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, 3 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_spline.hh b/source/blender/blenkernel/BKE_spline.hh
index 28f326a4ad4..767018ae0bb 100644
--- a/source/blender/blenkernel/BKE_spline.hh
+++ b/source/blender/blenkernel/BKE_spline.hh
@@ -15,7 +15,7 @@
 #include "BLI_math_vec_types.hh"
 #include "BLI_vector.hh"
 
-#include "BKE_attribute_access.hh"
+#include "BKE_attribute.hh"
 #include "BKE_attribute_math.hh"
 
 struct Curve;
@@ -646,6 +646,8 @@ struct CurveEval {
   void transform(const blender::float4x4 &matrix);
   bool bounds_min_max(blender::float3 &min, blender::float3 &max, bool use_evaluated) const;
 
+  blender::bke::MutableAttributeAccessor attributes_for_write();
+
   /**
    * Return the start indices for each of the curve spline's control points, if they were part
    * of a flattened array. This can be used to facilitate parallelism by avoiding the need to