Support for the third argument of useSyncExternalStore

[ssssota]

Describe the feature you'd love to see
A clear and concise description of what you'd love to see added to Preact.

By supporting getServerSnapshot, we improve compatibility with React, enabling detection of hydration steps like:
https://github.com/TanStack/router/blob/425626822255604efc9a0598c58baeebba5fb592/packages/react-router/src/ClientOnly.tsx#L44-L72

Additional context (optional)
Add any other context or screenshots about the feature request here.

• https://react.dev/reference/react/useSyncExternalStore#reference

[ssssota]

I understand that implementing this is difficult with only the existing options hooks.
If you are positive about supporting this, I am interested in implementing it.

I believe that the implementation itself is possible by adding a hook that can pass information during hydration to options and referencing it.
However, this would require moving useSyncExternalStore to preact/hooks module, which I suspect the you might not welcome.

[rschristian]

I believe that the implementation itself is possible by adding a hook that can pass information during hydration to options and referencing it.

We have flags that can be used to determine if the vnode is hydrating, see here for an example:

preact/src/render.js

Line 21 in 0999ff4

let isHydrating = vnode && vnode._flags & MODE_HYDRATE;

However, this would require moving useSyncExternalStore to preact/hooks module, which I suspect the you might not welcome.

Why would that be necessary? Maybe there's some minor duplication/overlap, but that'll likely be fine.

[ssssota]

@rschristian
Thank you for replying to my poor interpretation!

Currently, useSyncExternalStore is implemented in the preact/compat module. However, since we cannot access isHydratingor getHookState from the preact/compat module, I was thinking it would be necessary to move useSyncExternalStore to the preact/hooks module.

[rschristian]

However, since we cannot access isHydrating or getHookState from the preact/compat module

As linked above, isHydrating is derived from vnode flags... you absolutely can access it from anywhere, including your own apps.

Hook state might be a bit more work, but at the very least, it can be minimally replicated I believe. You don't need to directly access the existing impl necessarily.