User Content Feed Loader

[EricBAndrews]

This PR adds a feed loader for user content (mixed posts and comments, used in saved and profile).

For testing, you can use mlemgroup/mlem#1157, which implements the saved feed.

Due to the fact that the API does not provide a type-specific endpoint to load this content, we can't just use the parent/child loader setup; instead, I have implemented a similar architecture but using a much more lightweight "stream" in place of the child loaders. All of this is completely non-generic because as far as I know this is the only case where we have this API constraint, and for all other mixed feeds we can just use the parent/child architecture.

The basic loading concept works like this:

• UserContentStream stores a list of items, their loading thresholds, and their stream cursor. Interaction with them is done through three methods:
  • addItems: adds new items to the stream. The stream handles updating the loading thresholds.
  • nextItemSortVal: functions identically to the child feed loader; returns the sort value of the next item in the stream, but does not affect the cursor
  • consumeNextItem: returns the next item in the stream and updates the cursor
  • The UserContentStream also provides the convenience needsMoreItems bool, which is true when the stream believes there are more items to load but has reached the end of its items.
• UserContentFeedLoader maintains two UserContentStreams, one for posts and one for comments
• To load more items, the UserContentFeedLoader performs the same stream merge operation as the parent/child architecture. Unlike a parent tracker, however, at each iteration the UserContentFeedLoader checks whether either child needs more items and, if so, triggers a load.

To support single-type profile feeds, UserContentFeedLoader boasts some additional modifications from the standard feed loading architecture:

• loadIfThreshold supports checking a particular stream's thresholds. This is not currently used, but is designed to support single-item profile feeds.
• The streams themselves are private, since they are directly and intimately managed by the UserContentFeedLoader and have lots of thread-unsafe operations. Single-type feeds are therefore exposed through the posts and comments computed vars.
• To support refreshing without clearing on single-type feeds, two temporary streams are defined which supply the single-type streams while the main streams are reset. This is required because loading a fresh stream involves first clearing the children, then loading new items into them, then finally merging those items together into the parent stream.

This PR also makes a few minor changes:

• MiddlewareConstants.infiniteLoadThresholdOffset is now 10 instead of -10. Arithmetic has been updated accordingly.
• getContent now returns a tuple of (posts: [Post2], comments: [Comment2]) instead of a Person3, and takes parameters
• FeedLoaderSortVal.published has been renamed to .new. This will let us support .old in the future.
• FeedLoader thresholds are now handled using the new Thresholds struct.

[Sjmarf]

Nice! This is a clean solution 💯

[Sjmarf]

as far as I know this is the only case where we have this API constraint

There's also the Modlog

[EricBAndrews]

as far as I know this is the only case where we have this API constraint

There's also the Modlog

Modlog is all a single endpoint, but you can specify the specific type to fetch so we can paginate independently--this is the only endpoint that forces you to get everything

[Sjmarf]

LGTM!