pub fn watch<S>(
source: impl Fn() -> S + 'static,
cb: impl Fn(&S, Option<&S>) + 'static,
)where
S: Clone + 'static,Expand description
Derive-state without effect-write loops (§4.2): source is tracked; cb runs untracked
with (new, old). Does not fire for the initial value: a “load whatever is selected”
reaction has to run once by hand first, or use bind, which applies the initial value
and then every change.