Backends: Win32: rework to add ImGui_ImplWin32_WndProcHandlerEx() not using current context (experimental). (#8069, #6293, #5856, #586)

+ GetIOEx(). Amend fedf45c + cba656a. Amend 416cfdb9.

backends/imgui_impl_win32.cpp

@@ -133,12 +133,16 @@ static ImGui_ImplWin32_Data* ImGui_ImplWin32_GetBackendData()
 {
     return ImGui::GetCurrentContext() ? (ImGui_ImplWin32_Data*)ImGui::GetIO().BackendPlatformUserData : nullptr;
 }
+static ImGui_ImplWin32_Data* ImGui_ImplWin32_GetBackendData(ImGuiIO& io)
+{
+    return (ImGui_ImplWin32_Data*)io.BackendPlatformUserData;
+}
 
 // Functions
-static void ImGui_ImplWin32_UpdateKeyboardCodePage()
+static void ImGui_ImplWin32_UpdateKeyboardCodePage(ImGuiIO& io)
 {
     // Retrieve keyboard code page, required for handling of non-Unicode Windows.
-    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData();
+    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData(io);
     HKL keyboard_layout = ::GetKeyboardLayout(0);
     LCID keyboard_lcid = MAKELCID(HIWORD(keyboard_layout), SORT_DEFAULT);
     if (::GetLocaleInfoA(keyboard_lcid, (LOCALE_RETURN_NUMBER | LOCALE_IDEFAULTANSICODEPAGE), (LPSTR)&bd->KeyboardCodePage, sizeof(bd->KeyboardCodePage)) == 0)
@@ -168,7 +172,7 @@ static bool ImGui_ImplWin32_InitEx(void* hwnd, bool platform_has_own_dc)
     bd->TicksPerSecond = perf_frequency;
     bd->Time = perf_counter;
     bd->LastMouseCursor = ImGuiMouseCursor_COUNT;
-    ImGui_ImplWin32_UpdateKeyboardCodePage();
+    ImGui_ImplWin32_UpdateKeyboardCodePage(io);
 
     // Set platform dependent data in viewport
     ImGuiViewport* main_viewport = ImGui::GetMainViewport();
@@ -228,13 +232,11 @@ void    ImGui_ImplWin32_Shutdown()
     IM_DELETE(bd);
 }
 
-static bool ImGui_ImplWin32_UpdateMouseCursor()
+static bool ImGui_ImplWin32_UpdateMouseCursor(ImGuiIO& io, ImGuiMouseCursor imgui_cursor)
 {
-    ImGuiIO& io = ImGui::GetIO();
     if (io.ConfigFlags & ImGuiConfigFlags_NoMouseCursorChange)
         return false;
 
-    ImGuiMouseCursor imgui_cursor = ImGui::GetMouseCursor();
     if (imgui_cursor == ImGuiMouseCursor_None || io.MouseDrawCursor)
     {
         // Hide OS mouse cursor if imgui is drawing it or if it wants no cursor
@@ -266,42 +268,39 @@ static bool IsVkDown(int vk)
     return (::GetKeyState(vk) & 0x8000) != 0;
 }
 
-static void ImGui_ImplWin32_AddKeyEvent(ImGuiKey key, bool down, int native_keycode, int native_scancode = -1)
+static void ImGui_ImplWin32_AddKeyEvent(ImGuiIO& io, ImGuiKey key, bool down, int native_keycode, int native_scancode = -1)
 {
-    ImGuiIO& io = ImGui::GetIO();
     io.AddKeyEvent(key, down);
     io.SetKeyEventNativeData(key, native_keycode, native_scancode); // To support legacy indexing (<1.87 user code)
     IM_UNUSED(native_scancode);
 }
 
-static void ImGui_ImplWin32_ProcessKeyEventsWorkarounds()
+static void ImGui_ImplWin32_ProcessKeyEventsWorkarounds(ImGuiIO& io)
 {
     // Left & right Shift keys: when both are pressed together, Windows tend to not generate the WM_KEYUP event for the first released one.
     if (ImGui::IsKeyDown(ImGuiKey_LeftShift) && !IsVkDown(VK_LSHIFT))
-        ImGui_ImplWin32_AddKeyEvent(ImGuiKey_LeftShift, false, VK_LSHIFT);
+        ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_LeftShift, false, VK_LSHIFT);
     if (ImGui::IsKeyDown(ImGuiKey_RightShift) && !IsVkDown(VK_RSHIFT))
-        ImGui_ImplWin32_AddKeyEvent(ImGuiKey_RightShift, false, VK_RSHIFT);
+        ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_RightShift, false, VK_RSHIFT);
 
     // Sometimes WM_KEYUP for Win key is not passed down to the app (e.g. for Win+V on some setups, according to GLFW).
     if (ImGui::IsKeyDown(ImGuiKey_LeftSuper) && !IsVkDown(VK_LWIN))
-        ImGui_ImplWin32_AddKeyEvent(ImGuiKey_LeftSuper, false, VK_LWIN);
+        ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_LeftSuper, false, VK_LWIN);
     if (ImGui::IsKeyDown(ImGuiKey_RightSuper) && !IsVkDown(VK_RWIN))
-        ImGui_ImplWin32_AddKeyEvent(ImGuiKey_RightSuper, false, VK_RWIN);
+        ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_RightSuper, false, VK_RWIN);
 }
 
-static void ImGui_ImplWin32_UpdateKeyModifiers()
+static void ImGui_ImplWin32_UpdateKeyModifiers(ImGuiIO& io)
 {
-    ImGuiIO& io = ImGui::GetIO();
     io.AddKeyEvent(ImGuiMod_Ctrl, IsVkDown(VK_CONTROL));
     io.AddKeyEvent(ImGuiMod_Shift, IsVkDown(VK_SHIFT));
     io.AddKeyEvent(ImGuiMod_Alt, IsVkDown(VK_MENU));
     io.AddKeyEvent(ImGuiMod_Super, IsVkDown(VK_LWIN) || IsVkDown(VK_RWIN));
 }
 
-static void ImGui_ImplWin32_UpdateMouseData()
+static void ImGui_ImplWin32_UpdateMouseData(ImGuiIO& io)
 {
-    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData();
+    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData(io);
-    ImGuiIO& io = ImGui::GetIO();
     IM_ASSERT(bd->hWnd != 0);
 
     HWND focused_window = ::GetForegroundWindow();
@@ -328,11 +327,10 @@ static void ImGui_ImplWin32_UpdateMouseData()
 }
 
 // Gamepad navigation mapping
-static void ImGui_ImplWin32_UpdateGamepads()
+static void ImGui_ImplWin32_UpdateGamepads(ImGuiIO& io)
 {
 #ifndef IMGUI_IMPL_WIN32_DISABLE_GAMEPAD
-    ImGuiIO& io = ImGui::GetIO();
+    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData(io);
-    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData();
     //if ((io.ConfigFlags & ImGuiConfigFlags_NavEnableGamepad) == 0) // FIXME: Technically feeding gamepad shouldn't depend on this now that they are regular inputs.
     //    return;
 
@@ -381,7 +379,9 @@ static void ImGui_ImplWin32_UpdateGamepads()
     MAP_ANALOG(ImGuiKey_GamepadRStickDown,      gamepad.sThumbRY, -XINPUT_GAMEPAD_LEFT_THUMB_DEADZONE, -32768);
     #undef MAP_BUTTON
     #undef MAP_ANALOG
-#endif // #ifndef IMGUI_IMPL_WIN32_DISABLE_GAMEPAD
+#else // #ifndef IMGUI_IMPL_WIN32_DISABLE_GAMEPAD
+    IM_UNUSED(io);
+#endif
 }
 
 void    ImGui_ImplWin32_NewFrame()
@@ -402,21 +402,21 @@ void    ImGui_ImplWin32_NewFrame()
     bd->Time = current_time;
 
     // Update OS mouse position
-    ImGui_ImplWin32_UpdateMouseData();
+    ImGui_ImplWin32_UpdateMouseData(io);
 
     // Process workarounds for known Windows key handling issues
-    ImGui_ImplWin32_ProcessKeyEventsWorkarounds();
+    ImGui_ImplWin32_ProcessKeyEventsWorkarounds(io);
 
     // Update OS mouse cursor with the cursor requested by imgui
     ImGuiMouseCursor mouse_cursor = io.MouseDrawCursor ? ImGuiMouseCursor_None : ImGui::GetMouseCursor();
     if (bd->LastMouseCursor != mouse_cursor)
     {
         bd->LastMouseCursor = mouse_cursor;
-        ImGui_ImplWin32_UpdateMouseCursor();
+        ImGui_ImplWin32_UpdateMouseCursor(io, mouse_cursor);
     }
 
     // Update game controllers (if enabled and available)
-    ImGui_ImplWin32_UpdateGamepads();
+    ImGui_ImplWin32_UpdateGamepads(io);
 }
 
 // Map VK_xxx to ImGuiKey_xxx.
@@ -578,21 +578,27 @@ static ImGuiMouseSource ImGui_ImplWin32_GetMouseSourceFromMessageExtraInfo()
 // - When io.WantCaptureMouse is true, do not dispatch mouse input data to your main application, or clear/overwrite your copy of the mouse data.
 // - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application, or clear/overwrite your copy of the keyboard data.
 // Generally you may always pass all inputs to Dear ImGui, and hide them from your application based on those two flags.
-// PS: In this Win32 handler, we use the capture API (GetCapture/SetCapture/ReleaseCapture) to be able to read mouse coordinates when dragging mouse outside of our window bounds.
 // PS: We treat DBLCLK messages as regular mouse down messages, so this code will work on windows classes that have the CS_DBLCLKS flag set. Our own example app code doesn't set this flag.
 
-// Copy this line into your .cpp file to forward declare the function:
+// Copy either line into your .cpp file to forward declare the function:
-extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
+extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);                // Use ImGui::GetCurrentContext()
+extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandlerEx(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam, ImGuiIO& io); // Doesn't use ImGui::GetCurrentContext()
 
 IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 {
     // Most backends don't have silent checks like this one, but we need it because WndProc are called early in CreateWindow().
     // We silently allow both context or just only backend data to be nullptr.
-    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData();
+    if (ImGui::GetCurrentContext() == NULL)
-    if (bd == nullptr)
         return 0;
-    ImGuiIO& io = ImGui::GetIO();
+    return ImGui_ImplWin32_WndProcHandlerEx(hwnd, msg, wParam, lParam, ImGui::GetIO());
+}
 
+// This version is in theory thread-safe in the sense that no path should access ImGui::GetCurrentContext().
+IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandlerEx(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam, ImGuiIO& io)
+{
+    ImGui_ImplWin32_Data* bd = ImGui_ImplWin32_GetBackendData(io);
+    if (bd == NULL)
+        return 0;
     switch (msg)
     {
     case WM_MOUSEMOVE:
@@ -653,7 +659,7 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARA
         if (msg == WM_MBUTTONDOWN || msg == WM_MBUTTONDBLCLK) { button = 2; }
         if (msg == WM_XBUTTONDOWN || msg == WM_XBUTTONDBLCLK) { button = (GET_XBUTTON_WPARAM(wParam) == XBUTTON1) ? 3 : 4; }
         if (bd->MouseButtonsDown == 0 && ::GetCapture() == nullptr)
-            ::SetCapture(hwnd);
+            ::SetCapture(hwnd); // Allow us to read mouse coordinates when dragging mouse outside of our window bounds.
         bd->MouseButtonsDown |= 1 << button;
         io.AddMouseSourceEvent(mouse_source);
         io.AddMouseButtonEvent(button, true);
@@ -692,7 +698,7 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARA
         if (wParam < 256)
         {
             // Submit modifiers
-            ImGui_ImplWin32_UpdateKeyModifiers();
+            ImGui_ImplWin32_UpdateKeyModifiers(io);
 
             // Obtain virtual key code and convert to ImGuiKey
             const ImGuiKey key = ImGui_ImplWin32_KeyEventToImGuiKey(wParam, lParam);
@@ -701,28 +707,28 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARA
 
             // Special behavior for VK_SNAPSHOT / ImGuiKey_PrintScreen as Windows doesn't emit the key down event.
             if (key == ImGuiKey_PrintScreen && !is_key_down)
-                ImGui_ImplWin32_AddKeyEvent(key, true, vk, scancode);
+                ImGui_ImplWin32_AddKeyEvent(io, key, true, vk, scancode);
 
             // Submit key event
             if (key != ImGuiKey_None)
-                ImGui_ImplWin32_AddKeyEvent(key, is_key_down, vk, scancode);
+                ImGui_ImplWin32_AddKeyEvent(io, key, is_key_down, vk, scancode);
 
             // Submit individual left/right modifier events
             if (vk == VK_SHIFT)
             {
                 // Important: Shift keys tend to get stuck when pressed together, missing key-up events are corrected in ImGui_ImplWin32_ProcessKeyEventsWorkarounds()
-                if (IsVkDown(VK_LSHIFT) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(ImGuiKey_LeftShift, is_key_down, VK_LSHIFT, scancode); }
+                if (IsVkDown(VK_LSHIFT) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_LeftShift, is_key_down, VK_LSHIFT, scancode); }
-                if (IsVkDown(VK_RSHIFT) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(ImGuiKey_RightShift, is_key_down, VK_RSHIFT, scancode); }
+                if (IsVkDown(VK_RSHIFT) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_RightShift, is_key_down, VK_RSHIFT, scancode); }
             }
             else if (vk == VK_CONTROL)
             {
-                if (IsVkDown(VK_LCONTROL) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(ImGuiKey_LeftCtrl, is_key_down, VK_LCONTROL, scancode); }
+                if (IsVkDown(VK_LCONTROL) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_LeftCtrl, is_key_down, VK_LCONTROL, scancode); }
-                if (IsVkDown(VK_RCONTROL) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(ImGuiKey_RightCtrl, is_key_down, VK_RCONTROL, scancode); }
+                if (IsVkDown(VK_RCONTROL) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_RightCtrl, is_key_down, VK_RCONTROL, scancode); }
             }
             else if (vk == VK_MENU)
             {
-                if (IsVkDown(VK_LMENU) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(ImGuiKey_LeftAlt, is_key_down, VK_LMENU, scancode); }
+                if (IsVkDown(VK_LMENU) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_LeftAlt, is_key_down, VK_LMENU, scancode); }
-                if (IsVkDown(VK_RMENU) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(ImGuiKey_RightAlt, is_key_down, VK_RMENU, scancode); }
+                if (IsVkDown(VK_RMENU) == is_key_down) { ImGui_ImplWin32_AddKeyEvent(io, ImGuiKey_RightAlt, is_key_down, VK_RMENU, scancode); }
             }
         }
         return 0;
@@ -732,7 +738,7 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARA
         io.AddFocusEvent(msg == WM_SETFOCUS);
         return 0;
     case WM_INPUTLANGCHANGE:
-        ImGui_ImplWin32_UpdateKeyboardCodePage();
+        ImGui_ImplWin32_UpdateKeyboardCodePage(io);
         return 0;
     case WM_CHAR:
         if (::IsWindowUnicode(hwnd))
@@ -750,7 +756,7 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARA
         return 0;
     case WM_SETCURSOR:
         // This is required to restore cursor when transitioning from e.g resize borders to client area.
-        if (LOWORD(lParam) == HTCLIENT && ImGui_ImplWin32_UpdateMouseCursor())
+        if (LOWORD(lParam) == HTCLIENT && ImGui_ImplWin32_UpdateMouseCursor(io, bd->LastMouseCursor))
             return 1;
         return 0;
     case WM_DEVICECHANGE:

imgui.cpp

@@ -4748,6 +4748,13 @@ ImGuiIO& ImGui::GetIO()
     return GImGui->IO;
 }
 
+// This variant exists to facilitate backends experimenting with multi-threaded parallel context. (#8069, #6293, #5856)
+ImGuiIO& ImGui::GetIOEx(ImGuiContext* ctx)
+{
+    IM_ASSERT(ctx != NULL);
+    return ctx->IO;
+}
+
 ImGuiPlatformIO& ImGui::GetPlatformIO()
 {
     IM_ASSERT(GImGui != NULL && "No current context. Did you call ImGui::CreateContext() and ImGui::SetCurrentContext()?");

imgui_internal.h

@@ -2936,6 +2936,7 @@ namespace ImGui
     // If this ever crashes because g.CurrentWindow is NULL, it means that either:
     // - ImGui::NewFrame() has never been called, which is illegal.
     // - You are calling ImGui functions after ImGui::EndFrame()/ImGui::Render() and before the next ImGui::NewFrame(), which is also illegal.
+    IMGUI_API ImGuiIO&      GetIOEx(ImGuiContext* ctx);
     inline    ImGuiWindow*  GetCurrentWindowRead()      { ImGuiContext& g = *GImGui; return g.CurrentWindow; }
     inline    ImGuiWindow*  GetCurrentWindow()          { ImGuiContext& g = *GImGui; g.CurrentWindow->WriteAccessed = true; return g.CurrentWindow; }
     IMGUI_API ImGuiWindow*  FindWindowByID(ImGuiID id);