Function leptos::watch

pub fn watch<W, T>(
    deps: impl Fn() -> W + 'static,
    callback: impl Fn(&W, Option<&W>, Option<T>) -> T + Clone + 'static,
    immediate: bool
) -> impl Fn() + Clone
where
    W: Clone + 'static,
    T: 'static,

A version of create_effect that listens to any dependency that is accessed inside deps and returns a stop handler.

The return value of deps is passed into callback as an argument together with the previous value. Additionally the last return value of callback is provided as a third argument as is done in create_effect.

Usage

let (num, set_num) = create_signal(0);

let stop = watch(
    move || num.get(),
    move |num, prev_num, _| {
        log::debug!("Number: {}; Prev: {:?}", num, prev_num);
    },
    false,
);

set_num.set(1); // > "Number: 1; Prev: Some(0)"

stop(); // stop watching

set_num.set(2); // (nothing happens)

The callback itself doesn’t track any signal that is accessed within it.

let (num, set_num) = create_signal(0);
let (cb_num, set_cb_num) = create_signal(0);

watch(
    move || num.get(),
    move |num, _, _| {
        log::debug!("Number: {}; Cb: {}", num, cb_num.get());
    },
    false,
);

set_num.set(1); // > "Number: 1; Cb: 0"

set_cb_num.set(1); // (nothing happens)

set_num.set(2); // > "Number: 2; Cb: 1"

Immediate

If the final parameter immediate is true, the callback will run immediately. If it’s false, the callback will run only after the first change is detected of any signal that is accessed in deps.

let (num, set_num) = create_signal(0);

watch(
    move || num.get(),
    move |num, prev_num, _| {
        log::debug!("Number: {}; Prev: {:?}", num, prev_num);
    },
    true,
); // > "Number: 0; Prev: None"

set_num.set(1); // > "Number: 1; Prev: Some(0)"