Autodesk: Template class support for macro HD_DECLARE_DATASOURCE

[erikaharrison-adsk]

Description of Change(s)

Add template class support for the macro HD_DECLARE_DATASOURCE.

Add two classes UsdImagingDataSourceConvertedAttribute and UsdImagingDataSourceConvertedPrimvar for the primvar which doesn't directly get value from an attribute, but will do a conversion from the value of an attribute.

Link to proposal (if applicable)

• N/A

Fixes Issue(s)

• N/A

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

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[jesschimein]

Filed as internal issue #USD-11110

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[unhyperbolic]

UsdImagingDataSourceConvertedAttribute<A,B> is ultimately a subclass of HdTypedSampledDataSource<B> even though its GetValue returns a VtValue holding type A - violating the semantics.

This is a composition vs inheritance issue. Typically, a conversion data source subclasses from the appropriate HdTypedSampledDataSource and is instantiated from another HdTypedSampledDataSource. An example is the _ScalingTypedSampledDataSource in usdImaging/dataSourceCamera.cpp. This means we ultimately allocate two data sources, but that doesn't seem to be too much of overhead in the overall schema.

About HD_DECLARE_DATASOURCE_TEMPLATE: it is unfortunate, that the preprocessor thinks that HD_DECLARE_DATASOURCE(MySource<Arg1, Arg2>) has two arguments. UsdImagingDataSourceImplicitsPrim (in usdImaging/dataSourceImplicits-Impl.h) uses the following workaround:

using This = UsdImagingDataSourceImplicitsPrim<UsdSchemaType, HdSchemaType>;
HD_DECLARE_DATASOURCE(This);

I think that is an ok work-around and the amount of code duplication for HD_DECLARE_DATASOURCE_TEMPLATE isn't worth the merit.