Grease Pencil: Save Layer Group expanded state

Grease Pencil layer groups are drawn in the layer tree using
`AbstractTreeViewItem`. The problem is that the items can't
store their expand/collapse state. This meant that switching
e.g. the tabs in the properties editor would reset the collapsed
state of layer group items.

This PR uses the `GP_LAYER_TREE_NODE_EXPANDED` flag to store
the expand/collapse state. The tree view then reads from the flag to
expand/collapse the items.

This also adds an RNA property to layer groups `is_expanded`.
Allows to check from python if a group is expanded in the UI.

Pull Request: https://projects.blender.org/blender/blender/pulls/131159

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -703,6 +703,15 @@ class LayerGroup : public ::GreasePencilLayerTreeGroup {
   const TreeNode *find_node_by_name(StringRefNull name) const;
   TreeNode *find_node_by_name(StringRefNull name);
 
+  /**
+   * Returns true if the group is expanded in the UI.
+   */
+  bool is_expanded() const;
+  /**
+   * Expand/collapse the group in the UI.
+   */
+  void set_expanded(bool expanded);
+
   /**
    * Print the nodes. For debugging purposes.
    */

source/blender/blenkernel/intern/grease_pencil.cc

@@ -1748,6 +1748,16 @@ TreeNode *LayerGroup::find_node_by_name(const StringRefNull name)
   return nullptr;
 }
 
+bool LayerGroup::is_expanded() const
+{
+  return (this->base.flag & GP_LAYER_TREE_NODE_EXPANDED) != 0;
+}
+
+void LayerGroup::set_expanded(const bool expanded)
+{
+  SET_FLAG_FROM_TEST(this->base.flag, expanded, GP_LAYER_TREE_NODE_EXPANDED);
+}
+
 void LayerGroup::print_nodes(StringRefNull header) const
 {
   std::cout << header << std::endl;

source/blender/editors/animation/anim_channels_edit.cc

@@ -4404,16 +4404,18 @@ static int click_select_channel_grease_pencil_datablock(bAnimListElem *ale)
   return (ND_ANIMCHAN | NA_EDITED);
 }
 
-static int click_select_channel_grease_pencil_layer_group(bAnimListElem *ale)
+static int click_select_channel_grease_pencil_layer_group(bContext *C, bAnimListElem *ale)
 {
-  GreasePencilLayerTreeGroup *layer_group = static_cast<GreasePencilLayerTreeGroup *>(ale->data);
+  using namespace blender::bke::greasepencil;
+  LayerGroup &layer_group = static_cast<GreasePencilLayerTreeGroup *>(ale->data)->wrap();
 
   /* Toggle expand:
    * - Although the triangle widget already allows this,
    *   the whole channel can also be used for this purpose.
    */
-  layer_group->base.flag ^= GP_LAYER_TREE_NODE_EXPANDED;
-
+  layer_group.set_expanded(!layer_group.is_expanded());
+  WM_event_add_notifier(C, NC_SPACE | ND_SPACE_PROPERTIES | NA_EDITED, nullptr);
+  WM_event_add_notifier(C, NC_GPENCIL | ND_DATA | NA_EDITED, nullptr);
   return (ND_ANIMCHAN | NA_EDITED);
 }
 
@@ -4593,7 +4595,7 @@ static int mouse_anim_channels(bContext *C,
       notifierFlags |= click_select_channel_grease_pencil_datablock(ale);
       break;
     case ANIMTYPE_GREASE_PENCIL_LAYER_GROUP:
-      notifierFlags |= click_select_channel_grease_pencil_layer_group(ale);
+      notifierFlags |= click_select_channel_grease_pencil_layer_group(C, ale);
       break;
     case ANIMTYPE_GREASE_PENCIL_LAYER:
       notifierFlags |= click_select_channel_grease_pencil_layer(C, ac, ale, selectmode, filter);

source/blender/editors/animation/anim_filter.cc

@@ -2100,7 +2100,7 @@ static size_t animdata_filter_grease_pencil_layer_node_recursive(
     size_t tmp_items = 0;
 
     /* Add grease pencil layer channels. */
-    BEGIN_ANIMFILTER_SUBCHANNELS (layer_group.base.flag &GP_LAYER_TREE_NODE_EXPANDED) {
+    BEGIN_ANIMFILTER_SUBCHANNELS (layer_group.is_expanded()) {
       LISTBASE_FOREACH_BACKWARD (GreasePencilLayerTreeNode *, node_, &layer_group.children) {
         tmp_items += animdata_filter_grease_pencil_layer_node_recursive(
             ac, &tmp_data, grease_pencil, node_->wrap(), filter_mode);

source/blender/editors/interface/templates/interface_template_grease_pencil_layer_tree.cc

@@ -332,6 +332,35 @@ class LayerGroupViewItem : public AbstractTreeViewItem {
     this->label_ = group_.name();
   }
 
+  std::optional<bool> should_be_collapsed() const override
+  {
+    const bool is_collapsed = !group_.is_expanded();
+    return is_collapsed;
+  }
+
+  bool set_collapsed(const bool collapsed) override
+  {
+    if (!AbstractTreeViewItem::set_collapsed(collapsed)) {
+      return false;
+    }
+    group_.set_expanded(!collapsed);
+    return true;
+  }
+
+  void on_collapse_change(bContext &C, const bool is_collapsed) override
+  {
+    const bool is_expanded = !is_collapsed;
+
+    /* Let RNA handle the property change. This makes sure all the notifiers and DEG
+     * update calls are properly called. */
+    PointerRNA group_ptr = RNA_pointer_create(
+        &grease_pencil_.id, &RNA_GreasePencilLayerGroup, &group_);
+    PropertyRNA *prop = RNA_struct_find_property(&group_ptr, "is_expanded");
+
+    RNA_property_boolean_set(&group_ptr, prop, is_expanded);
+    RNA_property_update(&C, &group_ptr, prop);
+  }
+
   void build_row(uiLayout &row) override
   {
     build_layer_group_name(row);
@@ -478,7 +507,6 @@ void LayerTreeView::build_tree_node_recursive(TreeViewOrItem &parent, TreeNode &
   else if (node.is_group()) {
     LayerGroupViewItem &group_item = parent.add_tree_item<LayerGroupViewItem>(this->grease_pencil_,
                                                                               node.as_group());
-    group_item.uncollapse_by_default();
     LISTBASE_FOREACH_BACKWARD (GreasePencilLayerTreeNode *, node_, &node.as_group().children) {
       build_tree_node_recursive(group_item, node_->wrap());
     }

source/blender/makesrna/intern/rna_grease_pencil.cc

@@ -625,6 +625,12 @@ static void rna_GreasePencilLayerGroup_name_set(PointerRNA *ptr, const char *val
   grease_pencil->rename_node(*G_MAIN, group->wrap().as_node(), value);
 }
 
+static void rna_GreasePencilLayerGroup_is_expanded_set(PointerRNA *ptr, const bool value)
+{
+  GreasePencilLayerTreeGroup *group = static_cast<GreasePencilLayerTreeGroup *>(ptr->data);
+  group->wrap().set_expanded(value);
+}
+
 static void rna_iterator_grease_pencil_layer_groups_begin(CollectionPropertyIterator *iter,
                                                           PointerRNA *ptr)
 {
@@ -1173,6 +1179,16 @@ static void rna_def_grease_pencil_layer_group(BlenderRNA *brna)
       prop, "Onion Skinning", "Display onion skins before and after the current frame");
   RNA_def_property_update(prop, NC_GPENCIL | ND_DATA, "rna_grease_pencil_update");
 
+  /* Expanded */
+  prop = RNA_def_property(srna, "is_expanded", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(
+      prop, "GreasePencilLayerTreeNode", "flag", GP_LAYER_TREE_NODE_EXPANDED);
+  RNA_def_property_ui_text(prop, "Expanded", "The layer groups is expanded in the UI");
+  RNA_def_property_flag(prop, PROP_LIB_EXCEPTION);
+  RNA_def_property_override_flag(prop, PROPOVERRIDE_OVERRIDABLE_LIBRARY);
+  RNA_def_property_boolean_funcs(prop, nullptr, "rna_GreasePencilLayerGroup_is_expanded_set");
+  RNA_def_property_update(prop, NC_GPENCIL | ND_DATA, "rna_grease_pencil_update");
+
   /* Parent group. */
   prop = RNA_def_property(srna, "parent_group", PROP_POINTER, PROP_NONE);
   RNA_def_property_struct_type(prop, "GreasePencilLayerGroup");