feat: add fabric_read/fabric_write context managers to FabricFrameView

Add RAII-style context managers for safe raw Fabric access:

- fabric_write(): calls PrepareForReuse on entry, update_world_xforms +
  sync on exit.  Provides world_matrices fabricarray and view_to_fabric
  mapping for custom warp kernel launches.
- fabric_read(): calls PrepareForReuse on entry (ensures valid pointers
  after topology changes), no-op on exit.

Also exposes read-only properties:
- world_matrices: the raw fabricarray of omni:fabric:worldMatrix
- view_to_fabric_mapping: the view-index to fabric-index mapping

This addresses Piotr's Issue isaac-sim#6 (reader/writer pattern) by providing
a structured way to bracket Fabric operations that ensures
PrepareForReuse and hierarchy updates are never forgotten.

Tests added:
- test_fabric_write_context_manager: validates write + readback
- test_fabric_read_context_manager: validates read without side effects

Depends on: fix/fabric-prepare-for-reuse (PR isaac-sim#5380)

Author: pv-nvidia

source/isaaclab_physx/isaaclab_physx/sim/views/fabric_frame_view.py

@@ -352,6 +352,53 @@ def _rebuild_fabric_arrays(self) -> None:
 
         self._fabric_world_matrices = wp.fabricarray(self._fabric_selection, "omni:fabric:worldMatrix")
 
+    # ------------------------------------------------------------------
+    # Context managers for raw Fabric access
+    # ------------------------------------------------------------------
+
+    def fabric_write(self):
+        """Context manager for raw Fabric write operations.
+
+        Calls ``PrepareForReuse()`` on entry (notifying the renderer that
+        data is about to change) and ``update_world_xforms()`` +
+        ``PrepareForReuse()`` on exit (propagating changes through the
+        hierarchy).
+
+        Example::
+
+            with view.fabric_write() as fab:
+                # fab.world_matrices is the fabricarray
+                wp.launch(my_kernel, dim=N, inputs=[fab.world_matrices, ...])
+        """
+        return _FabricWriteContext(self)
+
+    def fabric_read(self):
+        """Context manager for raw Fabric read operations.
+
+        Calls ``PrepareForReuse()`` on entry to ensure the view’s
+        fabricarray pointers are still valid after potential topology
+        changes.
+
+        Example::
+
+            with view.fabric_read() as fab:
+                wp.launch(my_read_kernel, dim=N, inputs=[fab.world_matrices, ...])
+        """
+        return _FabricReadContext(self)
+
+    @property
+    def world_matrices(self) -> wp.fabricarray | None:
+        """The raw Fabric world-matrix array (read-only property).
+
+        Returns None if Fabric is not initialized.
+        """
+        return getattr(self, "_fabric_world_matrices", None)
+
+    @property
+    def view_to_fabric_mapping(self) -> wp.array | None:
+        """View-index → Fabric-index mapping array."""
+        return getattr(self, "_view_to_fabric", None)
+
     # ------------------------------------------------------------------
     # Internal — Fabric initialization
     # ------------------------------------------------------------------
@@ -456,3 +503,90 @@ def _resolve_indices_wp(self, indices: wp.array | None) -> wp.array:
         if indices.dtype != wp.uint32:
             return wp.array(indices.numpy().astype("uint32"), dtype=wp.uint32, device=self._device)
         return indices
+
+
+# ======================================================================
+# Context manager helpers (module-level, not inside FabricFrameView)
+# ======================================================================
+
+
+class _FabricWriteContext:
+    """RAII context manager for Fabric write operations.
+
+    On entry: ensures Fabric is initialized, calls PrepareForReuse.
+    On exit (no exception): synchronizes, propagates hierarchy, marks sync done.
+    """
+
+    __slots__ = ("_view",)
+
+    def __init__(self, view: FabricFrameView):
+        self._view = view
+
+    def __enter__(self):
+        if not self._view._fabric_initialized:
+            self._view._initialize_fabric()
+        if not self._view._fabric_usd_sync_done:
+            self._view._sync_fabric_from_usd_once()
+        self._view._prepare_for_reuse()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if exc_type is None:
+            wp.synchronize()
+            self._view._fabric_hierarchy.update_world_xforms()
+            self._view._fabric_usd_sync_done = True
+        return False
+
+    @property
+    def world_matrices(self) -> wp.fabricarray:
+        """The fabricarray of omni:fabric:worldMatrix."""
+        return self._view._fabric_world_matrices
+
+    @property
+    def view_to_fabric(self) -> wp.array:
+        """View-index to Fabric-index mapping."""
+        return self._view._view_to_fabric
+
+    @property
+    def count(self) -> int:
+        """Number of prims in the view."""
+        return self._view.count
+
+
+class _FabricReadContext:
+    """RAII context manager for Fabric read operations.
+
+    On entry: ensures Fabric is initialized, calls PrepareForReuse.
+    On exit: no-op.
+    """
+
+    __slots__ = ("_view",)
+
+    def __init__(self, view: FabricFrameView):
+        self._view = view
+
+    def __enter__(self):
+        if not self._view._fabric_initialized:
+            self._view._initialize_fabric()
+        if not self._view._fabric_usd_sync_done:
+            self._view._sync_fabric_from_usd_once()
+        self._view._prepare_for_reuse()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        return False
+
+    @property
+    def world_matrices(self) -> wp.fabricarray:
+        """The fabricarray of omni:fabric:worldMatrix."""
+        return self._view._fabric_world_matrices
+
+    @property
+    def view_to_fabric(self) -> wp.array:
+        """View-index to Fabric-index mapping."""
+        return self._view._view_to_fabric
+
+    @property
+    def count(self) -> int:
+        """Number of prims in the view."""
+        return self._view.count

source/isaaclab_physx/test/sim/test_views_xform_prim_fabric.py

@@ -23,12 +23,13 @@
 import torch  # noqa: E402
 import warp as wp  # noqa: E402
 from frame_view_contract_utils import *  # noqa: F401, F403, E402
-from frame_view_contract_utils import CHILD_OFFSET, ViewBundle, test_set_world_updates_local  # noqa: E402
+from frame_view_contract_utils import CHILD_OFFSET, ViewBundle  # noqa: E402
 from isaaclab_physx.sim.views import FabricFrameView as FrameView  # noqa: E402
 
 from pxr import Gf, UsdGeom  # noqa: E402
 
 import isaaclab.sim as sim_utils  # noqa: E402
+from isaaclab.utils.warp import fabric as fabric_utils  # noqa: E402
 
 PARENT_POS = (0.0, 0.0, 1.0)
 
@@ -201,6 +202,69 @@ def test_prepare_for_reuse_detects_topology_change(device, view_factory):
     assert not result, "PrepareForReuse should return False when no topology change"
 
 
+@pytest.mark.parametrize("device", ["cuda:0"])
+def test_fabric_write_context_manager(device, view_factory):
+    """Verify fabric_write() context manager correctly brackets writes.
+
+    After using the context manager to modify world matrices, the
+    resulting poses should be readable via get_world_poses.
+    """
+    bundle = view_factory(2, device)
+    view = bundle.view
+    view.get_world_poses()  # trigger Fabric init
+
+    # Write via context manager
+    with view.fabric_write() as fab:
+        assert fab.world_matrices is not None, "world_matrices should be available"
+        assert fab.view_to_fabric is not None, "view_to_fabric should be available"
+        assert fab.count == 2, f"Expected count=2, got {fab.count}"
+
+        # Move all prims to (42, 42, 42)
+        new_pos = wp.zeros((2, 3), dtype=wp.float32, device=device)
+        wp.launch(kernel=_fill_position, dim=2, inputs=[new_pos, 42.0, 42.0, 42.0], device=device)
+
+        wp.launch(
+            kernel=fabric_utils.compose_fabric_transformation_matrix_from_warp_arrays,
+            dim=2,
+            inputs=[
+                fab.world_matrices,
+                new_pos,
+                wp.zeros((0, 4), dtype=wp.float32, device=device),
+                wp.zeros((0, 3), dtype=wp.float32, device=device),
+                False,
+                False,
+                False,
+                view._default_view_indices,
+                fab.view_to_fabric,
+            ],
+            device=device,
+        )
+
+    # Verify via high-level API
+    pos, _ = view.get_world_poses()
+    pos_t = wp.to_torch(pos)
+    assert torch.allclose(pos_t, torch.tensor([[42.0, 42.0, 42.0]] * 2, device=device), atol=0.5), (
+        f"Expected ~(42,42,42) but got {pos_t}"
+    )
+
+
+@pytest.mark.parametrize("device", ["cuda:0"])
+def test_fabric_read_context_manager(device, view_factory):
+    """Verify fabric_read() context manager provides access without side effects."""
+    bundle = view_factory(1, device)
+    view = bundle.view
+    pos_before, _ = view.get_world_poses()  # trigger Fabric init
+    pos_before_t = wp.to_torch(pos_before).clone()
+
+    with view.fabric_read() as fab:
+        assert fab.world_matrices is not None
+        assert fab.count == 1
+
+    # Poses should be unchanged after a read context
+    pos_after, _ = view.get_world_poses()
+    assert torch.allclose(wp.to_torch(pos_after), pos_before_t, atol=0.01)
+
+
 @pytest.mark.parametrize("device", ["cuda:0"])
 def test_get_scales_fabric_path(device, view_factory):
     """Exercise the Fabric-native get_scales path."""