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_input_tangent.cc b/source/blender/nodes/geometry/nodes/node_geo_input_tangent.cc
index f5831941094..a2aab5464aa 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_input_tangent.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_input_tangent.cc
@@ -75,7 +75,7 @@ static VArray<float3> construct_curve_tangent_gvarray(const CurveComponent &comp
 
   const VArray<int8_t> types = curves.curve_types();
   if (curves.is_single_type(CURVE_TYPE_POLY)) {
-    return component.attribute_try_adapt_domain<float3>(
+    return component.attributes()->adapt_domain<float3>(
         VArray<float3>::ForSpan(curves.evaluated_tangents()), ATTR_DOMAIN_POINT, domain);
   }
 
@@ -86,7 +86,7 @@ static VArray<float3> construct_curve_tangent_gvarray(const CurveComponent &comp
   }
 
   if (domain == ATTR_DOMAIN_CURVE) {
-    return component.attribute_try_adapt_domain<float3>(
+    return component.attributes()->adapt_domain<float3>(
         VArray<float3>::ForContainer(std::move(tangents)), ATTR_DOMAIN_POINT, ATTR_DOMAIN_CURVE);
   }