Getting notified of changes in the scene

I am writing an Omniverse Extension. Part of the UI is a dropdown box with all of the stage’s Xforms. I want the contents of the dropdown box to be updated any time an Xform is added or deleted in the scene. I tried with subscribing to stage events but unfortunately it seems there is no event when an Xform is added, only if it is deleted.

  1. How can my code be notified properly about Xform added or deleted?
  2. How can I update the model’s items accordingly? I use isaac’s dropdown_builder to create it.

Thanks a lot

You might consider basing your extension off of the Stage extension since it accurately tracks changes to Xforms.

Stage uses isaac-sim/kit/extscore/omni.usd/omni/usd/_impl/, which gets changes and updates the cache (including deletion) for usd property changes, and creates the listener for changes. If possible I would interface this module rather than borrowing code from it.

The Stage extension also can apply updates to models, and the code to do so should live in either the omni.usdpackage path, the omni.kit.widget.stage package path, or the omni.kit.stage_column package path.

Thanks lapp0,

I think what you suggest is what I am looking for. Still I am a bit lost on how to proceed.

do you mean deriving my extension class from omni.kit.window.stage? Or omni.kit.widget.stage?

Which part of are taking care of monitoring changes. Is it PrimCaching?

Thanks again

Well, here is what I did, after looking at the code of omni.kit.widget.stage:

    self._stage_listener = Tf.Notice.Register(

def _on_objects_changed(self, notice, sender):
    paths = notice.GetResyncedPaths()
    prim_path = False
    for path in paths:
        if path.IsPrimPath():
            prim_path = True
    if not prim_path:

Now I can react on any prim changes, which is ok for the moment.
Kind regards

1 Like

@bruno.vetter you have the right idea. Have a look at this documentation in case on of the other UsdNotice functions fit your case better: Universal Scene Description: UsdNotice::ObjectsChanged Class Reference

Thanks for the help!

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