useMemo usage

[AjaxSolutions]

There is a lot of interesting code in React Query. This library is helpful and well done.

I'm curious why useMemo is used in the useHydrate hook as opposed to useEffect. What is the benefit?

useMemo is typically used to memoize the return value of a function, but the useMemo function is not returning any value.

React.useMemo(() => { if (state) { hydrate(queryClient, state, optionsRef.current) } }, [queryClient, state])

[TkDodo]

I'm not sure, but it really looks like we're executing a side-effect here (building the queryCache), which should probably not be done during render, because render should be a pure function... 🤔

/edit: to clarify: I think useEffect sounds right, but I'm not sure of the implications

[AjaxSolutions]

I think this should be changed to use useEffect, unless there is a very compelling reason to use useMemo.

The function used by useMemo should be a pure function, but in this case the function is not pure.

[TkDodo]

can you PR this and test if it still works? I have not used the hydration feature before... maybe this would result in one render-cycle where we have no cache yet (because effects run after render), triggering all the queries as if they were new (or so ...)

[AjaxSolutions]

Would it make more sense to use useLayoutEffect since useEffect didn't work?