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, 9 insertions, 9 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_string_to_curves.cc b/source/blender/nodes/geometry/nodes/node_geo_string_to_curves.cc
index 94d5d7f946f..afd7db6604d 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_string_to_curves.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_string_to_curves.cc
@@ -335,13 +335,14 @@ static void create_attributes(GeoNodeExecParams &params,
                               const TextLayout &layout,
                               InstancesComponent &instances)
 {
+  MutableAttributeAccessor attributes = *instances.attributes_for_write();
+
   if (params.output_is_required("Line")) {
     StrongAnonymousAttributeID line_id = StrongAnonymousAttributeID("Line");
-    OutputAttribute_Typed<int> line_attribute = instances.attribute_try_get_for_output_only<int>(
+    SpanAttributeWriter<int> line_attribute = attributes.lookup_or_add_for_write_only_span<int>(
         line_id.get(), ATTR_DOMAIN_INSTANCE);
-    MutableSpan<int> lines = line_attribute.as_span();
-    lines.copy_from(layout.line_numbers);
-    line_attribute.save();
+    line_attribute.span.copy_from(layout.line_numbers);
+    line_attribute.finish();
     params.set_output("Line",
                       AnonymousAttributeFieldInput::Create<int>(std::move(line_id),
                                                                 params.attribute_producer_name()));
@@ -349,15 +350,14 @@ static void create_attributes(GeoNodeExecParams &params,
 
   if (params.output_is_required("Pivot Point")) {
     StrongAnonymousAttributeID pivot_id = StrongAnonymousAttributeID("Pivot");
-    OutputAttribute_Typed<float3> pivot_attribute =
-        instances.attribute_try_get_for_output_only<float3>(pivot_id.get(), ATTR_DOMAIN_INSTANCE);
-    MutableSpan<float3> pivots = pivot_attribute.as_span();
+    SpanAttributeWriter<float3> pivot_attribute =
+        attributes.lookup_or_add_for_write_only_span<float3>(pivot_id.get(), ATTR_DOMAIN_INSTANCE);
 
     for (const int i : layout.char_codes.index_range()) {
-      pivots[i] = layout.pivot_points.lookup(layout.char_codes[i]);
+      pivot_attribute.span[i] = layout.pivot_points.lookup(layout.char_codes[i]);
     }
 
-    pivot_attribute.save();
+    pivot_attribute.finish();
     params.set_output("Pivot Point",
                       AnonymousAttributeFieldInput::Create<float3>(
                           std::move(pivot_id), params.attribute_producer_name()));