RFC: Support next/link prefetched data in next-urql

[yoshiwarab]

Summary

The Link component from next/link has prefetch turned on by default. For pages using static generation, this includes preloaded JSON data. Currently pages wrapped by withUrql ignore this data on client side transitions. I propose that support is added to take advantage of the prefetched SSG data.

Proposed Solution

Currently if an ssr exchange already exists, withUrql ignores pageProps.urqlState so it doesn't get the prefetched data on a client side transition. By updating this https://github.com/FormidableLabs/urql/blob/b6890ba1b0c9fa9e99301338555ea10d602cee22/packages/next-urql/src/with-urql-client.ts#L55-L58 to restore data to an existing ssr exchange it would be possible to hydrate the client with the prefetched data.
} else if (ssr) { ssr.restoreData(urqlState) }

Also, thank you for the amazing library :)! Hopefully this wouldn't break something else!

[kitten]

I think we'll need to test this; Personally, I don't have a huge overview of next-urql but @JoviDeCroock may know whether this will "just work". I don't think we've ever tested the prefetch behaviour, so once we see how it works we can probably test it properly. But in theory this does check out for me 👍