GPv3: Attribute panel to configure layer attributes on original data

This follows the same pattern as attribute panels for Mesh, Curves, and
Point Cloud. Only the layer domain is supported. The _add_ operator now
has to validate the domain setting on invoke, because layer attributes
don't support the default `Point` domain.

Pull Request: https://projects.blender.org/blender/blender/pulls/128471

scripts/startup/bl_ui/properties_data_grease_pencil.py

@@ -403,6 +403,73 @@ class DATA_PT_grease_pencil_custom_props(DataButtonsPanel, PropertyPanel, Panel)
     _property_type = bpy.types.GreasePencilv3
 
 
+class GREASE_PENCIL_UL_attributes(UIList):
+    def filter_items(self, _context, data, property):
+        attributes = getattr(data, property)
+        flags = []
+        indices = [i for i in range(len(attributes))]
+
+        # Filtering by name
+        if self.filter_name:
+            flags = bpy.types.UI_UL_list.filter_items_by_name(
+                self.filter_name, self.bitflag_filter_item, attributes, "name", reverse=self.use_filter_invert)
+        if not flags:
+            flags = [self.bitflag_filter_item] * len(attributes)
+
+        # Filtering internal attributes
+        for idx, item in enumerate(attributes):
+            flags[idx] = 0 if item.is_internal else flags[idx]
+
+        # Reorder by name.
+        if self.use_filter_sort_alpha:
+            indices = bpy.types.UI_UL_list.sort_items_by_name(attributes, "name")
+
+        return flags, indices
+
+    def draw_item(self, _context, layout, _data, attribute, _icon, _active_data, _active_propname, _index):
+        data_type = attribute.bl_rna.properties["data_type"].enum_items[attribute.data_type]
+
+        split = layout.split(factor=0.50)
+        split.emboss = 'NONE'
+        split.prop(attribute, "name", text="")
+        sub = split.row()
+        sub.alignment = 'RIGHT'
+        sub.active = False
+        sub.label(text=data_type.name)
+
+
+class DATA_PT_grease_pencil_attributes(DataButtonsPanel, Panel):
+    bl_label = "Attributes"
+    bl_options = {'DEFAULT_CLOSED'}
+    COMPAT_ENGINES = {
+        'BLENDER_RENDER',
+        'BLENDER_EEVEE',
+        'BLENDER_EEVEE_NEXT',
+        'BLENDER_WORKBENCH',
+    }
+
+    def draw(self, context):
+        grease_pencil = context.grease_pencil
+
+        layout = self.layout
+        row = layout.row()
+
+        col = row.column()
+        col.template_list(
+            "GREASE_PENCIL_UL_attributes",
+            "attributes",
+            grease_pencil,
+            "attributes",
+            grease_pencil.attributes,
+            "active_index",
+            rows=3,
+        )
+
+        col = row.column(align=True)
+        col.operator("geometry.attribute_add", icon='ADD', text="")
+        col.operator("geometry.attribute_remove", icon='REMOVE', text="")
+
+
 classes = (
     GREASE_PENCIL_UL_masks,
     GREASE_PENCIL_MT_layer_mask_add,
@@ -420,6 +487,8 @@ classes = (
     GREASE_PENCIL_MT_grease_pencil_add_layer_extra,
     GREASE_PENCIL_MT_group_context_menu,
     DATA_PT_grease_pencil_animation,
+    GREASE_PENCIL_UL_attributes,
+    DATA_PT_grease_pencil_attributes,
 )
 
 

source/blender/editors/geometry/geometry_attributes.cc

@@ -289,6 +289,21 @@ static int geometry_attribute_add_invoke(bContext *C, wmOperator *op, const wmEv
   if (!RNA_property_is_set(op->ptr, prop)) {
     RNA_property_string_set(op->ptr, prop, DATA_("Attribute"));
   }
+  /* Set a valid default domain, in case Point domain is not supported. */
+  prop = RNA_struct_find_property(op->ptr, "domain");
+  if (!RNA_property_is_set(op->ptr, prop)) {
+    EnumPropertyItem *items;
+    int totitems;
+    bool free;
+    RNA_property_enum_items(
+        C, op->ptr, prop, const_cast<const EnumPropertyItem **>(&items), &totitems, &free);
+    if (totitems > 0) {
+      RNA_property_enum_set(op->ptr, prop, items[0].value);
+    }
+    if (free) {
+      MEM_freeN(items);
+    }
+  }
   return WM_operator_props_popup_confirm_ex(
       C, op, event, IFACE_("Add Attribute"), CTX_IFACE_(BLT_I18NCONTEXT_OPERATOR_DEFAULT, "Add"));
 }

source/blender/makesrna/intern/rna_attribute.cc

@@ -332,7 +332,7 @@ const EnumPropertyItem *rna_enum_attribute_domain_itemf(const AttributeOwner &ow
       continue;
     }
     if (owner.type() == AttributeOwnerType::GreasePencil &&
-        ELEM(domain_item->value, int(AttrDomain::Layer)))
+        !ELEM(domain_item->value, int(AttrDomain::Layer)))
     {
       continue;
     }