implement _evaluate in state (#4018)

* implement _evaluate in state

* add warning

* use typing_extension

* add integration test

reflex/state.py

@@ -35,6 +35,7 @@ from typing import (
 
 import dill
 from sqlalchemy.orm import DeclarativeBase
+from typing_extensions import Self
 
 from reflex.config import get_config
 from reflex.vars.base import (
@@ -43,6 +44,7 @@ from reflex.vars.base import (
     Var,
     computed_var,
     dispatch,
+    get_unique_variable_name,
     is_computed_var,
 )
 
@@ -695,6 +697,36 @@ class BaseState(Base, ABC, extra=pydantic.Extra.allow):
             and hasattr(value, "__code__")
         )
 
+    @classmethod
+    def _evaluate(cls, f: Callable[[Self], Any]) -> Var:
+        """Evaluate a function to a ComputedVar. Experimental.
+
+        Args:
+            f: The function to evaluate.
+
+        Returns:
+            The ComputedVar.
+        """
+        console.warn(
+            "The _evaluate method is experimental and may be removed in future versions."
+        )
+        from reflex.components.base.fragment import fragment
+        from reflex.components.component import Component
+
+        unique_var_name = get_unique_variable_name()
+
+        @computed_var(_js_expr=unique_var_name, return_type=Component)
+        def computed_var_func(state: Self):
+            return fragment(f(state))
+
+        setattr(cls, unique_var_name, computed_var_func)
+        cls.computed_vars[unique_var_name] = computed_var_func
+        cls.vars[unique_var_name] = computed_var_func
+        cls._update_substate_inherited_vars({unique_var_name: computed_var_func})
+        cls._always_dirty_computed_vars.add(unique_var_name)
+
+        return getattr(cls, unique_var_name)
+
     @classmethod
     def _mixins(cls) -> List[Type]:
         """Get the mixin classes of the state.

reflex/vars/base.py

@@ -1559,8 +1559,9 @@ class ComputedVar(Var[RETURN_TYPE]):
         Raises:
             TypeError: If the computed var dependencies are not Var instances or var names.
         """
-        hints = get_type_hints(fget)
-        hint = hints.get("return", Any)
+        hint = kwargs.pop("return_type", None) or get_type_hints(fget).get(
+            "return", Any
+        )
 
         kwargs["_js_expr"] = kwargs.pop("_js_expr", fget.__name__)
         kwargs["_var_type"] = kwargs.pop("_var_type", hint)

tests/integration/test_dynamic_components.py

@@ -16,6 +16,8 @@ def DynamicComponents():
     import reflex as rx
 
     class DynamicComponentsState(rx.State):
+        value: int = 10
+
         button: rx.Component = rx.button(
             "Click me",
             custom_attrs={
@@ -52,11 +54,20 @@ def DynamicComponents():
 
     app = rx.App()
 
+    def factorial(n: int) -> int:
+        if n == 0:
+            return 1
+        return n * factorial(n - 1)
+
     @app.add_page
     def index():
         return rx.vstack(
             DynamicComponentsState.client_token_component,
             DynamicComponentsState.button,
+            rx.text(
+                DynamicComponentsState._evaluate(lambda state: factorial(state.value)),
+                id="factorial",
+            ),
         )
 
 
@@ -150,3 +161,7 @@ def test_dynamic_components(driver, dynamic_components: AppHarness):
         dynamic_components.poll_for_content(button, exp_not_equal="Click me")
         == "Clicked"
     )
+
+    factorial = poll_for_result(lambda: driver.find_element(By.ID, "factorial"))
+    assert factorial
+    assert factorial.text == "3628800"