Added support for defining an on_import() function in
bpy.types.USDHook subclasses. If on_import() is defined
on a given USD hook, it will be invoked in import_endjob().
The implementation closely follows the existing design of
export hooks. USDHook.on_import() takes as an argument
an instance of an internally defined USDSceneImportContext
class which provides an accessor to the USD stage.
Also updated the USDHook documentation with an example
on_import() callback implementation.
Pull Request: https://projects.blender.org/blender/blender/pulls/117822
Michael Kowalski
ecbf3385c5