Optimize Private Message loading via In-Memory Singleton Service

[Pinselohrkater]

• [x] 1. Create an singleton service that wraps the API call to get private messages for a user and caches the result in memory. Service should support:

  • [x] GetAll - to retrieve all cached messages.
  • [x] GetOne - to retrieve one cached message by id.
  • [x] If "GetOne" doesn't have the message in cache, it should transiently run an Update/Fetch and try again, before returning null worst case.
  • [x] Update/Fetch - make the API call to the backend and update the cache.
  • [x] Being observable, so that consumers can subscribe to the OnUpdate/OnFetch method when data (might) change.

• [x] 2. Hook up existing consumers to the service

  • [x] Home screen fragment ->GetAll
  • [x] Fragment subscribes to OnUpdate & refreshes accordingly
  • [x] PM list -> GetAll
  • [x] Optional (Fragment subscribes to OnUpdate & refreshes accordingly)
  • [x] PM detail -> GetOne

• [ ] 3. Set up cache population/invalidation. Update/Fetch should be called on:

  • [x] App start.
  • [x] "Refresh" via menu
  • [x] Receiving a FCM Notification for a private message
  • [ ] Implicit, if GetOne(id) asks for an id that's not present in the cache

[lukashaertel]

Currently at 1883ecf6c2e6c09c473ef38211aa0bf7c2e75207

[lukashaertel]

At 51ee60b5aafde808d968efbb71ea7b401e68e829, implemented the changes to update triggers