Compare variables based on their serialized value

[gregmartyn]

Apollo Client currently uses @wry/equality to determine whether variables has changed between invocations of useQuery. See: source 1, source 2, source 3 and maybe more.

That works fine for the specific classes that equality has special handling for (e.g. Date) and standard objects and primitives, but doesn't work for custom classes used by custom scalars. Unlike standard objects, equality compares instances of custom classes by reference, not by value.

equality doesn't have a special-case for the Temporal classes used by graphql-temporal, so useQuery(query, { variables: { aDate: Temporal.Instant.from('2020-01-01T00:00+00') } }) results in an infinite loop of identical requests, all with aDate=2020-01-01T00:00:00Z.

I propose that variables should be compared by JSON.stringify()'ing them instead. !equal(newOptions.variables, oldVariables) would become JSON.stringify(newOptions.variables) !== JSON.stringify(oldVariables). All variables have to be stringified to go over the wire anyway, so this should be compatible with all types. This should be backward compatible because any code relying on useQuery checking reference equality would have infinite-looped.

[jerelmiller]

Hey @gregmartyn 👋

Interesting idea! To be honest, I think in general we need a more holistic approach to custom scalars anyways (something there is a lot of interest in: #368). This might be a good test case for whatever solution we introduce for that feature to ensure we serialize variables before comparing them.

Thanks for bringing this case up! This is really good for us to be thinking about.