GPv3: Add API to copy and add a layer

This adds a core API to add a new layer by copying a source layer.

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -503,6 +503,7 @@ class LayerGroup : public ::GreasePencilLayerTreeGroup {
    * Adds a new layer named \a name at the end of this group and returns it.
    */
   Layer &add_layer(StringRefNull name);
+  Layer &add_layer(const Layer &duplicate_layer);
   /**
    * Adds a new group named \a name at the end of this group and returns it.
    */

source/blender/blenkernel/intern/grease_pencil.cc

@@ -869,6 +869,12 @@ Layer &LayerGroup::add_layer(StringRefNull name)
   return this->add_node(new_layer->as_node()).as_layer();
 }
 
+Layer &LayerGroup::add_layer(const Layer &duplicate_layer)
+{
+  Layer *new_layer = MEM_new<Layer>(__func__, duplicate_layer);
+  return this->add_node(new_layer->as_node()).as_layer();
+}
+
 LayerGroup &LayerGroup::add_group(StringRefNull name)
 {
   LayerGroup *new_group = MEM_new<LayerGroup>(__func__, name);
@@ -1664,6 +1670,24 @@ void GreasePencil::remove_drawings_with_no_users()
   remove_drawings_unchecked(*this, drawings_to_be_removed.as_span());
 }
 
+void GreasePencil::update_drawing_users_for_layer(const blender::bke::greasepencil::Layer &layer)
+{
+  using namespace blender;
+  for (auto [key, value] : layer.frames().items()) {
+    if (value.drawing_index > 0 && value.drawing_index < this->drawings().size()) {
+      GreasePencilDrawingBase *drawing_base = this->drawing(value.drawing_index);
+      if (drawing_base->type != GP_DRAWING) {
+        continue;
+      }
+      bke::greasepencil::Drawing &drawing =
+          reinterpret_cast<GreasePencilDrawing *>(drawing_base)->wrap();
+      if (!drawing.has_users()) {
+        drawing.add_user();
+      }
+    }
+  }
+}
+
 void GreasePencil::move_frames(blender::bke::greasepencil::Layer &layer,
                                const blender::Map<int, int> &frame_number_destinations)
 {
@@ -2029,23 +2053,42 @@ static int find_layer_insertion_index(
   return index;
 }
 
+static void grow_or_init_customdata(GreasePencil *grease_pencil,
+                                    const blender::bke::greasepencil::LayerGroup &parent_group)
+{
+  using namespace blender;
+  const Span<const bke::greasepencil::Layer *> layers = grease_pencil->layers();
+  if (layers.is_empty()) {
+    CustomData_realloc(&grease_pencil->layers_data, 0, 1);
+  }
+  else {
+    int insertion_index = find_layer_insertion_index(layers, parent_group, false);
+    grow_customdata(grease_pencil->layers_data, insertion_index, layers.size());
+  }
+}
+
 blender::bke::greasepencil::Layer &GreasePencil::add_layer(
     blender::bke::greasepencil::LayerGroup &parent_group, const blender::StringRefNull name)
 {
   using namespace blender;
   std::string unique_name = unique_layer_name(*this, name);
-
-  const Span<const bke::greasepencil::Layer *> layers = this->layers();
-  if (layers.is_empty()) {
-    CustomData_realloc(&this->layers_data, 0, 1);
-    return parent_group.add_layer(unique_name);
-  }
-  int insertion_index = find_layer_insertion_index(layers, parent_group, false);
-  grow_customdata(this->layers_data, insertion_index, layers.size());
-
+  grow_or_init_customdata(this, parent_group);
   return parent_group.add_layer(unique_name);
 }
 
+blender::bke::greasepencil::Layer &GreasePencil::add_layer(
+    blender::bke::greasepencil::LayerGroup &parent_group,
+    const blender::bke::greasepencil::Layer &duplicate_layer)
+{
+  using namespace blender;
+  std::string unique_name = unique_layer_name(*this, duplicate_layer.name());
+  grow_or_init_customdata(this, parent_group);
+  bke::greasepencil::Layer &new_layer = parent_group.add_layer(duplicate_layer);
+  this->update_drawing_users_for_layer(new_layer);
+  new_layer.set_name(unique_name);
+  return new_layer;
+}
+
 blender::bke::greasepencil::LayerGroup &GreasePencil::add_layer_group(
     blender::bke::greasepencil::LayerGroup &parent_group, const blender::StringRefNull name)
 {

source/blender/makesdna/DNA_grease_pencil_types.h

@@ -467,6 +467,9 @@ typedef struct GreasePencil {
   /* Adding layers and layer groups. */
   blender::bke::greasepencil::Layer &add_layer(
       blender::bke::greasepencil::LayerGroup &parent_group, blender::StringRefNull name);
+  blender::bke::greasepencil::Layer &add_layer(
+      blender::bke::greasepencil::LayerGroup &parent_group,
+      const blender::bke::greasepencil::Layer &duplicate_layer);
   blender::bke::greasepencil::LayerGroup &add_layer_group(
       blender::bke::greasepencil::LayerGroup &parent_group, blender::StringRefNull name);
 
@@ -545,6 +548,10 @@ typedef struct GreasePencil {
    * drawings array.
    */
   void remove_drawings_with_no_users();
+  /**
+   * Makes sure all the drawings that the layer points to have a user.
+   */
+  void update_drawing_users_for_layer(const blender::bke::greasepencil::Layer &layer);
 
   /**
    * Returns a drawing on \a layer at frame \a frame_number or `nullptr` if no such