exception handler draft

reflex/__init__.py

@@ -132,6 +132,7 @@
         "EventChain",
         "background",
         "call_script",
+        "client_error",
         "clear_local_storage",
         "console_log",
         "download",

reflex/app.py

@@ -141,6 +141,12 @@ class App(Base):
     # The radix theme for the entire app
     theme: Optional[Component] = themes.theme(accent_color="blue")
 
+    # Custom exception handler
+    exception_handler: Optional[Callable] = None
+
+    # client-side exception handler on error
+    load_error_handler: Optional[Callable] = None
+
     def __init__(self, *args, **kwargs):
         """Initialize the app.
 
@@ -391,6 +397,10 @@ def add_page(
         | None = None,
         meta: list[dict[str, str]] = constants.DefaultPage.META_LIST,
         script_tags: list[Component] | None = None,
+        on_error: EventHandler
+        | EventSpec
+        | list[EventHandler | EventSpec]
+        | None = None,
     ):
         """Add a page to the app.
 
@@ -464,6 +474,11 @@ def add_page(
             if not isinstance(on_load, list):
                 on_load = [on_load]
             self.load_events[route] = on_load
+
+        if on_error:
+            self.load_error_handler = on_error
+
+
 
     def get_load_events(self, route: str) -> list[EventHandler | EventSpec]:
         """Get the load events for a route.

reflex/event.py

@@ -18,7 +18,7 @@
 
 from reflex import constants
 from reflex.base import Base
-from reflex.utils import console, format
+from reflex.utils import console, format, prerequisites
 from reflex.utils.types import ArgsSpec
 from reflex.vars import BaseVar, Var
 
@@ -666,13 +666,34 @@ def call_script(
         callback_kwargs = {
             "callback": f"({arg_name}) => queueEvents([{format.format_event(event_spec)}], {constants.CompileVars.SOCKET})"
         }
+
+    error_event_spec = client_error('client error')
+
+    wrapped_code = f"""
+    try {{
+        {javascript_code}
+    }} catch (e) {{
+        queueEvents([{format.format_event(error_event_spec)}], {constants.CompileVars.SOCKET})       
+    }}
+    """
+    return server_side(
+            "_call_script",
+            get_fn_signature(call_script),
+            javascript_code=wrapped_code,
+            **callback_kwargs,
+        )
+
+def client_error(error: Any) -> EventSpec:
     return server_side(
-        "_call_script",
-        get_fn_signature(call_script),
-        javascript_code=javascript_code,
-        **callback_kwargs,
+        "_client_error",
+        get_fn_signature(client_error),
+        error = error
     )
 
+def client_error_handler(): # just for test
+    app = getattr(prerequisites.get_app(), constants.CompileVars.APP) 
+    exception_handler = app.load_error_handler  
+    return exception_handler
 
 def get_event(state, event):
     """Get the event from the given state.

reflex/state.py

@@ -1192,7 +1192,12 @@ async def _process_event(
         # If an error occurs, throw a window alert.
         except Exception:
             error = traceback.format_exc()
-            print(error)
+            app = getattr(prerequisites.get_app(), constants.CompileVars.APP) 
+            exception_handler = app.exception_handler    
+            if callable(exception_handler):
+                exception_handler(error)
+            else:
+                print(error)
             yield state._as_state_update(
                 handler,
                 window_alert("An error occurred. See logs for details."),

tests/states/__init__.py

@@ -28,6 +28,9 @@ def go(self, c: int):
         Yields:
             After each increment.
         """
+        if c < 0:
+            raise ValueError("Test exception")
+
         for _ in range(c):
             self.value += 1
             yield

tests/test_app.py

@@ -1131,6 +1131,24 @@ async def test_process_events(mocker, token: str):
         await app.state_manager.close()
 
 
+@pytest.mark.asyncio
+async def test_custom_exception_handler():
+
+    def custom_exception_handler(exception):
+        print(f"custom exception handler called: {exception}")
+
+    app = App(state=GenState, exception_handler=custom_exception_handler)
+    event = Event(
+        token="t",
+        name="gen_state.go",
+        payload={"c": -1},
+    )
+
+    with pytest.raises(ValueError, match="Test exception"):
+        async for _update in app.process(event, "mock_sid", {}, "127.0.0.1"):
+            pass
+
+
 @pytest.mark.parametrize(
     ("state", "overlay_component", "exp_page_child"),
     [