Allows partial response from cache

[halberto-deezer]

Discussion around #57 that is closed. Having partial response from the cache is now possible, but seems complex since it requires to override the Fetch Policy Interceptors, possibly having to handle multiple fetch policies where cache is involved.

Today to allows partial response from cache, we would have to do 3 custom Fetch Policy Interceptors (CacheFirst, NetworkFirst, CacheAndNetwork) and not use the default Apollo ones. It's a bummer since any version update we would have to check for update to do on them to align with the ones from Apollo.

As a side note, from our tests, it might also be an issue with CacheOnly Fetch Policy as empty data could now be returned if nothing is found on the cache, and it could break with .execute() usage.

[BoD]

The decision to not return partial results with the built-in policies was due to the definition of a "cache hit" depends on the application. Some projects may consider at least one field present means a cache hit, others may consider that at least one miss means a miss.

With that in mind, it's hard to decide what the built-in policies should do with partial results (should they fetch the network or not?) so we do not make that decision in their context. That also preserves the behavior of prior versions of the cache - always a plus for people migrating.

That being said, I wouldn't worry too much about maintaining your own cache policy interceptors, or that the built-in ones will evolve. There are no plans for their behavior to change and at least I can't think of a reason why they would.