Added empty event callback function. Implemented for Windows only at the moment

.gitignore

@@ -101,3 +101,6 @@ tests/triangle-vulkan
 tests/window
 tests/windows
 
+/.idea
+/cmake-build-debug
+/.gitignore

include/GLFW/glfw3.h

@@ -1974,6 +1974,23 @@ typedef void (* GLFWmonitorfun)(GLFWmonitor* monitor, int event);
  */
 typedef void (* GLFWjoystickfun)(int jid, int event);
 
+/*! @brief The function pointer type for empty event callbacks.
+ *
+ *  This is the function pointer type for empty event callbacks.
+ *  An empty event callback function has the following signature:
+ *  @code
+ *  void function_name()
+ *  @endcode
+ *
+ *  @sa @ref empty_event
+ *  @sa @ref glfwSetEmptyEventCallback
+ *
+ *  @since Added in version 3.2.
+ *
+ *  @ingroup input
+ */
+typedef void (* GLFWemptyeventfun)(void);
+
 /*! @brief Video mode type.
  *
  *  This describes a single video mode.
@@ -5697,7 +5714,36 @@ GLFWAPI int glfwJoystickIsGamepad(int jid);
  *
  *  @ingroup input
  */
-GLFWAPI GLFWjoystickfun glfwSetJoystickCallback(GLFWjoystickfun callback);
+GLFWAPI GLFWjoystickfun glfwSetJoystickCallback(GLFWjoystickfun callback);/*! @brief Sets the joystick configuration callback.
+ *
+ *
+ *  This function sets the empty event callback, or removes the currently
+ *  set callback. This is called on the GLFW thread at some point in the future
+ *  when glfwPostEmptyEvent is called from any thread.
+ *
+ *  @param[in] callback The new callback, or `NULL` to remove the currently set
+ *  callback.
+ *  @return The previously set callback, or `NULL` if no callback was set or the
+ *  library had not been [initialized](@ref intro_init).
+ *
+ *  @callback_signature
+ *  @code
+ *  void function_name()
+ *  @endcode
+ *  For more information about the callback parameters, see the
+ *  [function pointer type](@ref GLFWemptyeventfun).
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED.
+ *
+ *  @thread_safety This function may be called from any thread
+ *
+ *  @sa @ref empty_event
+ *
+ *  @since Added in version TODO
+ *
+ *  @ingroup input
+ */
+GLFWAPI GLFWemptyeventfun glfwSetEmptyEventCallback(GLFWemptyeventfun callback);
 
 /*! @brief Adds the specified SDL_GameControllerDB gamepad mappings.
  *

src/input.c

@@ -1253,6 +1253,13 @@ GLFWAPI GLFWjoystickfun glfwSetJoystickCallback(GLFWjoystickfun cbfun)
     return cbfun;
 }
 
+GLFWAPI GLFWemptyeventfun glfwSetEmptyEventCallback(GLFWemptyeventfun cbfun)
+{
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+    _GLFW_SWAP(GLFWemptyeventfun, _glfw.callbacks.emptyEvent, cbfun);
+    return cbfun;
+}
+
 GLFWAPI int glfwUpdateGamepadMappings(const char* string)
 {
     int jid;

src/internal.h

@@ -866,6 +866,7 @@ struct _GLFWlibrary
     struct {
         GLFWmonitorfun  monitor;
         GLFWjoystickfun joystick;
+        GLFWemptyeventfun emptyEvent;
     } callbacks;
 
     // These are defined in platform.h

src/win32_init.c

@@ -363,6 +363,19 @@ static LRESULT CALLBACK helperWindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LP
 
             break;
         }
+
+        default:
+        {
+            // this check may not be necessary, but it checks just in case the message
+            // has not been registered. this skips having to check uMsg != WM_NULL
+            UINT emptyEventMsg = _glfw.win32.emptyEventMessage;
+            if (emptyEventMsg != 0 && uMsg == emptyEventMsg)
+            {
+                if (_glfw.callbacks.emptyEvent)
+                    _glfw.callbacks.emptyEvent();
+            }
+            break;
+        }
     }
 
     return DefWindowProcW(hWnd, uMsg, wParam, lParam);
@@ -405,6 +418,14 @@ static GLFWbool createHelperWindow(void)
         return GLFW_FALSE;
     }
 
+    _glfw.win32.emptyEventMessage = RegisterWindowMessageW(L"GLFW.EmptyEventMessage");
+    if (!_glfw.win32.emptyEventMessage)
+    {
+        _glfwInputErrorWin32(GLFW_PLATFORM_ERROR,
+                             "Win32: Failed to register empty event message");
+        return GLFW_FALSE;
+    }
+
     // HACK: The command to the first ShowWindow call is ignored if the parent
     //       process passed along a STARTUPINFO, so clear that with a no-op call
     ShowWindow(_glfw.win32.helperWindowHandle, SW_HIDE);

src/win32_platform.h

@@ -457,6 +457,7 @@ typedef struct _GLFWlibraryWin32
     RAWINPUT*           rawInput;
     int                 rawInputSize;
     UINT                mouseTrailSize;
+    UINT                emptyEventMessage;
 
     struct {
         HINSTANCE                       instance;

src/win32_window.c

@@ -2128,7 +2128,7 @@ void _glfwWaitEventsTimeoutWin32(double timeout)
 
 void _glfwPostEmptyEventWin32(void)
 {
-    PostMessageW(_glfw.win32.helperWindowHandle, WM_NULL, 0, 0);
+    PostMessageW(_glfw.win32.helperWindowHandle, _glfw.win32.emptyEventMessage, 0, 0);
 }
 
 void _glfwGetCursorPosWin32(_GLFWwindow* window, double* xpos, double* ypos)

tests/window.c

@@ -23,6 +23,16 @@
 //
 //========================================================================
 
+// TODO: Implement the postEmptyEvent test on multiple platforms. Works for Windows so far
+#ifdef WIN32
+#define USE_WIN32_THREAD_EMPTY_EVENT_TEST
+#endif // WIN32
+
+#ifdef USE_WIN32_THREAD_EMPTY_EVENT_TEST
+#include <Windows.h>
+#endif // USE_WIN32_THREAD_EMPTY_EVENT_TEST
+
+
 #define GLAD_GL_IMPLEMENTATION
 #include <glad/gl.h>
 #define GLFW_INCLUDE_NONE
@@ -48,6 +58,35 @@
 #include <stdlib.h>
 #include <limits.h>
 
+#ifdef USE_WIN32_THREAD_EMPTY_EVENT_TEST
+
+DWORD WINAPI win32ThreadEmptyEventTest(void)
+{
+    // Sleep for 3 seconds, and then post an empty event. The onEmptyEventPosted
+    // method should then be fired shortly after, on the GLFW main thread.
+
+    Sleep(3000);
+    glfwPostEmptyEvent();
+    return 0;
+}
+
+void onEmptyEventPosted(void)
+{
+    // If you hold your LMB down on the window border (aka initialise the resize
+    // phase), you will see a big difference in stack trace here compared to
+    // when not resizing. This is because DefWindowProc seems to use its own
+    // message loop temporarily, and passes any unhandled messages back to the
+    // dummy window's wndproc, which detects the empty event message, firing the
+    // event callback.
+    // This is the only effective way to run code on the main thread during the
+    // window resize phase while the user isn't moving their mouse, apart from the
+    // actual resize events obviously
+    MessageBoxExW(NULL, L"Callback event received!", L"Empty Event Callback", NULL, NULL);
+    // printf("Empty event received! Put a break point here, and this will be fired on the main thread");
+}
+
+#endif // USE_WIN32_THREAD_EMPTY_EVENT_TEST
+
 int main(int argc, char** argv)
 {
     int windowed_x, windowed_y, windowed_width, windowed_height;
@@ -66,12 +105,14 @@ int main(int argc, char** argv)
     if (!glfwInit())
         exit(EXIT_FAILURE);
 
+    glfwSetEmptyEventCallback(onEmptyEventPosted);
+
     glfwWindowHint(GLFW_SCALE_TO_MONITOR, GLFW_TRUE);
     glfwWindowHint(GLFW_WIN32_KEYBOARD_MENU, GLFW_TRUE);
     glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
     glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 1);
 
-    GLFWwindow* window = glfwCreateWindow(600, 600, "Window Features", NULL, NULL);
+    GLFWwindow* window = glfwCreateWindow(750, 600, "Window Features", NULL, NULL);
     if (!window)
     {
         glfwTerminate();
@@ -121,7 +162,7 @@ int main(int argc, char** argv)
         nk_glfw3_new_frame();
         if (nk_begin(nk, "main", area, 0))
         {
-            nk_layout_row_dynamic(nk, 30, 5);
+            nk_layout_row_dynamic(nk, 30, 6);
 
             if (nk_button_label(nk, "Toggle Fullscreen"))
             {
@@ -156,10 +197,16 @@ int main(int argc, char** argv)
                 const double time = glfwGetTime() + 3.0;
                 while (glfwGetTime() < time)
                     glfwWaitEventsTimeout(1.0);
-
                 glfwShowWindow(window);
             }
 
+#ifdef USE_WIN32_THREAD_EMPTY_EVENT_TEST
+            if (nk_button_label(nk, "Empty Event(3s)"))
+            {
+                CreateThread(NULL, 0, win32ThreadEmptyEventTest, NULL, 0, NULL);
+            }
+#endif // USE_WIN32_THREAD_EMPTY_EVENT_TEST
+
             nk_layout_row_dynamic(nk, 30, 1);
 
             if (glfwGetWindowAttrib(window, GLFW_MOUSE_PASSTHROUGH))