GPv3: Add user count to drawings

Adds a user count to drawings to track how many frames use the drawing.
If the user count hits `0` the drawing should be deleted.

Also adds a function to delete a frame in a layer. This will
decrement the user count of the drawing and delete it if there are no
users left. This is consistent with the way GPv2 worked.

Pull Request: https://projects.blender.org/blender/blender/pulls/110579

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -9,6 +9,8 @@
  * \brief Low-level operations for grease pencil.
  */
 
+#include <atomic>
+
 #include "BLI_function_ref.hh"
 #include "BLI_map.hh"
 #include "BLI_math_vector_types.hh"
@@ -62,6 +64,13 @@ class DrawingRuntime {
    * Triangle cache for all the strokes in the drawing.
    */
   mutable SharedCache<Vector<uint3>> triangles_cache;
+
+  /**
+   * Number of users for this drawing. The users are the frames in the Grease Pencil layers.
+   * Different frames can refer to the same drawing, so we need to make sure we count these users
+   * and remove a drawing if it has zero users.
+   */
+  mutable std::atomic<int> user_count = 1;
 };
 
 class Drawing : public ::GreasePencilDrawing {
@@ -92,6 +101,22 @@ class Drawing : public ::GreasePencilDrawing {
    */
   VArray<float> opacities() const;
   MutableSpan<float> opacities_for_write();
+
+  /**
+   * Add a user for this drawing. When a drawing has multiple users, both users are allowed to
+   * modifify this drawings data.
+   */
+  void add_user() const;
+  /**
+   * Removes a user from this drawing. Note that this does not handle deleting the drawing if it
+   * has not users.
+   */
+  void remove_user() const;
+  /**
+   * Returns true for when this drawing has more than one user.
+   */
+  bool is_instanced() const;
+  bool has_users() const;
 };
 
 class LayerGroup;
@@ -504,6 +529,26 @@ class GreasePencilRuntime {
 
 }  // namespace blender::bke
 
+inline void blender::bke::greasepencil::Drawing::add_user() const
+{
+  this->runtime->user_count.fetch_add(1, std::memory_order_relaxed);
+}
+
+inline void blender::bke::greasepencil::Drawing::remove_user() const
+{
+  this->runtime->user_count.fetch_sub(1, std::memory_order_relaxed);
+}
+
+inline bool blender::bke::greasepencil::Drawing::is_instanced() const
+{
+  return this->runtime->user_count.load(std::memory_order_relaxed) > 1;
+}
+
+inline bool blender::bke::greasepencil::Drawing::has_users() const
+{
+  return this->runtime->user_count.load(std::memory_order_relaxed) > 0;
+}
+
 inline blender::bke::greasepencil::Drawing &GreasePencilDrawing::wrap()
 {
   return *reinterpret_cast<blender::bke::greasepencil::Drawing *>(this);

source/blender/blenkernel/intern/grease_pencil.cc

@@ -109,7 +109,6 @@ static void grease_pencil_copy_data(Main * /*bmain*/,
         break;
       }
     }
-    /* TODO: Update drawing user counts. */
   }
 
   /* Duplicate layer tree. */
@@ -1384,8 +1383,6 @@ void GreasePencil::add_empty_drawings(const int add_num)
     new_drawings[i] = reinterpret_cast<GreasePencilDrawingBase *>(
         MEM_new<blender::bke::greasepencil::Drawing>(__func__));
   }
-
-  /* TODO: Update drawing user counts. */
 }
 
 bool GreasePencil::insert_blank_frame(blender::bke::greasepencil::Layer &layer,
@@ -1403,6 +1400,32 @@ bool GreasePencil::insert_blank_frame(blender::bke::greasepencil::Layer &layer,
   return true;
 }
 
+void GreasePencil::remove_frame_at(blender::bke::greasepencil::Layer &layer,
+                                   const int frame_number)
+{
+  using namespace blender::bke::greasepencil;
+  if (!layer.frames().contains(frame_number)) {
+    return;
+  }
+  const GreasePencilFrame &frame_to_remove = layer.frames().lookup(frame_number);
+  const int drawing_index_to_remove = frame_to_remove.drawing_index;
+  if (!layer.remove_frame(frame_number)) {
+    /* If removing the frame was not successful, return early. */
+    return;
+  }
+  GreasePencilDrawingBase *drawing_base = this->drawings(drawing_index_to_remove);
+  if (drawing_base->type != GP_DRAWING) {
+    /* If the drawing is referenced from another object, we don't track it's users because we
+     * cannot delete drawings from another object. Return early. */
+    return;
+  }
+  Drawing &drawing = reinterpret_cast<GreasePencilDrawing *>(drawing_base)->wrap();
+  drawing.remove_user();
+  if (!drawing.has_users()) {
+    this->remove_drawing(drawing_index_to_remove);
+  }
+}
+
 void GreasePencil::remove_drawing(const int index_to_remove)
 {
   using namespace blender::bke::greasepencil;

source/blender/makesdna/DNA_grease_pencil_types.h

@@ -485,6 +485,7 @@ typedef struct GreasePencil {
                           int frame_number,
                           int duration,
                           eBezTriple_KeyframeType keytype);
+  void remove_frame_at(blender::bke::greasepencil::Layer &layer, int frame_number);
 
   void remove_drawing(int index);