Making a custom PrimUpdater in python - editAsMaya is not triggered

[daniele-niero]

Hello,

I have a custom IsA schema that defines a LOD primitive.
I’d like to convert this into Maya’s lodGroup when using the editAsMaya command (and later, of course, handle the reverse conversion—but one step at a time).

I thought the right way to do this was to implement a new PrimUpdater.

Here’s the basic implementation:

import mayaUsd.lib as mayaUsdLib
from maya import cmds

class LodPrimUpdater(mayaUsdLib.PrimUpdater):
    @classmethod
    def sRegister(cls):
        print(f'Registering {cls.__qualname__}')
        mayaUsdLib.PrimUpdater.Register(
            cls, "MyLodGroup", "lodGroup", LodPrimUpdater.Supports.All.value
        )

    @classmethod
    def sUnregister(cls):
        print(f'Un-registering {cls.__qualname__}')
        mayaUsdLib.PrimUpdater.Unregister(
            cls, "MyLodGroup", "lodGroup", LodPrimUpdater.Supports.All.value
        )

    def canEditAsMaya(self):
        print('canEditAsMaya', self.__class__.__name__)
        return True
    
    def editAsMaya(self):
        print('editAsMaya')
        cmds.createNode("transform", name="myCustomNode")
        return True

Here’s a simple USD file you can use for testing:

#usda 1.0

def Scope "Scope"
{
    def LodGroup "LodGroup"
    {}
}

The interesting part is that after registering LodPrimUpdater, when I right-click in Maya on the LodGroup prim, I clearly see:

canEditAsMaya LodPrimUpdater

This means the class is registered and recognizes my custom prim.
As a double check, if I unregister it, the “Edit as Maya” option disappears.

However, when I try to convert it, nothing happens, and Maya simply prints:

mayaUsdEditAsMaya "|UsdTest|UsdTestShape,/Scope/LodGroup";
// Warning: Nothing to edit in the selection.
// Warning: line 1: waitCursor stack empty

It seems Maya never even calls my editAsMaya method.

What am I missing? What am I doing wrong?

[pierrebai-adsk]

You could try to use a prim reader instead of updater, The problem right now with your example is that Maya does not even know what to do with the LodGroup prim.

Here is an example PrimReader that does read your prim (can currently creates a Maya sphere). The updater is still not called though, but I suppose you an do what you can in the reader?

import mayaUsd.lib as mayaUsdLib
from maya import cmds


class LodGroupReader(mayaUsdLib.PrimReader):
    def Read(self, context):
        usdPrim = self._GetArgs().GetUsdPrim()
        print(f'Read {usdPrim.GetName()}')
        sphere = cmds.polySphere(r=7, name=usdPrim.GetName())[0]
        selectionList = OpenMaya.MSelectionList()
        selectionList.add(sphere)
        sphereObj = selectionList.getDependNode(0)
        context.RegisterNewMayaNode(usdPrim.GetPath().pathString, sphereObj)
        return True


class LodPrimUpdater(mayaUsdLib.PrimUpdater):
    @classmethod
    def sRegister(cls):
        print(f'Registering {cls.__qualname__}')
        mayaUsdLib.PrimUpdater.Register(
            cls, "LodGroup", "lodGroup", LodPrimUpdater.Supports.All.value
        )

    @classmethod
    def sUnregister(cls):
        print(f'Un-registering {cls.__qualname__}')
        mayaUsdLib.PrimUpdater.Unregister(
            cls, "LodGroup", "lodGroup", LodPrimUpdater.Supports.All.value
        )

    def canEditAsMaya(self):
        print('canEditAsMaya', self.__class__.__name__)
        return True
    
    def editAsMaya(self):
        print('editAsMaya')
        cmds.createNode("transform", name="myCustomNode")
        return True

def test():
    LodPrimUpdater.sRegister()
    mayaUsdLib.PrimReader.Register(LodGroupReader, "LodGroup")

test()

[daniele-niero]

Hi, and thanks for the reply.

Yes, using that approach I can create a sphere (or whatever object I want), but that’s essentially all the PrimReader does. It doesn’t perform all the other small but important steps that happen when I call editAsMaya on a supported prim—things like:

• Creating a hidden parent under the hidden object __mayaUsd__
• Connecting attributes from the proxy shape to the newly created objects
• Adding those objects to a set
• And probably other internal steps I’m not aware of

Without those steps, I don’t end up with a proper working setup that’s indistinguishable from the other prims supported by MayaUSD. Even if I tried to manually implement them, any changes in MayaUSD’s internal implementation could easily break my work.

Also, even when using a PrimReader, I still don’t understand why the PrimUpdater’s editAsMaya method is never called. This is the same for other methods too, such as mergeToUsd and discardEdits.

I can see how mergeToUsd might be replaced by a PrimWriter, but what about discardEdits?

It’s really not clear to me how all these components are supposed to work together. Honestly, it feels incomplete.

[pierrebai-adsk]

No doubt there is a mystery why the PrimUpdater is not being called. I was presenting this as a work-around to get the data into Maya. It did work for me when I edited-as-Maya the parent scope, so that could be a work-around. You would also need to also implement a PrimWriter to do the conversion back to USD.

[ilpoldo]

Hi, I just stumbled on this discussion while researching the same topic... Following along the example above (and looking at the test suite), the PrimReader was erroring before completing until I parented the sphere directly:

class LodGroupReader(mayaUsdLib.PrimReader):
    def Read(self, context):
        usdPrim = self._GetArgs().GetUsdPrim()
        print(f'Read {usdPrim.GetName()}')
        sphere = cmds.polySphere(r=7, name=usdPrim.GetName())[0]
        parent = context.GetMayaNode(usdPrim.GetPath().GetParentPath(), True)
        parentDagPath = OpenMaya.MFnDagNode(parent).fullPathName()
        cmds.parent(sphere, parentDagPath)
       # ...

The reader seems to produce the expected result: set membership, hierarchy under __mayaUsd__ and cleanup when discarding Maya edits all appear to work.

I am still wrapping my head around this and have the same follow-up questions about the prim updater...

[daniele-niero]

Thank you @ilpoldo this is exactly what was missing.

context.GetMayaNode(usdPrim.GetPath().GetParentPath(), True) returns an utomatically generated parent nodes that must be used (something needs to be parented under it) in order to get averythign working.
Just context.RegisterNewMayaNode(usdPrim.GetPath().pathString, sphereObj) is not enough but needed as well. This is also the same dag node that need to be parented under the parent node given by context.GetMayaNode

After the Reader is in a working state, the PrimUpdater will work as intended.
This should be specified somewhere to avoid people to scratch their heads in confusion.
context.GetMayaNode should also have a more descriptive name, something like GetMayaParentNode or something on that line.

[ilpoldo]

you are right! The prim updater is now running as expected... In my tests I was trying to deal with the children of the group too...

Imagine that the composed stage looks like this:

def Scope "Scope"
{
    def LodGroup "LodGroup"
    {
        def Xform "transform_01"
        {
        }
    }
}

when I edit "/Scope/LodGroup" as Maya my prim reader creates the new Maya node for that prim. However, by the time the prim updater's editAsMaya is called, something has created a group node for "transform_01" under the new Maya node (a transform, in my example).

In my custom prim reader I try to stop the reader job from reading the child prims using context.SetPruneChildren(True), but that doesn't seem to have an effect. Any suggestions?