reflex/docs/vars/computed_vars.md

import random
import time

import reflex as rx

Computed Vars

Computed vars have values derived from other properties on the backend. They are defined as methods in your State class with the @rx.var decorator. A computed var is recomputed whenever an event is processed against the state.

Try typing in the input box and clicking out.

class UppercaseState(rx.State):
    text: str = "hello"

    @rx.var
    def upper_text(self) -> str:
        # This will be recomputed whenever `text` changes.
        return self.text.upper()


def uppercase_example():
    return rx.vstack(
        rx.heading(UppercaseState.upper_text),
        rx.chakra.input(on_blur=UppercaseState.set_text, placeholder="Type here..."),
    )

Here, upper_text is a computed var that always holds the upper case version of text.

We recommend always using type annotations for computed vars.

Cached Vars

A cached var, decorated as @rx.cached_var is a special type of computed var that is only recomputed when the other state vars it depends on change. This is useful for expensive computations, but in some cases it may not update when you expect it to.

class CachedVarState(rx.State):
    counter_a: int = 0
    counter_b: int = 0

    @rx.var
    def last_touch_time(self) -> str:
        # This is updated anytime the state is updated.
        return time.strftime("%H:%M:%S")

    def increment_a(self):
        self.counter_a += 1

    @rx.cached_var
    def last_counter_a_update(self) -> str:
        # This is updated only when `counter_a` changes.
        return f"{self.counter_a} at {time.strftime('%H:%M:%S')}"

    def increment_b(self):
        self.counter_b += 1

    @rx.cached_var
    def last_counter_b_update(self) -> str:
        # This is updated only when `counter_b` changes.
        return f"{self.counter_b} at {time.strftime('%H:%M:%S')}"


def cached_var_example():
    return rx.vstack(
        rx.text(f"State touched at: {CachedVarState.last_touch_time}"),
        rx.text(f"Counter A: {CachedVarState.last_counter_a_update}"),
        rx.text(f"Counter B: {CachedVarState.last_counter_b_update}"),
        rx.hstack(
            rx.button("Increment A", on_click=CachedVarState.increment_a),
            rx.button("Increment B", on_click=CachedVarState.increment_b),
        ),
    )

In this example last_touch_time is a normal computed var, which updates any time the state is modified. last_counter_a_update is a computed var that only depends on counter_a, so it only gets recomputed when counter_a has changes. Similarly last_counter_b_update only depends on counter_b, and thus is updated only when counter_b changes.