Fix for unnecessary updates of joysticks and gamepads in GLFW backend

src/Input/Silk.NET.Input.Glfw/GlfwGamepad.cs

@@ -23,7 +23,7 @@ public GlfwGamepad(int i)
             Thumbsticks = new Thumbstick[GamepadThumbstickCount];
             Triggers = new Trigger[GamepadTriggerCount];
 
-            _connected = hasState;
+            IsConnected = _connected = hasState;
 
             for (int j = 0; j < GamepadButtonCount; j++)
             {
@@ -44,7 +44,12 @@ public GlfwGamepad(int i)
 
         public string Name => GlfwProvider.GLFW.Value.GetGamepadName(Index) ?? "Silk.NET Gamepad (via GLFW)";
         public int Index { get; }
-        public bool IsConnected => GlfwProvider.GLFW.Value.JoystickIsGamepad(Index);
+
+        /// <summary>
+        /// Cached connection status from GLFW's joystick connection callback.
+        /// </summary>
+        public bool IsConnected { get; internal set; }
+
         public IReadOnlyList<Button> Buttons { get; }
         public IReadOnlyList<Thumbstick> Thumbsticks { get; }
         public IReadOnlyList<Trigger> Triggers { get; }
@@ -57,8 +62,10 @@ public GlfwGamepad(int i)
 
         public void Update()
         {
-            if (!GlfwProvider.GLFW.Value.GetGamepadState(Index, out var state))
+            if (!IsConnected || !GlfwProvider.GLFW.Value.GetGamepadState(Index, out var state))
             {
+                IsConnected = false;
+
                 // Detect when this gamepad disconnects
                 if (_connected)
                 {

src/Input/Silk.NET.Input.Glfw/GlfwInputContext.cs

@@ -49,6 +49,7 @@ public unsafe GlfwInputContext(IView window) : base(window)
             Mice = _mice;
 
             GlfwInputPlatform.RegisterWindow((WindowHandle*) Handle, _subscribers);
+            GlfwInputPlatform.JoystickConnectionChanged += OnJoystickConnectionChanged;
         }
 
         public override void ProcessEvents()
@@ -69,8 +70,24 @@ public override void ProcessEvents()
             }
         }
 
+        // Cache connection status on the devices so that ProcessEvents only polls slots that actually have something plugged
+        // in, rather than asking GLFW about all gamepads and all joysticks every single frame.
+        private void OnJoystickConnectionChanged(int jid, ConnectedState state)
+        {
+            if (jid < 0 || jid >= _joysticks.Length)
+            {
+                return;
+            }
+
+            var connected = state == ConnectedState.Connected;
+            var isGamepad = connected && GlfwProvider.GLFW.Value.JoystickIsGamepad(jid);
+            _gamepads[jid].IsConnected = connected && isGamepad;
+            _joysticks[jid].IsConnected = connected && !isGamepad;
+        }
+
         public override unsafe void CoreDispose()
         {
+            GlfwInputPlatform.JoystickConnectionChanged -= OnJoystickConnectionChanged;
             GlfwInputPlatform.UnregisterWindow((WindowHandle*) Handle, _subscribers);
             foreach (var gamepad in _gamepads)
             {

src/Input/Silk.NET.Input.Glfw/GlfwInputPlatform.cs

@@ -18,6 +18,17 @@ internal class GlfwInputPlatform : IInputPlatform
     {
         private static readonly Dictionary<nint, GlfwEvents> _subs = new Dictionary<nint, GlfwEvents>();
 
+        // Unlike the keyboard/mouse callbacks, GLFW's joystick callback is global rather than per window, so it is
+        // owned here instead of by GlfwEvents.cs and available to every live input context.
+        private static readonly GlfwCallbacks.JoystickCallback _joystickCallback =
+            (jid, state) => JoystickConnectionChanged?.Invoke(jid, state);
+
+        /// <summary>
+        /// Raised when GLFW reports that a joystick has been connected to or disconnected from the system. The
+        /// underlying GLFW callback is only installed while at least one input context is alive.
+        /// </summary>
+        internal static event GlfwCallbacks.JoystickCallback? JoystickConnectionChanged;
+
         /// <inheritdoc />
         public bool IsApplicable(IView window) => window is GlfwWindow;
 
@@ -30,6 +41,12 @@ internal static unsafe void RegisterWindow(WindowHandle* handle, IEnumerable<IGl
             {
                 throw new InvalidOperationException($"More than one input context for window {(nint) handle}.");
             }
+
+            if (_subs.Count == 0)
+            {
+                GlfwProvider.GLFW.Value.SetJoystickCallback(_joystickCallback);
+            }
+
             var events = _subs[(nint) handle] = new GlfwEvents(handle);
             foreach (var subscriber in subscribers)
             {
@@ -48,6 +65,11 @@ internal static unsafe void UnregisterWindow(WindowHandle* handle, IEnumerable<I
 
                 events.Dispose();
                 _subs.Remove((nint) handle);
+
+                if (_subs.Count == 0)
+                {
+                    GlfwProvider.GLFW.Value.SetJoystickCallback(null);
+                }
             }
             else
             {

src/Input/Silk.NET.Input.Glfw/GlfwJoystick.cs

@@ -18,14 +18,17 @@ public GlfwJoystick(int i)
             Buttons = new Button[0];
             Hats = new Hat[0];
 
-            _connected = IsConnected;
+            IsConnected = _connected = GlfwProvider.GLFW.Value.JoystickPresent(i) &&
+                                       !GlfwProvider.GLFW.Value.JoystickIsGamepad(i);
         }
 
         public string Name => GlfwProvider.GLFW.Value.GetJoystickName(Index) ?? "Silk.NET Joystick (via GLFW)";
         public int Index { get; }
 
-        public bool IsConnected => GlfwProvider.GLFW.Value.JoystickPresent(Index) &&
-                                   !GlfwProvider.GLFW.Value.JoystickIsGamepad(Index);
+        /// <summary>
+        /// Cached connection status from GLFW's joystick connection callback.
+        /// </summary>
+        public bool IsConnected { get; internal set; }
 
         public IReadOnlyList<Axis> Axes { get; private set; }
         public IReadOnlyList<Button> Buttons { get; private set; }