From 96f2ca5443c3f87a5456cf38320631769952da6b Mon Sep 17 00:00:00 2001
From: Cody Mullins <1738479+codymullins@users.noreply.github.com>
Date: Tue, 16 Jun 2026 20:44:07 -0400
Subject: [PATCH] feat(window): window focus/blur (activation) events

Add AppWindow.FocusChanged (true = active) so a shell can pause
animation, timers, and media while the window is blurred and resume on
focus.

- GLFW backend forwards Silk.NET's IWindow.FocusChanged.
- AppKit backend raises it from windowDidBecomeKey / windowDidResignKey.

Addresses #10 (the focus/blur part). The occlusion signal is macOS-only
(NSWindow.occlusionState) with no GLFW equivalent, so it does not fit the
shared backend seam and is left as follow-up.
---
 src/Skyline.Apple/AppKitWindowBackend.cs         | 7 +++++++
 src/Skyline/AppWindow.cs                         | 7 +++++++
 src/Skyline/GlfwWindowBackend.cs                 | 2 ++
 src/Skyline/IWindowBackend.cs                    | 3 +++
 tests/Skyline.Tests/WindowBackendFactoryTests.cs | 1 +
 5 files changed, 20 insertions(+)

diff --git a/src/Skyline.Apple/AppKitWindowBackend.cs b/src/Skyline.Apple/AppKitWindowBackend.cs
index 91524de..09b11dc 100644
--- a/src/Skyline.Apple/AppKitWindowBackend.cs
+++ b/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);
     }
 }
diff --git a/src/Skyline/AppWindow.cs b/src/Skyline/AppWindow.cs
index 962a94a..08db5d0 100644
--- a/src/Skyline/AppWindow.cs
+++ b/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
diff --git a/src/Skyline/GlfwWindowBackend.cs b/src/Skyline/GlfwWindowBackend.cs
index 3d0d94f..5a5cc35 100644
--- a/src/Skyline/GlfwWindowBackend.cs
+++ b/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();
diff --git a/src/Skyline/IWindowBackend.cs b/src/Skyline/IWindowBackend.cs
index e1b01c9..b80da32 100644
--- a/src/Skyline/IWindowBackend.cs
+++ b/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;
 
diff --git a/tests/Skyline.Tests/WindowBackendFactoryTests.cs b/tests/Skyline.Tests/WindowBackendFactoryTests.cs
index a7b3ff2..649a793 100644
--- a/tests/Skyline.Tests/WindowBackendFactoryTests.cs
+++ b/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;