Geometry Nodes: new geometry attribute API

Currently, there are two attribute API. The first, defined in `BKE_attribute.h` is
accessible from RNA and C code. The second is implemented with `GeometryComponent`
and is only accessible in C++ code. The second is widely used, but only being
accessible through the `GeometrySet` API makes it awkward to use, and even impossible
for types that don't correspond directly to a geometry component like `CurvesGeometry`.

This patch adds a new attribute API, designed to replace the `GeometryComponent`
attribute API now, and to eventually replace or be the basis of the other one.

The basic idea is that there is an `AttributeAccessor` class that allows code to
interact with a set of attributes owned by some geometry. The accessor itself has
no ownership. `AttributeAccessor` is a simple type that can be passed around by
value. That makes it easy to return it from functions and to store it in containers.

For const-correctness, there is also a `MutableAttributeAccessor` that allows
changing individual and can add or remove attributes.

Currently, `AttributeAccessor` is composed of two pointers. The first is a pointer
to the owner of the attribute data. The second is a pointer to a struct with
function pointers, that is similar to a virtual function table. The functions
know how to access attributes on the owner.

The actual attribute access for geometries is still implemented with the `AttributeProvider`
pattern, which makes it easy to support different sources of attributes on a
geometry and simplifies dealing with built-in attributes.

There are different ways to get an attribute accessor for a geometry:
* `GeometryComponent.attributes()`
* `CurvesGeometry.attributes()`
* `bke::mesh_attributes(const Mesh &)`
* `bke::pointcloud_attributes(const PointCloud &)`

All of these also have a `_for_write` variant that returns a `MutabelAttributeAccessor`.

Differential Revision: https://developer.blender.org/D15280

1 files changed, 2 insertions, 2 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_curve_trim.cc b/source/blender/nodes/geometry/nodes/node_geo_curve_trim.cc
index 2b90428acb1..00a79168087 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_curve_trim.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_curve_trim.cc
@@ -506,9 +506,9 @@ static void geometry_set_curve_trim(GeometrySet &geometry_set,
 
   CurveComponent &component = geometry_set.get_component_for_write<CurveComponent>();
   GeometryComponentFieldContext field_context{component, ATTR_DOMAIN_CURVE};
-  const int domain_num = component.attribute_domain_num(ATTR_DOMAIN_CURVE);
+  const int domain_size = component.attribute_domain_size(ATTR_DOMAIN_CURVE);
 
-  fn::FieldEvaluator evaluator{field_context, domain_num};
+  fn::FieldEvaluator evaluator{field_context, domain_size};
   evaluator.add(start_field);
   evaluator.add(end_field);
   evaluator.evaluate();