Added GetDrawData() alternative to setting a Render function.

imgui.cpp

@@ -96,6 +96,7 @@
         io.DisplaySize.y = 1280.0f;
         io.DeltaTime = 1.0f/60.0f;
         io.IniFilename = "imgui.ini";
+        io.RenderDrawListsFn = my_render_function;  // Setup a render function, or set to NULL and call GetDrawData() after Render() to access the render data.
         // TODO: Fill others settings of the io structure
 
         // Load texture
@@ -1769,6 +1770,12 @@ ImGuiStyle& ImGui::GetStyle()
     return GImGui->Style;
 }
 
+// Same value as passed to your RenderDrawListsFn() function. valid after Render() and until the next call to NewFrame()
+ImDrawData* ImGui::GetDrawData()
+{
+    return GImGui->RenderDrawData.Valid ? &GImGui->RenderDrawData : NULL;
+}
+
 float ImGui::GetTime()
 {
     return GImGui->Time;
@@ -1786,7 +1793,6 @@ void ImGui::NewFrame()
     // Check user data
     IM_ASSERT(g.IO.DeltaTime >= 0.0f);
     IM_ASSERT(g.IO.DisplaySize.x >= 0.0f && g.IO.DisplaySize.y >= 0.0f);
-    IM_ASSERT(g.IO.RenderDrawListsFn != NULL);       // Must be implemented
     IM_ASSERT(g.IO.Fonts->Fonts.Size > 0);           // Font Atlas not created. Did you call io.Fonts->GetTexDataAsRGBA32 / GetTexDataAsAlpha8 ?
     IM_ASSERT(g.IO.Fonts->Fonts[0]->IsLoaded());     // Font Atlas not created. Did you call io.Fonts->GetTexDataAsRGBA32 / GetTexDataAsAlpha8 ?
     IM_ASSERT(g.Style.CurveTessellationTol > 0.0f);  // Invalid
@@ -1812,6 +1818,11 @@ void ImGui::NewFrame()
     g.OverlayDrawList.PushClipRectFullScreen();
     g.OverlayDrawList.AddDrawCmd();
 
+    // Mark rendering data as invalid to prevent user who may have a handle on it to use it
+    g.RenderDrawData.Valid = false;
+    g.RenderDrawData.CmdLists = NULL;
+    g.RenderDrawData.CmdListsCount = g.RenderDrawData.TotalVtxCount = g.RenderDrawData.TotalIdxCount = 0;
+
     // Update inputs state
     if (g.IO.MousePos.x < 0 && g.IO.MousePos.y < 0)
         g.IO.MousePos = ImVec2(-9999.0f, -9999.0f);
@@ -2366,15 +2377,17 @@ void ImGui::Render()
         if (!g.OverlayDrawList.VtxBuffer.empty())
             AddDrawListToRenderList(g.RenderDrawLists[0], &g.OverlayDrawList);
 
-        // Render
-        if (!g.RenderDrawLists[0].empty())
+        // Setup draw data
+        g.RenderDrawData.Valid = true;
+        g.RenderDrawData.CmdLists = &g.RenderDrawLists[0][0];
+        g.RenderDrawData.CmdListsCount = g.RenderDrawLists[0].Size;
+        g.RenderDrawData.TotalVtxCount = g.IO.MetricsRenderVertices;
+        g.RenderDrawData.TotalIdxCount = g.IO.MetricsRenderIndices;
+
+        // 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)
         {
-            ImDrawData data;
-            data.CmdLists = &g.RenderDrawLists[0][0];
-            data.CmdListsCount = g.RenderDrawLists[0].Size;
-            data.TotalVtxCount = g.IO.MetricsRenderVertices;
-            data.TotalIdxCount = g.IO.MetricsRenderIndices;
-            g.IO.RenderDrawListsFn(&data);
+            g.IO.RenderDrawListsFn(&g.RenderDrawData);
         }
     }
 }

imgui.h

@@ -105,6 +105,7 @@ namespace ImGui
     // Main
     IMGUI_API ImGuiIO&      GetIO();
     IMGUI_API ImGuiStyle&   GetStyle();
+    IMGUI_API ImDrawData*   GetDrawData();                              // same value as passed to your RenderDrawListsFn() function. valid after Render() and until the next call to NewFrame()
     IMGUI_API void          NewFrame();
     IMGUI_API void          Render();
     IMGUI_API void          Shutdown();
@@ -694,8 +695,9 @@ struct ImGuiIO
     // User Functions
     //------------------------------------------------------------------
 
-    // REQUIRED: rendering function.
-    // See example code if you are unsure of how to implement this.
+    // Rendering function, will be called in Render(). 
+    // Alternatively you can keep this to NULL and call GetDrawData() after Render() to get the same pointer.
+    // See example applications if you are unsure of how to implement this.
     void        (*RenderDrawListsFn)(ImDrawData* data);
 
     // Optional: access OS clipboard
@@ -1149,14 +1151,16 @@ struct ImDrawList
 // All draw data to render an ImGui frame
 struct ImDrawData
 {
+    bool            Valid;                  // Only valid after Render() is called and before the next NewFrame() is called.
     ImDrawList**    CmdLists;
     int             CmdListsCount;
     int             TotalVtxCount;          // For convenience, sum of all cmd_lists vtx_buffer.Size
     int             TotalIdxCount;          // For convenience, sum of all cmd_lists idx_buffer.Size
 
     // Functions
-    void DeIndexAllBuffers();               // For backward compatibility: convert all buffers from indexed to de-indexed, in case you cannot render indexed. Note: this is slow and most likely a waste of resources. Always prefer indexed rendering!
-    void ScaleClipRects(const ImVec2& sc);  // Helper to scale the ClipRect field of each ImDrawCmd. Use if your final output buffer is at a different scale than ImGui expects, or if there is a difference between your window resolution and framebuffer resolution.
+    IMGUI_API ImDrawData() { Valid = false; CmdLists = NULL; CmdListsCount = TotalVtxCount = TotalIdxCount = 0; }
+    IMGUI_API void DeIndexAllBuffers();               // For backward compatibility: convert all buffers from indexed to de-indexed, in case you cannot render indexed. Note: this is slow and most likely a waste of resources. Always prefer indexed rendering!
+    IMGUI_API void ScaleClipRects(const ImVec2& sc);  // Helper to scale the ClipRect field of each ImDrawCmd. Use if your final output buffer is at a different scale than ImGui expects, or if there is a difference between your window resolution and framebuffer resolution.
 };
 
 struct ImFontConfig

imgui_internal.h

@@ -371,6 +371,7 @@ struct ImGuiState
     ImGuiSetCond            SetNextTreeNodeOpenedCond;
 
     // Render
+    ImDrawData              RenderDrawData;                     // Main ImDrawData instance to pass render information to the user
     ImVector<ImDrawList*>   RenderDrawLists[3];
     float                   ModalWindowDarkeningRatio;
     ImDrawList              OverlayDrawList;                    // Optional software render of mouse cursors, if io.MouseDrawCursor is set + a few debug overlays