ImDrawCallback: Allow to override the signature of ImDrawCallback by #define-ing it. This is meant to facilitate custom rendering back-ends passing local render-specific data to the draw callback.

docs/CHANGELOG.txt

@@ -84,6 +84,8 @@ Other Changes:
   support 32-bits indices. Most examples back-ends have been modified to support the VtxOffset field.
 - ImDrawList: Added ImDrawCmd::IdxOffset value, equivalent to summing element count for each draw command.
   This is provided for convenience and consistency with VtxOffset.
+- ImDrawCallback: Allow to override the signature of ImDrawCallback by #define-ing it. This is meant to
+  facilitate custom rendering back-ends passing local render-specific data to the draw callback.
 - ImFontAtlas: FreeType: Added RasterizerFlags::Monochrome flag to disable font anti-aliasing. (#2545)
   Combine with RasterizerFlags::MonoHinting for best results.
 - ImFontGlyphRangesBuilder: Fixed unnecessarily over-sized buffer, which incidentally was also not

imconfig.h

@@ -68,6 +68,12 @@
 // Read about ImGuiBackendFlags_RendererHasVtxOffset for details.
 //#define ImDrawIdx unsigned int
 
+//---- Override ImDrawCallback signature (will need to modify renderer back-ends accordingly)
+//struct ImDrawList;
+//struct ImDrawCmd;
+//typedef void (*MyImDrawCallback)(const ImDrawList* draw_list, const ImDrawCmd* cmd, void* my_renderer_user_data);
+//#define ImDrawCallback MyImDrawCallback
+
 //---- Tip: You can add extra functions within the ImGui:: namespace, here or in your own headers files.
 /*
 namespace ImGui

imgui.h

@@ -1771,7 +1771,10 @@ struct ImColor
 //  A) Change your GPU render state,
 //  B) render a complex 3D scene inside a UI element without an intermediate texture/render target, etc.
 // The expected behavior from your rendering function is 'if (cmd.UserCallback != NULL) { cmd.UserCallback(parent_list, cmd); } else { RenderTriangles() }'
+// If you want to override the signature of ImDrawCallback, you can simply use e.g. '#define ImDrawCallback MyDrawCallback' (in imconfig.h) + update rendering back-end accordingly.
+#ifndef ImDrawCallback
 typedef void (*ImDrawCallback)(const ImDrawList* parent_list, const ImDrawCmd* cmd);
+#endif
 
 // Special Draw callback value to request renderer back-end to reset the graphics/render state.
 // The renderer back-end needs to handle this special value, otherwise it will crash trying to call a function at this address.