feat: trigger a calculated property on mutation

* feat: implement fixture to isolate test of global context

tests/backend/fixtures/writer_fixtures.py

@@ -0,0 +1,40 @@
+import contextlib
+import copy
+
+from writer import WriterState, core, core_ui
+from writer.core import Config, SessionManager
+
+
+@contextlib.contextmanager
+def new_app_context():
+    """
+    Creates a new application context for testing, independent of the global state.
+
+    This fixture avoids conflicts between tests that use the same global state.
+    At the end of the context, the global state is restored to its original state.
+
+    >>> with writer_fixtures.new_app_context():
+    >>>     initial_state = wf.init_state({
+    >>>            "counter": 0,
+    >>>            "total": 0
+    >>>     }, schema=MyState)
+    """
+    saved_context_vars = {}
+    core_context_vars = ['initial_state', 'base_component_tree', 'session_manager']
+    core_config_vars = copy.deepcopy(core.Config)
+
+    for var in core_context_vars:
+        saved_context_vars[var] = getattr(core, var)
+
+    core.initial_state = WriterState()
+    core.base_component_tree = core_ui.build_base_component_tree()
+    core.session_manager = SessionManager()
+    Config.mode = "run"
+    Config.logger = None
+
+    yield
+
+    for var in core_context_vars:
+        setattr(core, var, saved_context_vars[var])
+
+    core.Config = core_config_vars

tests/backend/test_core.py

@@ -26,7 +26,7 @@
 from writer.core_ui import Component
 from writer.ss_types import WriterEvent
 
-from backend.fixtures import core_ui_fixtures
+from backend.fixtures import core_ui_fixtures, writer_fixtures
 from tests.backend import test_app_dir
 
 raw_state_dict = {
@@ -439,27 +439,28 @@ def test_subscribe_mutation_with_typed_state_should_manage_mutation(self):
         """
         Tests that a mutation handler is triggered on a typed state and can use attributes directly.
         """
-        # Assign
-        class MyState(wf.WriterState):
-            counter: int
-            total: int
+        with writer_fixtures.new_app_context():
+            # Assign
+            class MyState(wf.WriterState):
+                counter: int
+                total: int
 
-        def cumulative_sum(state: MyState):
-            state.total += state.counter
+            def cumulative_sum(state: MyState):
+                state.total += state.counter
 
-        initial_state = wf.init_state({
-            "counter": 0,
-            "total": 0
-        }, schema=MyState)
+            initial_state = wf.init_state({
+                "counter": 0,
+                "total": 0
+            }, schema=MyState)
 
-        initial_state.subscribe_mutation('counter', cumulative_sum)
+            initial_state.subscribe_mutation('counter', cumulative_sum)
 
-        # Acts
-        initial_state['counter'] = 1
-        initial_state['counter'] = 3
+            # Acts
+            initial_state['counter'] = 1
+            initial_state['counter'] = 3
 
-        # Assert
-        assert initial_state['total'] == 4
+            # Assert
+            assert initial_state['total'] == 4
 
 class TestWriterState: