void OnLoaded()
+    {
+        base.OnLoaded();
+        UpdateBadgePosition();
+    }
+
+    protected override Size ArrangeOverride(Size finalSize)
+    {
+        UpdateBadgePosition();
+        return base.ArrangeOverride(finalSize);
+    }
+
+    private void UpdateBadgePosition()
+    {
+        var vertical = CornerPosition is CornerPosition.BottomLeft or CornerPosition.BottomRight ? 1 : -1;
+        var horizontal = CornerPosition is CornerPosition.TopRight or CornerPosition.BottomRight ? 1 : -1;
+        if (_badgeContainer is not null && _content?.Child is not null)
+        {
+            _badgeContainer.RenderTransform = new TransformGroup()
+            {
+                Children = new Transforms()
+                {
+                    new TranslateTransform(horizontal*_badgeContainer.Bounds.Width/2,vertical*_badgeContainer.Bounds.Height/2)
+                }
+            };
+        }
+    }
+}

src/Ursa/Converters/BadgeContentOverflowConverter.cs

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+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace Ursa.Converters;
+
+public class BadgeContentOverflowConverter: IMultiValueConverter
+{
+    public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+    {
+        string overflowMark = parameter is string s ? s : "+";
+        if (double.TryParse(values[0]?.ToString(), out var b) && values[1] is int i and > 0)
+        {
+            if (b > i)
+            {
+                return i + overflowMark;
+            }
+        }
+        return values[0];
+    }
+}