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, 7 insertions, 4 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_dual_mesh.cc b/source/blender/nodes/geometry/nodes/node_geo_dual_mesh.cc
index 8a256a9b91b..bf5479d552e 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_dual_mesh.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_dual_mesh.cc
@@ -146,9 +146,12 @@ static void transfer_attributes(
     const GeometryComponent &src_component,
     GeometryComponent &dst_component)
 {
+  const AttributeAccessor src_attributes = *src_component.attributes();
+  MutableAttributeAccessor dst_attributes = *dst_component.attributes_for_write();
+
   for (Map<AttributeIDRef, AttributeKind>::Item entry : attributes.items()) {
     const AttributeIDRef attribute_id = entry.key;
-    ReadAttributeLookup src_attribute = src_component.attribute_try_get_for_read(attribute_id);
+    GAttributeReader src_attribute = src_attributes.lookup(attribute_id);
     if (!src_attribute) {
       continue;
     }
@@ -166,7 +169,7 @@ static void transfer_attributes(
     }
     const eCustomDataType data_type = bke::cpp_type_to_custom_data_type(
         src_attribute.varray.type());
-    OutputAttribute dst_attribute = dst_component.attribute_try_get_for_output_only(
+    GSpanAttributeWriter dst_attribute = dst_attributes.lookup_or_add_for_write_only_span(
         attribute_id, out_domain, data_type);
 
     if (!dst_attribute) {
@@ -176,7 +179,7 @@ static void transfer_attributes(
     attribute_math::convert_to_static_type(data_type, [&](auto dummy) {
       using T = decltype(dummy);
       VArraySpan<T> span{src_attribute.varray.typed<T>()};
-      MutableSpan<T> dst_span = dst_attribute.as_span<T>();
+      MutableSpan<T> dst_span = dst_attribute.span.typed<T>();
       if (src_attribute.domain == ATTR_DOMAIN_FACE) {
         dst_span.take_front(span.size()).copy_from(span);
         if (keep_boundaries) {
@@ -193,7 +196,7 @@ static void transfer_attributes(
         copy_data_based_on_new_to_old_map(span, dst_span, new_to_old_face_corners_map);
       }
     });
-    dst_attribute.save();
+    dst_attribute.finish();
   }
 }