Backends: Win32 + Viewports: tweak ImGui_ImplWin32_WndProcHandler_PlatformWindow() to be easier to rework in a parallal friendly way. (#8069)

backends/imgui_impl_win32.cpp

@@ -23,6 +23,7 @@
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
 //  2024-XX-XX: Platform: Added support for multiple windows via the ImGuiPlatformIO interface.
+//  2024-10-28: [Docking] Rely on property stored inside HWND to retrieve context/viewport, should facilitate attempt to use this for parallel contexts. (#8069)
 //  2024-09-16: [Docking] Inputs: fixed an issue where a viewport destroyed while clicking would hog mouse tracking and temporary lead to incorrect update of HoveredWindow. (#7971)
 //  2024-07-08: Inputs: Fixed ImGuiMod_Super being mapped to VK_APPS instead of VK_LWIN||VK_RWIN. (#7768)
 //  2023-10-05: Inputs: Added support for extra ImGuiKey values: F13 to F24 function keys, app back/forward keys.
@@ -187,6 +188,8 @@ static bool ImGui_ImplWin32_InitEx(void* hwnd, bool platform_has_own_dc)
     // Our mouse update function expect PlatformHandle to be filled for the main viewport
     ImGuiViewport* main_viewport = ImGui::GetMainViewport();
     main_viewport->PlatformHandle = main_viewport->PlatformHandleRaw = (void*)bd->hWnd;
+    ::SetPropA(bd->hWnd, "IMGUI_CONTEXT", ImGui::GetCurrentContext());
+    ::SetPropA(bd->hWnd, "IMGUI_VIEWPORT", main_viewport);
     ImGui_ImplWin32_InitMultiViewportSupport(platform_has_own_dc);
 
     // Dynamically load XInput library
@@ -230,6 +233,8 @@ void    ImGui_ImplWin32_Shutdown()
     IM_ASSERT(bd != nullptr && "No platform backend to shutdown, or already shutdown?");
     ImGuiIO& io = ImGui::GetIO();
 
+    ::SetPropA(bd->hWnd, "IMGUI_CONTEXT", nullptr);
+    ::SetPropA(bd->hWnd, "IMGUI_VIEWPORT", nullptr);
     ImGui_ImplWin32_ShutdownMultiViewportSupport();
 
     // Unload XInput library
@@ -314,6 +319,14 @@ static void ImGui_ImplWin32_UpdateKeyModifiers()
     io.AddKeyEvent(ImGuiMod_Super, IsVkDown(VK_LWIN) || IsVkDown(VK_RWIN));
 }
 
+static ImGuiViewport* ImGui_ImplWin32_FindViewportByPlatformHandle(HWND hwnd)
+{
+    // We could use ImGui::FindViewportByPlatformHandle() but GetPropA() gets us closer to parallel multi-contexts,
+    // until we can pass an explicit context to FindViewportByPlatformHandle().
+    //return ImGui::FindViewportByPlatformHandle((void*)hwnd);
+    return (ImGuiViewport*)::GetPropA(hwnd, "IMGUI_VIEWPORT");
+}
+
 // This code supports multi-viewports (multiple OS Windows mapped into different Dear ImGui viewports)
 // Because of that, it is a little more complicated than your typical single-viewport binding code!
 static void ImGui_ImplWin32_UpdateMouseData()
@@ -326,7 +339,7 @@ static void ImGui_ImplWin32_UpdateMouseData()
     bool has_mouse_screen_pos = ::GetCursorPos(&mouse_screen_pos) != 0;
 
     HWND focused_window = ::GetForegroundWindow();
-    const bool is_app_focused = (focused_window && (focused_window == bd->hWnd || ::IsChild(focused_window, bd->hWnd) || ImGui::FindViewportByPlatformHandle((void*)focused_window)));
+    const bool is_app_focused = (focused_window && (focused_window == bd->hWnd || ::IsChild(focused_window, bd->hWnd) || ImGui_ImplWin32_FindViewportByPlatformHandle(focused_window)));
     if (is_app_focused)
     {
         // (Optional) Set OS mouse position from Dear ImGui if requested (rarely used, only when io.ConfigNavMoveSetMousePos is enabled by user)
@@ -364,7 +377,7 @@ static void ImGui_ImplWin32_UpdateMouseData()
     ImGuiID mouse_viewport_id = 0;
     if (has_mouse_screen_pos)
         if (HWND hovered_hwnd = ::WindowFromPoint(mouse_screen_pos))
-            if (ImGuiViewport* viewport = ImGui::FindViewportByPlatformHandle((void*)hovered_hwnd))
+            if (ImGuiViewport* viewport = ImGui_ImplWin32_FindViewportByPlatformHandle(hovered_hwnd))
                 mouse_viewport_id = viewport->ID;
     io.AddMouseViewportEvent(mouse_viewport_id);
 }
@@ -1072,6 +1085,7 @@ static void ImGui_ImplWin32_CreateWindow(ImGuiViewport* viewport)
 
     // Secondary viewports store their imgui context
     ::SetPropA(vd->Hwnd, "IMGUI_CONTEXT", ImGui::GetCurrentContext());
+    ::SetPropA(vd->Hwnd, "IMGUI_VIEWPORT", viewport);
 }
 
 static void ImGui_ImplWin32_DestroyWindow(ImGuiViewport* viewport)
@@ -1294,7 +1308,7 @@ static LRESULT CALLBACK ImGui_ImplWin32_WndProcHandler_PlatformWindow(HWND hWnd,
     LRESULT result = 0;
     if (ImGui_ImplWin32_WndProcHandler(hWnd, msg, wParam, lParam))
         result = true;
-    else if (ImGuiViewport* viewport = ImGui::FindViewportByPlatformHandle((void*)hWnd))
+    else if (ImGuiViewport* viewport = ImGui_ImplWin32_FindViewportByPlatformHandle(hWnd))
     {
         switch (msg)
         {