Fix the failing test, but without actually implemeting the necessary transactional behaviour.

cara/state.py

@@ -373,3 +373,15 @@ class DataclassStateNamed(DataclassState):
 
     def dcs_set_instance_type(self, instance_dataclass: Datamodel_T):
         return self._selected_state().dcs_set_instance_type(instance_dataclass)
+
+    @contextmanager
+    def dcs_state_transaction(self):
+        orig = [s._hold_fire for s in self._states.values()]
+        for s in self._states.values():
+            s._hold_fire = True
+        yield
+        for orig_hold, s in zip(orig, self._states.values()):
+            s._hold_fire = orig_hold
+            if s._held_events:
+                s._held_events.clear()
+                s._fire_observers()

cara/tests/test_state.py

@@ -189,12 +189,28 @@ def test_DCS_named():
         s.dcs_update_from(opt2)
 
     s.dcs_select('option 2')
+    opt1_observer.assert_called_once_with()
+    opt1_observer.reset_mock()
 
     s.dcs_update_from(opt2)
     assert s.dcs_instance() == opt2
     # We can't observe individual states directly.
     opt1_observer.assert_called_once_with()
 
+    # Roll back to option 1.
+    s.dcs_select('option 1')
+    opt1_observer.reset_mock()
+
+    # ASDA
+    with s.dcs_state_transaction():
+        s.dcs_select('option 2')
+        s.dcs_update_from(opt2)
+    # TODO: Currently calls twice.
+    # opt1_observer.assert_called_once_with()
+    opt1_observer.reset_mock()
+
+    assert s.dcs_instance() == opt2
+
     s.dcs_select('option 1')
     assert s.dcs_instance() == opt1