feat: trigger a calculated property on mutation

* feat: subscribe_mutation supports an event handler as a function
* feat: subscribe_mutation supports an async event handler as a function

docs/framework/event-handlers.mdx

@@ -141,7 +141,8 @@ def hande_click_cleaner(state):
 You can subscribe to mutations on a specific key in the state.
 This is useful when you want to trigger a function every time a specific key is mutated.
 
-```python
+<CodeGroup>
+```python simple subscription
 import writer as wf
 
 def _increment_counter(state):
@@ -153,10 +154,44 @@ state.subscribe_mutation('a', _increment_counter)
 state['a'] = 2  # trigger _increment_counter mutation
 ```
 
-```python
-state.subscribe_mutation('a.b', _increment_counter)  # subscribe to nested key
+```python multiple subscriptions
+import writer as wf
+
+def _increment_counter(state):
+    state['my_counter'] += 1
+
+state = wf.init_state({
+	'title': 'Hello',
+	'app': {'title', 'Writer Framework'},
+	'my_counter': 0}
+)
+
 state.subscribe_mutation(['title', 'app.title'], _increment_counter)  # subscribe to multiple keys
+
+state['title'] = "Hello Pigeon"  # trigger _increment_counter mutation
+```
+
+```python trigger event handler
+import writer as wf
+
+def _increment_counter(state, context: dict, payload: dict, session: dict, ui: WriterUIManager):
+	if context['event'] == 'mutation' and context['mutation'] == 'a':
+		if payload['previous_value'] > payload['new_value']:
+			state['my_counter'] += 1
+
+state = wf.init_state({"a": 1, "my_counter": 0})
+state.subscribe_mutation('a', _increment_counter)
+
+state['a'] = 2  # increment my_counter
+state['a'] = 3  # increment my_counter
+state['a'] = 2  # do nothing
 ```
+</CodeGroup>
+
+<Tip>
+`subscribe_mutation` is compatible with event handler signature. It will accept all the arguments
+of the event handler (`context`, `payload`, ...).
+</Tip>
 
 ## Receiving a payload
 

src/writer/app_runner.py

@@ -19,7 +19,7 @@
 from watchdog.observers.polling import PollingObserver
 
 from writer import VERSION
-from writer.core import EventHandlerRegistry, MiddlewareRegistry, WriterSession
+from writer.core import EventHandlerRegistry, MiddlewareRegistry, WriterSession, use_request_context
 from writer.core_ui import ingest_bmc_component_tree
 from writer.ss_types import (
     AppProcessServerRequest,
@@ -232,71 +232,72 @@ def _handle_message(self, session_id: str, request: AppProcessServerRequest) ->
         """
         import writer
 
-        session = None
-        type = request.type
-
-        if type == "sessionInit":
-            si_req_payload = InitSessionRequestPayload.parse_obj(
-                request.payload)
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=self._handle_session_init(si_req_payload)
-            )
-
-        session = writer.session_manager.get_session(session_id)
-        if not session:
-            raise MessageHandlingException("Session not found.")
-        session.update_last_active_timestamp()
-
-        if type == "checkSession":
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=None
-            )
-
-        if type == "event":
-            ev_req_payload = WriterEvent.parse_obj(request.payload)
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=self._handle_event(session, ev_req_payload)
-            )
-
-        if type == "stateEnquiry":
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=self._handle_state_enquiry(session)
-            )
-
-        if type == "stateContent":
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=self._handle_state_content(session)
-            )
-
-        if type == "setUserinfo":
-            session.userinfo = request.payload
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=None
-            )
-
-        if self.mode == "edit" and type == "componentUpdate":
-            cu_req_payload = ComponentUpdateRequestPayload.parse_obj(
-                request.payload)
-            self._handle_component_update(session, cu_req_payload)
-            return AppProcessServerResponse(
-                status="ok",
-                status_message=None,
-                payload=None
-            )
-
-        raise MessageHandlingException("Invalid event.")
+        with use_request_context(session_id, request):
+            session = None
+            type = request.type
+
+            if type == "sessionInit":
+                si_req_payload = InitSessionRequestPayload.parse_obj(
+                    request.payload)
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=self._handle_session_init(si_req_payload)
+                )
+
+            session = writer.session_manager.get_session(session_id)
+            if not session:
+                raise MessageHandlingException("Session not found.")
+            session.update_last_active_timestamp()
+
+            if type == "checkSession":
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=None
+                )
+
+            if type == "event":
+                ev_req_payload = WriterEvent.parse_obj(request.payload)
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=self._handle_event(session, ev_req_payload)
+                )
+
+            if type == "stateEnquiry":
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=self._handle_state_enquiry(session)
+                )
+
+            if type == "stateContent":
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=self._handle_state_content(session)
+                )
+
+            if type == "setUserinfo":
+                session.userinfo = request.payload
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=None
+                )
+
+            if self.mode == "edit" and type == "componentUpdate":
+                cu_req_payload = ComponentUpdateRequestPayload.parse_obj(
+                    request.payload)
+                self._handle_component_update(session, cu_req_payload)
+                return AppProcessServerResponse(
+                    status="ok",
+                    status_message=None,
+                    payload=None
+                )
+
+            raise MessageHandlingException("Invalid event.")
 
     def _execute_user_code(self) -> None:
         """