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/sample/implot.py

@@ -108,7 +108,7 @@ def _setup_angle_axis(plot_size: imgui.Vec2) -> None:
   )
 
 
-class BodyInspectorHandler:
+class BodyInspector:
   """Handler that draws body-inspection plots using ImGui/ImPlot."""
 
   def __init__(self) -> None:
@@ -124,7 +124,7 @@ class BodyInspectorHandler:
     self._app = event.viewer_app
 
   @messages.handler
-  def inspect_body(self, _: viewer_app.BuildGuiEvent) -> None:
+  def inspect_body(self, _: messages.BuildGuiEvent) -> None:
     """Renders the body-inspection charts in ImGui/ImPlot."""
     app = self._app
     if app is None:
@@ -215,8 +215,7 @@ def main(argv: list[str]) -> None:
     print('Usage: implot <model_path.xml>')
     sys.exit(1)
 
-  data = parser.parse(argv[1])
-  if data is None:
+  if (data := parser.parse(argv[1])) is None:
     print(f'Error loading model from {argv[1]!r}')
     sys.exit(1)
   model = data.model
@@ -232,7 +231,8 @@ def main(argv: list[str]) -> None:
   )
 
   with launch_passive.launch_passive(
-      config, viewer_handlers=[BodyInspectorHandler()]
+      config,
+      viewer_handlers=[viewer_app.ViewerApp(), BodyInspector()],
   ) as handle:
     handle.send_to_viewer(messages.ModelEvent(model=model))