Refactor Viewer to be a base class owning the communication with the simulation, handler registry, core visualization objects and the render function

ViewerApp is simplified, interacting with the endpoint and handlers through the Viewer instance. This change makes it possible to write simulation viewers without using the UI/UX provided by ViewerApp (which should be renamed StudioApp)

PiperOrigin-RevId: 943642660
Change-Id: Id8eb3d4e6a27ceda93833367fec4699fa6bc838e

python/mujoco/experimental/studio/viewer.py

@@ -19,6 +19,7 @@ from mujoco.experimental.studio import launch_passive
 from mujoco.experimental.studio import messages
 from mujoco.experimental.studio import parser
 from mujoco.experimental.studio import sim
+from mujoco.experimental.studio import viewer_app
 from mujoco.experimental.studio import viewer_protocol
 
 vp = viewer_protocol
@@ -40,24 +41,23 @@ def main(argv: list[str]) -> None:
       viewer_mode=_VIEWER.value,
   )
 
-  # Get the model path, if provided.
-  model_path = None
-  if _MJCF_PATH.value is not None:
-    model_path = _MJCF_PATH.value
-  elif len(argv) > 1 and not argv[1].startswith('--'):
-    model_path = argv[1]
+  # Resolve model path, if provided.
+  model_path = _MJCF_PATH.value or (
+      argv[1] if len(argv) > 1 and not argv[1].startswith('--') else None
+  )
 
-  with launch_passive.launch_passive(config) as handle:
-    # Load the model if we have a path.
-    model, data = None, None
-    if model_path is not None:
-      data = parser.parse(model_path)
-      if data is not None:
-        model = data.model
+  # Load model if path was provided.
+  data, model = None, None
+  if model_path and (data := parser.parse(model_path)):
+    model = data.model
 
+  with launch_passive.launch_passive(
+      config,
+      viewer_handlers=[viewer_app.ViewerApp()],
+  ) as handle:
     # Send the model to the viewer, if we have a model.
     if model is not None:
-      handle.send_to_viewer(messages.ModelEvent(model=model))
+      handle.send_to_viewer(messages.ModelEvent(model=model, path=model_path))
 
     # Run the simulation.
     step_control = sim.StepControl()