Provide an API to watch fragments

[jpvajda]

Relates to

apollographql/apollo-client#8236
apollographql/apollo-feature-requests#198

Details

This relates to our work in our web Client to offer a useFragment hook, and there has been some interest in this same level of functionality in iOS.

useFragment is a read-only reactive/live binding into the cache, providing an always-up-to-date view of whatever data the cache currently contains for a given fragment.

It makes sense to want to watch just a fragment but it's not possible in iOS at the moment; it looks like our watcher functions are restricted to queries only.

[jpvajda]

From a discussion I had with the requester of this feature:

There is sort of a work around where you can create a getFoo(uuid: ID!): Foo query then have the cache know how to lookup that entity even if it came in a list. But would be nice to have it as a first class concept and it seems like it could be possible given things are already effectively stored this way in the normalized cache.

[kroussevrb]

@jpvajda what do you mean "even if it came in a list". Doesn't cacheKeyForObject = { $0["id"] } suffice if the Foo id is included in both the list query and the getFoo query?

[cerupcat]

I'm curious how to achieve this. I don't quite get the Foo workaround @jpvajda.

I'm trying to observe changes to an array of objects that are modified from a different screen. The objects are a shared fragment.

Screen 1: Query that returns a list of objects
Screen 2: Can add or remove objects
Screen 1: Would like to receive updates when objects are added or removed from Screen 2.