Create a @serve decorator to let developers configure routes manually.

mesop/features/__init__.py

@@ -1 +1,2 @@
 from .page import page as page
+from .serve import server as server

mesop/features/serve.py

@@ -0,0 +1,17 @@
+from typing import Callable
+
+from mesop.runtime import runtime
+
+
+def serve(
+  *,
+  rule: str,
+) -> Callable[[Callable[[], None]], Callable[[], None]]:
+  def decorator(func: Callable[[], None]) -> Callable[[], None]:
+    runtime().register_handler(
+      rule=rule,
+      handler=func,
+    )
+    return func
+
+  return decorator

mesop/runtime/runtime.py

@@ -1,6 +1,6 @@
 from copy import deepcopy
 from dataclasses import dataclass
-from typing import Any, Callable, Generator, Type, TypeVar, cast
+from typing import Any, Callable, Dict, Generator, Tuple, Type, TypeVar, cast
 
 from flask import g, request
 
@@ -49,6 +49,9 @@ def __init__(self):
     # clients polling whether to request a hot reload.
     self.hot_reload_counter = 0
     self._path_to_page_config: dict[str, PageConfig] = {}
+    self._rule_to_handler: dict[
+      str, Callable[[], Tuple[str, int, Dict[str, str]]]
+    ] = {}
     self.component_fns: set[Callable[..., Any]] = set()
     self.event_mappers: dict[Type[Any], Callable[[pb.UserEvent, Key], Any]] = {}
     self._state_classes: list[type[Any]] = []
@@ -131,9 +134,27 @@ def register_page(self, *, path: str, page_config: PageConfig) -> None:
       )
     self._path_to_page_config[path] = page_config
 
+  def register_handler(
+    self, *, rule: str, handler: Callable[[], Tuple[str, int, Dict[str, str]]]
+  ):
+    if self._has_served_traffic:
+      raise MesopDeveloperException(
+        "Cannot register a handler after traffic has been served. You must register all handlers upon server startup before any traffic has been served. This prevents security issues."
+      )
+    if rule in self._rule_to_handler:
+      raise MesopDeveloperException(
+        f"Handler for rule '{rule}' already registered."
+      )
+    self._rule_to_handler[rule] = handler
+
   def get_page_config(self, *, path: str) -> PageConfig | None:
     return self._path_to_page_config.get(path)
 
+  def get_rule_to_handlers(
+    self,
+  ) -> dict[str, Callable[[], Tuple[str, int, Dict[str, str]]]]:
+    return deepcopy(self._rule_to_handler)
+
   def get_path_to_page_configs(self) -> dict[str, PageConfig]:
     return deepcopy(self._path_to_page_config)
 

mesop/server/static_file_serving.py

@@ -46,6 +46,10 @@ def configure_static_file_serving(
   disable_gzip_cache: bool = False,
   default_allowed_iframe_parents: str = "'self'",
 ):
+  # Register @me.serve handlers.
+  for rule, handler_func in runtime().get_rule_to_handlers().items():
+    app.add_url_rule(rule, view_func=handler_func)
+
   def get_path(path: str):
     safe_path = safe_join(static_file_runfiles_base, path)
     assert safe_path