use get_event_handler in upload, cherry-pick from #6100

reflex/app.py

@@ -77,13 +77,13 @@
 from reflex.event import (
     _EVENT_FIELDS,
     Event,
-    EventHandler,
     EventSpec,
     EventType,
     IndividualEventType,
     get_hydrate_event,
     noop,
 )
+from reflex.istate.proxy import StateProxy
 from reflex.page import DECORATED_PAGES
 from reflex.route import (
     get_route_args,
@@ -1619,6 +1619,8 @@ def _process_background(
         if not handler.is_background:
             return None
 
+        substate = StateProxy(substate)
+
         async def _coro():
             """Coroutine to process the event and emit updates inside an asyncio.Task.
 
@@ -1934,21 +1936,14 @@ async def upload_file(request: Request):
         substate_token = _substate_key(token, handler.rpartition(".")[0])
         state = await app.state_manager.get_state(substate_token)
 
-        # get the current session ID
-        # get the current state(parent state/substate)
-        path = handler.split(".")[:-1]
-        current_state = state.get_substate(path)
         handler_upload_param = ()
 
-        # get handler function
-        func = getattr(type(current_state), handler.split(".")[-1])
+        _current_state, event_handler = state._get_event_handler(handler)
 
-        # check if there exists any handler args with annotation, list[UploadFile]
-        if isinstance(func, EventHandler):
-            if func.is_background:
-                msg = f"@rx.event(background=True) is not supported for upload handler `{handler}`."
-                raise UploadTypeError(msg)
-            func = func.fn
+        if event_handler.is_background:
+            msg = f"@rx.event(background=True) is not supported for upload handler `{handler}`."
+            raise UploadTypeError(msg)
+        func = event_handler.fn
         if isinstance(func, functools.partial):
             func = func.func
         for k, v in get_type_hints(func).items():

reflex/state.py

@@ -1709,13 +1709,11 @@ async def get_var_value(self, var: Var[VAR_TYPE]) -> VAR_TYPE:
         )
         return getattr(other_state, var_data.field_name)
 
-    def _get_event_handler(
-        self, event: Event
-    ) -> tuple[BaseState | StateProxy, EventHandler]:
+    def _get_event_handler(self, event: Event | str) -> tuple[BaseState, EventHandler]:
         """Get the event handler for the given event.
 
         Args:
-            event: The event to get the handler for.
+            event: The event to get the handler for, or a dotted handler name string.
 
 
         Returns:
@@ -1725,18 +1723,15 @@ def _get_event_handler(
             ValueError: If the event handler or substate is not found.
         """
         # Get the event handler.
-        path = event.name.split(".")
+        name = event.name if isinstance(event, Event) else event
+        path = name.split(".")
         path, name = path[:-1], path[-1]
         substate = self.get_substate(path)
         if not substate:
             msg = "The value of state cannot be None when processing an event."
             raise ValueError(msg)
         handler = substate.event_handlers[name]
 
-        # For background tasks, proxy the state
-        if handler.is_background:
-            substate = StateProxy(substate)
-
         return substate, handler
 
     async def _process(self, event: Event) -> AsyncIterator[StateUpdate]:
@@ -1751,6 +1746,10 @@ async def _process(self, event: Event) -> AsyncIterator[StateUpdate]:
         # Get the event handler.
         substate, handler = self._get_event_handler(event)
 
+        # For background tasks, proxy the state.
+        if handler.is_background:
+            substate = StateProxy(substate)
+
         # Run the event generator and yield state updates.
         async for update in self._process_event(
             handler=handler,