Ability to derive a `Signal` while dropping some values

[srid]

Specific Demand

Analogous to map (#1537), I'd like a way to produce a new signal that contains only a subset of values set in the original signal, potentially mapped over to a new type, U. This analogous to Rust's filter_map semantics

cf. discussion at https://github.com/DioxusLabs/dioxus/pull/1537#issuecomment-1769205030

Implement Suggestion

fn signal_filter_map<T, U, F>(cx: Scope, sig: Signal<T>, initial: U, f: F) -> Signal<U>
where
    F: Fn(T) -> Option<U> + 'static,
    T: Copy,
{
    let res: Signal<U> = use_signal(cx, || initial);
    dioxus_signals::use_effect(cx, move || {
        let value = *sig.read();
        if let Some(value) = f(value) {
            res.set(value);
        }
    });
    res
}