feat: Theme toggler

src/Ursa/Controls/ThemeSelector/ThemeToggleButton.cs

@@ -0,0 +1,45 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Primitives;
+using Avalonia.Interactivity;
+using Avalonia.Styling;
+using Ursa.Common;
+
+namespace Ursa.Controls;
+
+[TemplatePart(PART_ThemeToggleButton, typeof(ToggleButton))]
+public class ThemeToggleButton: ThemeSelectorBase
+{
+    public const string PART_ThemeToggleButton = "PART_ThemeToggleButton";
+
+    /// <summary>
+    /// This button IsChecked=true means ThemeVariant.Light, IsChecked=false means ThemeVariant.Dark.
+    /// </summary>
+    private ToggleButton? _button;
+    private ThemeVariant? _currentTheme;
+    
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        _currentTheme = this.ActualThemeVariant;
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        EventHelper.UnregisterEvent(ToggleButton.IsCheckedChangedEvent, OnButtonCheckedChanged, _button);
+        _button = e.NameScope.Get<ToggleButton>(PART_ThemeToggleButton);
+        EventHelper.RegisterEvent(ToggleButton.IsCheckedChangedEvent, OnButtonCheckedChanged, _button);
+        PropertyHelper.SetValue(ToggleButton.IsCheckedProperty, _currentTheme == ThemeVariant.Light, _button);
+    }
+
+    private void OnButtonCheckedChanged(object sender, RoutedEventArgs e)
+    {
+        var newTheme = (sender as ToggleButton)!.IsChecked;
+        if (newTheme is null) return;
+        SelectedTheme = newTheme.Value ? ThemeVariant.Light : ThemeVariant.Dark;
+    }
+    
+    
+}