feat(WPF): Observe system theme during runtime

Author: trungnt2910

src/Uno.UI.Runtime.Skia.Wpf/WpfApplicationExtension.cs

@@ -1,20 +1,75 @@
 using System;
+using Microsoft.Win32;
+using Uno.Helpers.Theming;
+using Uno.UI.Runtime.Skia.Wpf.WPF.Extensions.Helper.Theming;
 using Uno.UI.Xaml;
 using Windows.UI.Xaml;
 
 namespace Uno.UI.Runtime.Skia.Wpf
 {
-	public class WpfApplicationExtension : IApplicationExtension
+	public class WpfApplicationExtension : IApplicationExtension, IDisposable
 	{
 		private readonly Application _owner;
+		private WpfSystemThemeHelperExtension _themeHelper;
+		private SystemTheme _currentTheme;
+
+		private bool _disposedValue;
 
 		public WpfApplicationExtension(Application owner)
 		{
 			_owner = owner ?? throw new ArgumentNullException(nameof(owner));
+
+			_themeHelper = new WpfSystemThemeHelperExtension(null);
+			_currentTheme = _themeHelper.GetSystemTheme();
+
+			SystemEvents.UserPreferenceChanged += SystemEvents_UserPreferenceChanged;
 		}
 
 		public bool CanExit => true;
 
 		public void Exit() => System.Windows.Application.Current.Shutdown();
+
+		public event EventHandler SystemThemeChanged;
+
+		private void SystemEvents_UserPreferenceChanged(object sender, UserPreferenceChangedEventArgs e)
+		{
+			// This event is raised when the theme is changed,
+			// but also for various other irrelevant reasons.
+			// Check the registry first, before forcing the application
+			// to refresh everything.
+			var newTheme = _themeHelper.GetSystemTheme();
+
+			if (newTheme != _currentTheme)
+            {
+				_currentTheme = newTheme;
+				SystemThemeChanged?.Invoke(this, EventArgs.Empty);
+			}
+		}
+
+		#region IDisposable
+		protected virtual void Dispose(bool disposing)
+		{
+			if (!_disposedValue)
+			{
+				// According to Microsoft, https://docs.microsoft.com/en-us/dotnet/api/microsoft.win32.systemevents.userpreferencechanged,
+				// this event must be manully unhooked when the application is disposed.
+				SystemEvents.UserPreferenceChanged -= SystemEvents_UserPreferenceChanged;
+				_disposedValue = true;
+			}
+		}
+
+		~WpfApplicationExtension()
+		{
+			// Do not change this code. Put cleanup code in 'Dispose(bool disposing)' method
+			Dispose(disposing: false);
+		}
+
+		public void Dispose()
+		{
+			// Do not change this code. Put cleanup code in 'Dispose(bool disposing)' method
+			Dispose(disposing: true);
+			GC.SuppressFinalize(this);
+		}
+		#endregion
 	}
 }

src/Uno.UI/UI/Xaml/Application.skia.cs

@@ -78,11 +78,21 @@ private void Initialize()
 				// Force init
 				Window.Current.ToString();
 
+				InitializationCompleted();
+
 				OnLaunched(new LaunchActivatedEventArgs(ActivationKind.Launch, string.Join(";", _args)));
 			}
 		}
 
 		internal void ForceSetRequestedTheme(ApplicationTheme theme) => _requestedTheme = theme;
+
+		partial void ObserveSystemThemeChanges()
+		{
+			_applicationExtension.SystemThemeChanged += SystemThemeChanged;
+			_systemThemeChangesObserved = true;
+		}
+
+		private void SystemThemeChanged(object sender, EventArgs e) => OnSystemThemeChanged();
 	}
 
 	internal interface IApplicationEvents

src/Uno.UI/UI/Xaml/IApplicationExtension.cs

@@ -1,9 +1,13 @@
-namespace Uno.UI.Xaml
+using System;
+
+namespace Uno.UI.Xaml
 {
 	internal interface IApplicationExtension
 	{
 		bool CanExit { get; }
 
 		void Exit();
+
+		event EventHandler SystemThemeChanged;
 	}
 }