Alex Rickabaugh
bf1c12cd93
Previously, navigating a `FieldTree` in signal forms involved reactive reads
of the value of the parent field(s), both directly and via `.childrenMap()`.
This meant that on _any_ change to the value of a field, reactive
notifications would trigger updates of computeds, reruns of effects, etc.
So for example, this effect would run on every change to the form:
```ts
const f = form(signal({data: 'abc', unrelated: 0}));
effect(() => {
// accessing f.data incurs a dependency on f().value() which changes
// on every change in the whole form
console.log(f.data().value());
});
```
This is deeply counterintuitive and troublesome when attempting to write
effect logic, and also results in `computed`s unnecessarily updating.
This change introduces the concept of a "reader" computed, which memoizes
the access of a field at a given key via the reactive graph. With this, the
same `f.data` access above now depends on the `data` reader in `f` only,
which is effectively a constant computed. As a result, the effect only
reruns on changes to `data`'s value, as intended.
PR Close #65802
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