During an MPA form submission, useFormState should only reuse the form
state if same action is passed both times. (We also compare the key
paths.)
We compare the identity of the inner closure function, disregarding the
value of the bound arguments. That way you can pass an inline Server
Action closure:
```js
function FormContainer({maxLength}) {
function submitAction(prevState, formData) {
'use server'
if (formData.get('field').length > maxLength) {
return { errorMsg: 'Too many characters' };
}
// ...
}
return <Form submitAction={submitAction} />
}
```
If a Server Action is passed to useFormState, the action may be
submitted before it has hydrated. This will trigger a full page
(MPA-style) navigation. We can transfer the form state to the next page
by comparing the key path of the hook instance.
`ReactServerDOMServer.decodeFormState` is used by the server to extract
the form state from the submitted action. This value can then be passed
as an option when rendering the new page. It must be passed during both
SSR and hydration.
```js
const boundAction = await decodeAction(formData, serverManifest);
const result = await boundAction();
const formState = decodeFormState(result, formData, serverManifest);
// SSR
const response = createFromReadableStream(<App />);
const ssrStream = await renderToReadableStream(response, { formState })
// Hydration
hydrateRoot(container, <App />, { formState });
```
If the `formState` option is omitted, then the state won't be
transferred to the next page. However, it must be passed in both places,
or in neither; misconfiguring will result in a hydration mismatch.
(The `formState` option is currently prefixed with `experimental_`)
This automatically exposes `$$FORM_ACTIONS` on Server References coming
from Flight. So that when they're used in a form action, we can encode
the ID for the server reference as a hidden field or as part of the name
of a button.
If the Server Action is a bound function it can have complex data
associated with it. In this case this additional data is encoded as
additional form fields.
To process a POST on the server there's now a `decodeAction` helper that
can take one of these progressive posts from FormData and give you a
function that is prebound with the correct closure and FormData so that
you can just invoke it.
I updated the fixture which now has a "Server State" that gets
automatically refreshed. This also lets us visualize form fields.
There's no "Action State" here for showing error messages that are not
thrown, that's still up to user space.
