Function leptos::create_slice

pub fn create_slice<T, O, S>(
    signal: RwSignal<T>,
    getter: impl Fn(&T) -> O + Copy + 'static,
    setter: impl Fn(&mut T, S) + Copy + 'static,
) -> (Signal<O>, SignalSetter<S>)
where
    O: PartialEq,

Derives a reactive slice of an RwSignal.

Slices have the same guarantees as Memos: they only emit their value when it has actually been changed.

Slices need a getter and a setter, and you must make sure that the setter and getter only touch their respective field and nothing else. They optimally should not have any side effects.

You can use slices whenever you want to react to only parts of a bigger signal. The prime example would be state management, where you want all state variables grouped together, but also need fine-grained signals for each or some of these variables. In the example below, setting an auth token will only trigger the token signal, but none of the other derived signals.

// some global state with independent fields
#[derive(Default, Clone, Debug)]
struct GlobalState {
    count: u32,
    name: String,
}

let state = create_rw_signal(GlobalState::default());

// `create_slice` lets us create a "lens" into the data
let (count, set_count) = create_slice(
    // we take a slice *from* `state`
    state,
    // our getter returns a "slice" of the data
    |state| state.count,
    // our setter describes how to mutate that slice, given a new value
    |state, n| state.count = n,
);

// this slice is completely independent of the `count` slice
// neither of them will cause the other to rerun
let (name, set_name) = create_slice(
    // we take a slice *from* `state`
    state,
    // our getter returns a "slice" of the data
    |state| state.name.clone(),
    // our setter describes how to mutate that slice, given a new value
    |state, n| state.name = n,
);

create_effect(move |_| {
    // note: in the browser, use leptos::log! instead
    println!("name is {}", name.get());
});
create_effect(move |_| {
    println!("count is {}", count.get());
});

// setting count only causes count to log, not name
set_count.set(42);

// setting name only causes name to log, not count
set_name.set("Bob".into());