Function leptos::create_selector

pub fn create_selector<T>(
    source: impl Fn() -> T + Clone + 'static
) -> Selector<T>
where
    T: PartialEq + Eq + Clone + Hash + 'static,

Creates a conditional signal that only notifies subscribers when a change in the source signal’s value changes whether it is equal to the key value (as determined by PartialEq.)

You probably don’t need this, but it can be a very useful optimization in certain situations (e.g., “set the class selected if selected() == this_row_index) because it reduces them from O(n) to O(1).

let (a, set_a) = create_signal(0);
let is_selected = create_selector(move || a.get());
let total_notifications = Rc::new(RefCell::new(0));
let not = Rc::clone(&total_notifications);
create_isomorphic_effect({
    let is_selected = is_selected.clone();
    move |_| {
        if is_selected.selected(5) {
            *not.borrow_mut() += 1;
        }
    }
});

assert_eq!(is_selected.selected(5), false);
assert_eq!(*total_notifications.borrow(), 0);
set_a.set(5);
assert_eq!(is_selected.selected(5), true);
assert_eq!(*total_notifications.borrow(), 1);
set_a.set(5);
assert_eq!(is_selected.selected(5), true);
assert_eq!(*total_notifications.borrow(), 1);
set_a.set(4);
assert_eq!(is_selected.selected(5), false);