Vars
/
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 every time an event is processed in your app.
Try typing in the input box and clicking out.
HELLO
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.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.
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.
State touched at: 21:54:22
Counter A: 0 at 21:54:22
Counter B: 0 at 21:54:22
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.
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