quadrotor example: add getter for geometry frame

bindings/pydrake/examples/quadrotor_py.cc

@@ -38,6 +38,8 @@ PYBIND11_MODULE(quadrotor, m) {
 
   py::class_<QuadrotorGeometry, LeafSystem<double>>(
       m, "QuadrotorGeometry", doc.QuadrotorGeometry.doc)
+      .def("get_frame_id", &QuadrotorGeometry::get_frame_id,
+          doc.QuadrotorGeometry.get_frame_id.doc)
       .def_static("AddToBuilder", &QuadrotorGeometry::AddToBuilder,
           py::arg("builder"), py::arg("quadrotor_state_port"),
           py::arg("scene_graph"), py::return_value_policy::reference,

bindings/pydrake/examples/test/quadrotor_test.py

@@ -25,4 +25,5 @@ def test_geometry(self):
         quadrotor = builder.AddSystem(QuadrotorPlant())
         scene_graph = builder.AddSystem(SceneGraph())
         state_port = quadrotor.get_output_port(0)
-        QuadrotorGeometry.AddToBuilder(builder, state_port, scene_graph)
+        geom = QuadrotorGeometry.AddToBuilder(builder, state_port, scene_graph)
+        self.assertTrue(geom.get_frame_id().is_valid())

examples/quadrotor/quadrotor_geometry.h

@@ -25,6 +25,10 @@ class QuadrotorGeometry final : public systems::LeafSystem<double> {
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(QuadrotorGeometry);
   ~QuadrotorGeometry() final;
 
+  /// Returns the frame of the geometry registered with a SceneGraph.  This can
+  /// be useful, e.g., if one would like to add a camera to the quadrotor.
+  geometry::FrameId get_frame_id() const { return frame_id_; }
+
   /// Creates, adds, and connects a QuadrotorGeometry system into the given
   /// `builder`.  Both the `quadrotor_state.get_system()` and `scene_graph`
   /// systems must have been added to the given `builder` already.

examples/quadrotor/test/quadrotor_geometry_test.cc

@@ -20,6 +20,8 @@ GTEST_TEST(QuadrotorGeometryTest, AcceptanceTest) {
       &builder, plant->get_output_port(0), scene_graph);
   auto diagram = builder.Build();
   ASSERT_NE(geom, nullptr);
+
+  EXPECT_TRUE(geom->get_frame_id().is_valid());
 }
 
 }  // namespace