Custom adaptor not triggering when exporting a skeleton with a self-generated API

[BeardedPlatypus]

Hi all,

I have a question regarding exporting skeletons with a self-generated API, which I hope someone can help me with. I am completely new to Maya and USD scripting so I hope this is just a matter of me not having a complete understanding of the concepts involved.

Basically, I have the following (simplified) set up:

I will paste all the code at the bottom of this post

• A usd file containing among other things a skeleton with a self-generated HelloWorldAPI schema
• A codeless definition of this HelloWorldAPI
• An importer derived from the SchemaApiAdaptor registered to trigger on the combination of a joint maya object and the HelloWorldAPI
• An exporter derived from the SchemaApiAdaptor registered to trigger on the combination of a joint maya object and the HelloWorldAPI

Now I would expect that when I run the import statement, the importer adaptor would get triggered, and when I run the export statement on the data that was imported, the exporter adaptor would get triggered.
When running the code however, only the importer gets triggered, and the exporter is always ignored. I am curious what I am doing wrong, or how this could be solved.

For reference, I am using the 0.19 release with Maya 2022.

Edit: When trying to export the same HelloWorldAPI on a mesh, I don't have any problems, and the exporter would get properly called. So far I only have encountered this behaviour when dealing with skeletons/joints.

I hope this clearly explains my question, if any additional information is required, I'd gladly provide it!

Python script

import os

import maya
import mayaUsd.lib as mayaUsdLib
from maya import cmds


class HelloWorldSchemaApiImporter(mayaUsdLib.SchemaApiAdaptor):
    def CanAdaptForImport(self, jobArgs):
        return True

    def ApplySchemaForImport(self, primReaderArgs, context):
        return True

    def CopyFromPrim(self, prim, args, context):
        # This actually gets called correctly
        print("Called importer.")
        return True


class HelloWorldSchemaApiExporter(mayaUsdLib.SchemaApiAdaptor):
    def CanAdaptForExport(self, jobArgs):
        return True

    def CopyToPrim(self, prim, usdTime, valueWriter):
        # This doesn't get called at all
        print("Called exporter.")
        return True


maya.cmds.loadPlugin("mayaUsdPlugin")

def maya_useNewAPI():
    pass

mayaUsdLib.SchemaApiAdaptor.Register(HelloWorldSchemaApiImporter, "joint", "HelloWorldAPI")
mayaUsdLib.SchemaApiAdaptor.Register(HelloWorldSchemaApiExporter, "joint", "HelloWorldAPI")


# NOTE: This would have to be set to the correct folder:
parent_dir = r"<path/to/usd/file>"

example_asset = os.path.join(parent_dir, "skel.usda")
export_path = os.path.join(parent_dir, "exported_skel.usda")

cmds.file(new=True, f=True)
cmds.mayaUSDImport(file=example_asset, apiSchema=["HelloWorldAPI"], primPath="/")
cmds.mayaUSDExport(file=export_path, apiSchema=["HelloWorldAPI"], exportSkels="auto")

USD file with a skeleton with HelloWorldAPI

skel.zip

Schema definition of the HelloWorldAPI

schema.zip