refactor: move Fabric/USD dispatch from FabricFrameView to FrameView factory

[pv-nvidia]

Problem

FabricFrameView has an internal _use_fabric flag that falls back to UsdFrameView when Fabric is disabled or the device is unsupported. This violates single-responsibility: the class pretends to be a Fabric-accelerated view but sometimes silently delegates everything to a different implementation. It's also a Liskov substitution violation — callers can't reason about which code path runs.

Solution

Move the Fabric-enabled + device-supported check from FabricFrameView.__init__ up to the FrameView factory's _get_backend() method. The factory now dispatches to three implementations:

Condition	Backend key	Class
PhysX + Fabric enabled + supported device	physx	FabricFrameView
PhysX without Fabric or unsupported device	usd	UsdFrameView
Newton	newton	NewtonSiteFrameView

UsdFrameView is eagerly registered on the factory since it lives in isaaclab (not a backend package like isaaclab_physx), so FactoryBase's dynamic import (isaaclab_{backend}.sim.views) can't discover it.

Changes

• FrameView._get_backend(): checks /physics/fabricEnabled setting + device support, returns "physx", "usd", or "newton"
• FabricFrameView: stripped of all _use_fabric conditionals, SettingsManager import, _fabric_supported_devices constant — it just does Fabric, nothing else
• UsdFrameView: eagerly registered as "usd" backend on the factory

Result

Each FrameView implementation now has a single responsibility. FabricFrameView assumes Fabric is available (the factory guarantees this). No runtime behavior change for existing callers — the same code paths execute, just dispatched from a cleaner location.

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Merge Order

This PR is part of a stacked series for Fabric-accelerated local poses:

1. feat: add typed service locator to SimulationContext #5672 — service locator ← this depends on
2. feat: add FabricStageCache service for shared hierarchy handles #5676 — FabricStageCache
3. feat: add indexed fabric transform kernels for local/world matrix propagation #5675 — indexed fabric kernels
4. refactor: move Fabric/USD dispatch from FabricFrameView to FrameView factory #5673 — factory dispatch ← this PR (depends on feat: add typed service locator to SimulationContext #5672)
5. refactor: fuse set_world_poses/set_scales into single _compose_fabric_transform #5674 — fused compose
6. PR C+D — local poses (depends on all above)

[isaaclab-review-bot]

🤖 Isaac Lab Review Bot

PR #5673: refactor: move Fabric/USD dispatch from FabricFrameView to FrameView factory

Summary

This PR contains 3 commits that introduce a service locator pattern and refactor the FrameView backend dispatch logic:

1. 596bedb - Add typed ServiceLocator to SimulationContext
2. dac5a16 - Add unit tests and changelog fragment
3. 4c71366 - Move Fabric/USD dispatch from FabricFrameView to FrameView factory

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✅ What's Good

1. Clean Service Locator Implementation (service_locator.py)

   • Well-designed subscript API (services[cls] = instance)
   • Proper lifecycle management with close() handling
   • Good defensive coding with _try_close() checking callable

2. Improved Separation of Concerns

   • Factory pattern in FrameView now handles backend selection logic
   • FabricFrameView is now simpler - only handles Fabric operations
   • Removed conditional checks inside FabricFrameView methods

3. Comprehensive Tests

   • test_service_locator.py covers edge cases (non-callable close, missing keys, etc.)
   • Good coverage of the service locator API

4. Proper Integration

   • SimulationContext.clear_instance() now calls services.close_all()
   • Services property exposed for external use

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🔍 Review Items

1. _FABRIC_SUPPORTED_DEVICES Duplication (Minor)

# In frame_view.py:
_FABRIC_SUPPORTED_DEVICES = ("cpu", "cuda", "cuda:0")

This was previously in fabric_frame_view.py. Consider:

• Making this a module-level constant that can be imported
• Or document why the factory owns this knowledge

2. Device Argument Extraction (frame_view.py:58-60)

device = kwargs.get("device", "cpu")
if len(args) >= 2:
    device = args[1]

This relies on positional argument order. Consider adding a comment documenting the expected signature or using keyword-only enforcement.

3. Return Type Consistency (ServiceLocator.__getitem__)

def __getitem__(self, cls: type[_T]) -> _T | None:
    return self._services.get(cls)  # returns None if missing

Returning None for missing keys differs from standard dict behavior (which raises KeyError). This is documented but worth noting - callers must always handle None.

4. No SettingsManager Import Removed from FabricFrameView

The import of SettingsManager was removed from fabric_frame_view.py since Fabric checks moved to the factory. 👍

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📋 Checklist

• Code follows Isaac Lab style guidelines
• Type hints present and correct
• Docstrings provided for public APIs
• Unit tests added for new functionality
• Changelog fragment included
• No obvious regressions

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Verdict: Approve ✅

This is a clean refactor that improves code organization by moving dispatch logic to the factory where it belongs. The service locator is a useful addition for managing backend-specific singletons. Minor suggestions above are non-blocking.

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Reviewed at commit: 4c71366