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, 7 deletions

diff --git a/source/blender/blenkernel/BKE_mesh_sample.hh b/source/blender/blenkernel/BKE_mesh_sample.hh
index cbbd0249f4f..356709d8942 100644
--- a/source/blender/blenkernel/BKE_mesh_sample.hh
+++ b/source/blender/blenkernel/BKE_mesh_sample.hh
@@ -13,6 +13,7 @@
 #include "DNA_meshdata_types.h"
 
 #include "BKE_attribute.h"
+#include "BKE_attribute.hh"
 
 struct Mesh;
 struct BVHTreeFromMesh;
@@ -21,11 +22,6 @@ namespace blender {
 class RandomNumberGenerator;
 }
 
-namespace blender::bke {
-struct ReadAttributeLookup;
-class OutputAttribute;
-}  // namespace blender::bke
-
 namespace blender::bke::mesh_surface_sample {
 
 void sample_point_attribute(const Mesh &mesh,
@@ -81,8 +77,8 @@ class MeshAttributeInterpolator {
                    eAttributeMapMode mode,
                    const GMutableSpan dst);
 
-  void sample_attribute(const ReadAttributeLookup &src_attribute,
-                        OutputAttribute &dst_attribute,
+  void sample_attribute(const GAttributeReader &src_attribute,
+                        GSpanAttributeWriter &dst_attribute,
                         eAttributeMapMode mode);
 
  protected: