Extracted a EndFrame() function out of Render() but kept it internal/private + clarified some asserts (#335)

imgui.cpp

@@ -2262,16 +2262,15 @@ static void PopClipRect()
     window->ClipRect = window->DrawList->_ClipRectStack.back();
 }
 
-void ImGui::Render()
+// This is normally called by Render(). You may want to call it directly if you want to avoid calling Render() but the gain will be very minimal.
+void ImGui::EndFrame()
 {
     ImGuiState& g = *GImGui;
     IM_ASSERT(g.Initialized);                       // Forgot to call ImGui::NewFrame()
+    IM_ASSERT(g.FrameCountEnded != g.FrameCount);   // ImGui::EndFrame() called multiple times, or forgot to call ImGui::NewFrame() again
 
-    const bool first_render_of_the_frame = (g.FrameCountRendered != g.FrameCount);
+    g.FrameCountEnded = g.FrameCount;
-    g.FrameCountRendered = g.FrameCount;
 
-    if (first_render_of_the_frame)
-    {
     // Hide implicit "Debug" window if it hasn't been used
     IM_ASSERT(g.CurrentWindowStack.Size == 1);    // Mismatched Begin/End
     if (g.CurrentWindow && !g.CurrentWindow->Accessed)
@@ -2322,8 +2321,17 @@ void ImGui::Render()
     memset(g.IO.InputCharacters, 0, sizeof(g.IO.InputCharacters));
 }
 
+void ImGui::Render()
+{
+    ImGuiState& g = *GImGui;
+    IM_ASSERT(g.Initialized);   // Forgot to call ImGui::NewFrame()
+
+    if (g.FrameCountEnded != g.FrameCount)
+        ImGui::EndFrame();
+    g.FrameCountRendered = g.FrameCount;
+
     // Skip render altogether if alpha is 0.0
-    // Note that vertex buffers have been created and are wasted, so it is best practice that you don't create windows in the first place, or respond to Begin() returning false.
+    // Note that vertex buffers have been created and are wasted, so it is best practice that you don't create windows in the first place, or consistently respond to Begin() returning false.
     if (g.Style.Alpha > 0.0f)
     {
         // Render tooltip
@@ -2363,13 +2371,13 @@ void ImGui::Render()
         for (int i = 1; i < IM_ARRAYSIZE(g.RenderDrawLists); i++)
         {
             ImVector<ImDrawList*>& layer = g.RenderDrawLists[i];
-            if (!layer.empty())
+            if (layer.empty())
-            {
+                continue;
             memcpy(&g.RenderDrawLists[0][n], &layer[0], layer.Size * sizeof(ImDrawList*));
             n += layer.Size;
         }
-        }
 
+        // Draw software mouse cursor if requested
         if (g.IO.MouseDrawCursor)
         {
             const ImGuiMouseCursorData& cursor_data = g.MouseCursorData[g.MouseCursor];
@@ -2395,11 +2403,9 @@ void ImGui::Render()
 
         // Render. If user hasn't set a callback then they may retrieve the draw data via GetDrawData()
         if (g.RenderDrawData.CmdListsCount > 0 && g.IO.RenderDrawListsFn != NULL)
-        {
             g.IO.RenderDrawListsFn(&g.RenderDrawData);
     }
 }
-}
 
 // Find the optional ## from which we stop displaying text.
 static const char*  FindTextDisplayEnd(const char* text, const char* text_end)
@@ -3433,8 +3439,9 @@ bool ImGui::Begin(const char* name, bool* p_opened, const ImVec2& size_on_first_
 {
     ImGuiState& g = *GImGui;
     const ImGuiStyle& style = g.Style;
+    IM_ASSERT(name != NULL);                        // Window name required
     IM_ASSERT(g.Initialized);                       // Forgot to call ImGui::NewFrame()
-    IM_ASSERT(name != NULL);                        // Must pass a name
+    IM_ASSERT(g.FrameCountEnded != g.FrameCount);   // Called ImGui::Render() or ImGui::EndFrame() and haven't called ImGui::NewFrame() again yet
 
     if (flags & ImGuiWindowFlags_NoInputs)
         flags |= ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoResize;

imgui_internal.h

@@ -331,6 +331,7 @@ struct ImGuiState
 
     float                   Time;
     int                     FrameCount;
+    int                     FrameCountEnded;
     int                     FrameCountRendered;
     ImVector<ImGuiWindow*>  Windows;
     ImVector<ImGuiWindow*>  WindowsSortBuffer;
@@ -416,7 +417,7 @@ struct ImGuiState
 
         Time = 0.0f;
         FrameCount = 0;
-        FrameCountRendered = -1;
+        FrameCountEnded = FrameCountRendered = -1;
         CurrentWindow = NULL;
         FocusedWindow = NULL;
         HoveredWindow = NULL;
@@ -631,7 +632,11 @@ public:
 
 namespace ImGui
 {
-    inline    ImGuiWindow*  GetCurrentWindowRead()      { ImGuiState& g = *GImGui; return g.CurrentWindow; }        // If this ever crash it means that ImGui::NewFrame() has never been called (which is illegal). We should always have a CurrentWindow in the stack (there is an implicit "Debug" window)
+    // We should always have a CurrentWindow in the stack (there is an implicit "Debug" window)
+    // If this ever crash 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::Render() and before the next ImGui::NewFrame(), which is also illegal.
+    inline    ImGuiWindow*  GetCurrentWindowRead()      { ImGuiState& g = *GImGui; return g.CurrentWindow; }
     inline    ImGuiWindow*  GetCurrentWindow()          { ImGuiState& g = *GImGui; g.CurrentWindow->Accessed = true; return g.CurrentWindow; }
     IMGUI_API ImGuiWindow*  GetParentWindow();
     IMGUI_API void          FocusWindow(ImGuiWindow* window);
@@ -639,6 +644,8 @@ namespace ImGui
     IMGUI_API void          SetActiveID(ImGuiID id, ImGuiWindow* window);
     IMGUI_API void          KeepAliveID(ImGuiID id);
 
+    IMGUI_API void          EndFrame();                 // This automatically called by Render()
+
     IMGUI_API void          ItemSize(const ImVec2& size, float text_offset_y = 0.0f);
     IMGUI_API void          ItemSize(const ImRect& bb, float text_offset_y = 0.0f);
     IMGUI_API bool          ItemAdd(const ImRect& bb, const ImGuiID* id);