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, 4 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_curves.hh b/source/blender/blenkernel/BKE_curves.hh
index 3e00dc78b74..fb97e52f6da 100644
--- a/source/blender/blenkernel/BKE_curves.hh
+++ b/source/blender/blenkernel/BKE_curves.hh
@@ -20,7 +20,7 @@
 #include "BLI_vector.hh"
 #include "BLI_virtual_array.hh"
 
-#include "BKE_attribute_access.hh"
+#include "BKE_attribute.hh"
 
 namespace blender::bke {
 
@@ -401,6 +401,9 @@ class CurvesGeometry : public ::CurvesGeometry {
    */
   void remove_attributes_based_on_types();
 
+  AttributeAccessor attributes() const;
+  MutableAttributeAccessor attributes_for_write();
+
   /* --------------------------------------------------------------------
    * Attributes.
    */