DrawList: AddCallback() added an optional size parameter allowing to copy and store any amount of user data for usage by callbacks: (#6969, #4770, #7665)

docs/CHANGELOG.txt

@@ -60,6 +60,14 @@ Other changes:
 - IO: added 'void* platform_io.Renderer_RenderState' which is set during the
   ImGui_ImplXXXX_RenderDrawData() of standard backend to expose selected render
   state to draw callbacks. (#6969, #5834, #7468, #3590)
+- DrawList: AddCallback() added an optional size parameter allowing to copy and
+  store any amount of user data for usage by callbacks: (#6969, #4770, #7665)
+  - If userdata_size == 0: we copy/store the 'userdata' argument as-is (existing behavior).
+    It will be available unmodified in ImDrawCmd::UserCallbackData during render.
+  - If userdata_size > 0, we copy/store 'userdata_size' bytes pointed to by 'userdata' (new behavior).
+    We store them in a buffer stored inside the drawlist. ImDrawCmd::UserCallbackData 
+    will point inside that buffer so you have to retrieve data from there. Your callback
+    may need to use ImDrawCmd::UserCallbackDataSize if you expect dynamically-sized data.
 - Tables: fixed initial auto-sizing issue with synched-instances. (#8045, #7218)
 - InputText: fixed an issue with not declaring ownership of Delete/Backspace/Arrow keys,
   preventing use of external shortcuts not guarded by an ActiveId check. (#8048) [@geertbleyen]

docs/TODO.txt

@@ -9,7 +9,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - doc: add a proper documentation system (maybe relying on automation? #435)
  - doc: checklist app to verify backends/integration of imgui (test inputs, rendering, callback, etc.).
  - doc/tips: tips of the day: website? applet in imgui_club?
- - doc/wiki: work on the wiki https://github.com/ocornut/imgui/wiki
  
  - window: preserve/restore relative focus ordering (persistent or not), and e.g. of multiple reappearing windows (#2304) -> also see docking reference to same #.
  - window: calling SetNextWindowSize() every frame with <= 0 doesn't do anything, may be useful to allow (particularly when used for a single axis). (#690)
@@ -44,7 +43,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - drawlist: maintaining bounding box per command would allow to merge draw command when clipping isn't relied on (typical non-scrolling window or non-overflowing column would merge with previous command). (WIP branch)
  - drawlist: make it easier to toggle AA per primitive, so we can use e.g. non-AA fill + AA borders more naturally
  - drawlist: non-AA strokes have gaps between points (#593, #288), glitch especially on RenderCheckmark() and ColorPicker4().
- - drawlist: callback: add an extra void* in ImDrawCallback to allow passing render-local data to the callback (would break API).
+ - drawlist: callback: add an extra void* in ImDrawCallback to expose render state instead of pulling from Renderer_RenderState (would break API).
  - drawlist: AddRect vs AddLine position confusing (#2441)
  - drawlist/opt: store rounded corners in texture to use 1 quad per corner (filled and wireframe) to lower the cost of rounding. (#1962)
  - drawlist/opt: AddRect() axis aligned pixel aligned (no-aa) could use 8 triangles instead of 16 and no normal calculation.

imgui.h

@@ -29,7 +29,7 @@
 // Library Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals, e.g. '#if IMGUI_VERSION_NUM >= 12345')
 #define IMGUI_VERSION       "1.91.4 WIP"
-#define IMGUI_VERSION_NUM   19132
+#define IMGUI_VERSION_NUM   19133
 #define IMGUI_HAS_TABLE
 
 /*
@@ -2953,7 +2953,9 @@ struct ImDrawCmd
     unsigned int    IdxOffset;          // 4    // Start offset in index buffer.
     unsigned int    ElemCount;          // 4    // Number of indices (multiple of 3) to be rendered as triangles. Vertices are stored in the callee ImDrawList's vtx_buffer[] array, indices in idx_buffer[].
     ImDrawCallback  UserCallback;       // 4-8  // If != NULL, call the function instead of rendering the vertices. clip_rect and texture_id will be set normally.
-    void*           UserCallbackData;   // 4-8  // The draw callback code can access this.
+    void*           UserCallbackData;   // 4-8  // Callback user data (when UserCallback != NULL). If called AddCallback() with size == 0, this is a copy of the AddCallback() argument. If called AddCallback() with size > 0, this is pointing to a buffer where data is stored.
+    int             UserCallbackDataSize;  // 4 // Size of callback user data when using storage, otherwise 0.
+    int             UserCallbackDataOffset;// 4 // [Internal] Offset of callback user data when using storage, otherwise -1.
 
     ImDrawCmd()     { memset(this, 0, sizeof(*this)); } // Also ensure our padding fields are zeroed
 
@@ -3068,6 +3070,7 @@ struct ImDrawList
     ImDrawListSplitter      _Splitter;          // [Internal] for channels api (note: prefer using your own persistent instance of ImDrawListSplitter!)
     ImVector<ImVec4>        _ClipRectStack;     // [Internal]
     ImVector<ImTextureID>   _TextureIdStack;    // [Internal]
+    ImVector<ImU8>          _CallbacksDataBuf;  // [Internal]
     float                   _FringeScale;       // [Internal] anti-alias fringe is scaled by this value, this helps to keep things sharp while zooming at vertex buffer content
     const char*             _OwnerName;         // Pointer to owner window's name for debugging
 
@@ -3140,8 +3143,18 @@ struct ImDrawList
     IMGUI_API void  PathBezierQuadraticCurveTo(const ImVec2& p2, const ImVec2& p3, int num_segments = 0);               // Quadratic Bezier (3 control points)
     IMGUI_API void  PathRect(const ImVec2& rect_min, const ImVec2& rect_max, float rounding = 0.0f, ImDrawFlags flags = 0);
 
-    // Advanced
+    // Advanced: Draw Callbacks
-    IMGUI_API void  AddCallback(ImDrawCallback callback, void* callback_data);  // Your rendering function must check for 'UserCallback' in ImDrawCmd and call the function instead of rendering triangles.
+    // - May be used to alter render state (change sampler, blending, current shader). May be used to emit custom rendering commands (difficult to do correctly, but possible).
+    // - Use special ImDrawCallback_ResetRenderState callback to instruct backend to reset its render state to the default.
+    // - Your rendering loop must check for 'UserCallback' in ImDrawCmd and call the function instead of rendering triangles. All standard backends are honoring this.
+    // - For some backends, the callback may access selected render-states exposed by the backend in a ImGui_ImplXXXX_RenderState structure pointed to by platform_io.Renderer_RenderState.
+    // - IMPORTANT: please be mindful of the different level of indirection between using size==0 (copying argument) and using size>0 (copying pointed data into a buffer).
+    //   - If userdata_size == 0: we copy/store the 'userdata' argument as-is. It will be available unmodified in ImDrawCmd::UserCallbackData during render.
+    //   - If userdata_size > 0,  we copy/store 'userdata_size' bytes pointed to by 'userdata'. We store them in a buffer stored inside the drawlist. ImDrawCmd::UserCallbackData will point inside that buffer so you have to retrieve data from there. Your callback may need to use ImDrawCmd::UserCallbackDataSize if you expect dynamically-sized data.
+    //   - Support for userdata_size > 0 was added in v1.91.4, October 2024. So earlier code always only allowed to copy/store a simple void*.
+    IMGUI_API void  AddCallback(ImDrawCallback callback, void* userdata, size_t userdata_size = 0);
+
+    // Advanced: Miscellaneous
     IMGUI_API void  AddDrawCmd();                                               // This is useful if you need to forcefully create a new draw call (to allow for dependent rendering / blending). Otherwise primitives are merged into the same draw-call as much as possible
     IMGUI_API ImDrawList* CloneOutput() const;                                  // Create a clone of the CmdBuffer/IdxBuffer/VtxBuffer.
 

imgui_draw.cpp

@@ -414,6 +414,7 @@ void ImDrawList::_ResetForNewFrame()
     _IdxWritePtr = NULL;
     _ClipRectStack.resize(0);
     _TextureIdStack.resize(0);
+    _CallbacksDataBuf.resize(0);
     _Path.resize(0);
     _Splitter.Clear();
     CmdBuffer.push_back(ImDrawCmd());
@@ -431,6 +432,7 @@ void ImDrawList::_ClearFreeMemory()
     _IdxWritePtr = NULL;
     _ClipRectStack.clear();
     _TextureIdStack.clear();
+    _CallbacksDataBuf.clear();
     _Path.clear();
     _Splitter.ClearFreeMemory();
 }
@@ -470,7 +472,7 @@ void ImDrawList::_PopUnusedDrawCmd()
     }
 }
 
-void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
+void ImDrawList::AddCallback(ImDrawCallback callback, void* userdata, size_t userdata_size)
 {
     IM_ASSERT_PARANOID(CmdBuffer.Size > 0);
     ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
@@ -480,8 +482,26 @@ void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
         AddDrawCmd();
         curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
     }
+
     curr_cmd->UserCallback = callback;
-    curr_cmd->UserCallbackData = callback_data;
+    if (userdata_size == 0)
+    {
+        // Store user data directly in command (no indirection)
+        curr_cmd->UserCallbackData = userdata;
+        curr_cmd->UserCallbackDataSize = 0;
+        curr_cmd->UserCallbackDataOffset = -1;
+    }
+    else
+    {
+        // Copy and store user data in a buffer
+        IM_ASSERT(userdata != NULL);
+        IM_ASSERT(userdata_size < (1u << 31));
+        curr_cmd->UserCallbackData = NULL; // Will be resolved during Render()
+        curr_cmd->UserCallbackDataSize = (int)userdata_size;
+        curr_cmd->UserCallbackDataOffset = _CallbacksDataBuf.Size;
+        _CallbacksDataBuf.resize(_CallbacksDataBuf.Size + (int)userdata_size);
+        memcpy(_CallbacksDataBuf.Data + (size_t)curr_cmd->UserCallbackDataOffset, userdata, userdata_size);
+    }
 
     AddDrawCmd(); // Force a new command after us (see comment below)
 }
@@ -2222,6 +2242,12 @@ void ImGui::AddDrawListToDrawDataEx(ImDrawData* draw_data, ImVector<ImDrawList*>
     if (sizeof(ImDrawIdx) == 2)
         IM_ASSERT(draw_list->_VtxCurrentIdx < (1 << 16) && "Too many vertices in ImDrawList using 16-bit indices. Read comment above");
 
+    // Resolve callback data pointers
+    if (draw_list->_CallbacksDataBuf.Size > 0)
+        for (ImDrawCmd& cmd : draw_list->CmdBuffer)
+            if (cmd.UserCallback != NULL && cmd.UserCallbackDataOffset != -1 && cmd.UserCallbackDataSize > 0)
+                cmd.UserCallbackData = draw_list->_CallbacksDataBuf.Data + cmd.UserCallbackDataOffset;
+
     // Add to output list + records state in ImDrawData
     out_list->push_back(draw_list);
     draw_data->CmdListsCount++;