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, 5 insertions, 7 deletions

diff --git a/source/blender/geometry/intern/mesh_primitive_cuboid.cc b/source/blender/geometry/intern/mesh_primitive_cuboid.cc
index 07ac2419ad9..486d8adbf39 100644
--- a/source/blender/geometry/intern/mesh_primitive_cuboid.cc
+++ b/source/blender/geometry/intern/mesh_primitive_cuboid.cc
@@ -7,7 +7,6 @@
 #include "DNA_mesh_types.h"
 #include "DNA_meshdata_types.h"
 
-#include "BKE_attribute_access.hh"
 #include "BKE_geometry_set.hh"
 #include "BKE_mesh.h"
 
@@ -322,11 +321,10 @@ static void calculate_polys(const CuboidConfig &config,
 
 static void calculate_uvs(const CuboidConfig &config, Mesh *mesh, const bke::AttributeIDRef &uv_id)
 {
-  MeshComponent mesh_component;
-  mesh_component.replace(mesh, GeometryOwnershipType::Editable);
-  bke::OutputAttribute_Typed<float2> uv_attribute =
-      mesh_component.attribute_try_get_for_output_only<float2>(uv_id, ATTR_DOMAIN_CORNER);
-  MutableSpan<float2> uvs = uv_attribute.as_span();
+  bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(*mesh);
+  bke::SpanAttributeWriter<float2> uv_attribute =
+      attributes.lookup_or_add_for_write_only_span<float2>(uv_id, ATTR_DOMAIN_CORNER);
+  MutableSpan<float2> uvs = uv_attribute.span;
 
   int loop_index = 0;
 
@@ -394,7 +392,7 @@ static void calculate_uvs(const CuboidConfig &config, Mesh *mesh, const bke::Att
     }
   }
 
-  uv_attribute.save();
+  uv_attribute.finish();
 }
 
 Mesh *create_cuboid_mesh(const float3 &size,