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, 6 deletions

diff --git a/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc b/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc
index dedc880988d..aabe6fd93e4 100644
--- a/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc
+++ b/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc
@@ -148,12 +148,9 @@ struct SlideOperationExecutor {
                          BKE_mesh_runtime_looptri_len(surface_)};
 
     if (curves_id_->surface_uv_map != nullptr) {
-      MeshComponent surface_component;
-      surface_component.replace(surface_, GeometryOwnershipType::ReadOnly);
-      surface_uv_map_ = surface_component
-                            .attribute_try_get_for_read(curves_id_->surface_uv_map,
-                                                        ATTR_DOMAIN_CORNER)
-                            .typed<float2>();
+      const bke::AttributeAccessor surface_attributes = bke::mesh_attributes(*surface_);
+      surface_uv_map_ = surface_attributes.lookup<float2>(curves_id_->surface_uv_map,
+                                                          ATTR_DOMAIN_CORNER);
     }
 
     if (stroke_extension.is_first) {