Window: focus/blur (activation) events

src/Skyline.Apple/AppKitWindowBackend.cs

@@ -28,6 +28,7 @@ internal sealed class AppKitWindowBackend : IWindowBackend
 
     public event Action<(int Width, int Height)>? FramebufferResized;
     public event Action<bool>? MinimizedChanged;
+    public event Action<bool>? FocusChanged;
     public event Action<double>? Render;
     public event Action<PointerEvent>? Pointer;
     public event Action<KeyEvent>? Key;
@@ -162,6 +163,8 @@ public void Run()
 
     internal void OnMinimizedChanged(bool minimized) => MinimizedChanged?.Invoke(minimized);
 
+    internal void OnFocusChanged(bool focused) => FocusChanged?.Invoke(focused);
+
     public void Dispose()
     {
         _window.Delegate = null;
@@ -181,5 +184,9 @@ private sealed class WindowDelegate(AppKitWindowBackend backend) : NSWindowDeleg
         public override void DidMiniaturize(NSNotification notification) => backend.OnMinimizedChanged(true);
 
         public override void DidDeminiaturize(NSNotification notification) => backend.OnMinimizedChanged(false);
+
+        public override void DidBecomeKey(NSNotification notification) => backend.OnFocusChanged(true);
+
+        public override void DidResignKey(NSNotification notification) => backend.OnFocusChanged(false);
     }
 }

src/Skyline/AppWindow.cs

@@ -97,6 +97,7 @@ public AppWindow(AppWindowOptions? options = null)
         _backend.Pointer += e => PointerInput?.Invoke(e);
         _backend.Key += e => KeyInput?.Invoke(e);
         _backend.Text += e => TextInput?.Invoke(e);
+        _backend.FocusChanged += focused => FocusChanged?.Invoke(focused);
     }
 
     internal void RaisePointer(PointerEventKind kind, float x, float y, int button, float wheelDx, float wheelDy) =>
@@ -139,6 +140,12 @@ internal void RaiseText(char character) =>
     public event Action<KeyEvent>? KeyInput;
     public event Action<TextEvent>? TextInput;
 
+    /// <summary>
+    /// The window gained or lost focus (true = active). Pause animation,
+    /// timers, and media while blurred; resume on focus.
+    /// </summary>
+    public event Action<bool>? FocusChanged;
+
     /// <summary>
     /// When set and returning false, the frame is skipped and the loop
     /// sleeps briefly instead of rendering. Continuous-render apps leave

src/Skyline/GlfwWindowBackend.cs

@@ -22,6 +22,7 @@ internal sealed class GlfwWindowBackend : IWindowBackend
 
     public event Action<(int Width, int Height)>? FramebufferResized;
     public event Action<bool>? MinimizedChanged;
+    public event Action<bool>? FocusChanged;
     public event Action<double>? Render;
     public event Action<PointerEvent>? Pointer;
     public event Action<KeyEvent>? Key;
@@ -51,6 +52,7 @@ public unsafe GlfwWindowBackend(AppWindowOptions options)
 
         _window.FramebufferResize += sz => FramebufferResized?.Invoke((sz.X, sz.Y));
         _window.StateChanged += state => MinimizedChanged?.Invoke(state == WindowState.Minimized);
+        _window.FocusChanged += focused => FocusChanged?.Invoke(focused);
         _window.Render += delta => Render?.Invoke(delta);
 
         _input = _window.CreateInput();

src/Skyline/IWindowBackend.cs

@@ -31,6 +31,9 @@ internal interface IWindowBackend : IDisposable
     /// <summary>Minimized state changed; true when minimized.</summary>
     event Action<bool>? MinimizedChanged;
 
+    /// <summary>Focus changed; true when the window became active.</summary>
+    event Action<bool>? FocusChanged;
+
     /// <summary>A frame tick from the backend's own loop, with the delta seconds.</summary>
     event Action<double>? Render;
 

tests/Skyline.Tests/WindowBackendFactoryTests.cs

@@ -21,6 +21,7 @@ private sealed class FakeBackend : IWindowBackend
 
         public event Action<(int Width, int Height)>? FramebufferResized;
         public event Action<bool>? MinimizedChanged;
+        public event Action<bool>? FocusChanged;
         public event Action<double>? Render;
         public event Action<PointerEvent>? Pointer;
         public event Action<KeyEvent>? Key;