[cloud_firestore] Make Futures work in offline-mode

[oodavid]

The docs and examples recommend making use of Futures, but they remain uncompleted when offline. This will be unexpected for most users.

Offline behaviour is described as using a local database, with server sync happening in the background:

Firestore provides out of the box support for offline capabilities. When reading and writing data, Firestore uses a local database which automatically synchronizes with the server. Cloud Firestore functionality continues when users are offline, and automatically handles data migration when they regain connectivity.

Source: https://firebase.flutter.dev/docs/firestore/usage#access-data-offline

I know this is described as intended behaviour, however it makes using the Futures unusable for apps that use offline behaviour with write operations.

Proposal

Allow us to decide how Futures are completed. On a global, or per-query basis. The Source enum springs to mind:

// Default behaviour, the future completes after being written to the server
docRef.set(
  data,
  SetOptions(completionFrom: Source.server),
); 

// The future completes after being written to the cache
docRef.set(
  data,
  SetOptions(completionFrom: Source.cache),
);

// The future completes after being written to the server AND cache
docRef.set(
  data,
  SetOptions(completionFrom: Source.serverAndCache),
);

That way we can cater for existing behaviour with defaults, and also improve the offline functionality.

Video

I originally thought this was a bug. It's since change to a feature request. Here's my original:

https://www.loom.com/share/30f4f0fa6d184ebe8df1751c526b2f4e

[pbouttier]

Hello @oodavid ,

I've encountered the same problem recently, and I haven't found a solution, either by searching here or on the android sdk directly.
Have you found a solution or, failing that, a good “safe” implementation?

Thanks