fix: timeline service uninitialised across routes

[shenlong-tanwen]

Description

Using a ProviderScope to override the timelineService for a particular sub-tree works for most cases except when we need access to the overridden service in other sub-trees like from the parent / across routes. It is addressed by making the service always initialised with the main timeline segments.

Whenever a new route with a timeline is pushed, we've to now update the global service provider to use the buckets for the new route. The new route will revert the provider to the main timeline segments before being disposed

[mertalev]

Is there a particular reason these create a new service every time?

Also, the assetSource and bucketSource seem odd to me. Shouldn't the service be the one interacting with the repositories?

[shenlong-tanwen]

Is there a particular reason these create a new service every time?

Yes, we use an internal buffer inside the service to preload assets when being scrolled. So when the timeline changes, we drop the older service and create a new one, essentially resetting everything.

Also, the assetSource and bucketSource seem odd to me. Shouldn't the service be the one interacting with the repositories?

The service is still the one that interacts with the repositories, yes. But since each timeline requires a different number of parameters, I've handled it by abstracting the service to rely on the following two methods instead:

typedef TimelineAssetSource = Future<List<BaseAsset>> Function(
  int index,
  int count,
);

typedef TimelineBucketSource = Stream<List<Bucket>> Function();

Now, based on the timeline being viewed, we pass in a closure to the service that resolves the other additional arguments.

[mertalev]

Perhaps a broader discussion, but are we sure that the buffer is actually beneficial? SQLite already caches pages in-memory (relevant pragma), unlike Isar which only relies on OS-level paging. If we need to reset this buffer or store multiple buffers in memory (doing power of two allocations for the lists each time), then scan these buffers in Dart, etc., it makes me wonder if this is actually better than what SQLite is doing.