Feed Patterns

Feed Patterns

Amethyst's "feed" abstraction is: a FeedFilter that decides which notes belong in a list, plus a FeedViewModel that exposes the current state reactively to the UI. Every scrollable list — home, profile, hashtag, bookmarks, notifications, DMs — is a variant of this.

When to Use This Skill

• Adding a new screen that shows a list of notes.
• Modifying an existing feed's filtering / ordering / inclusion rules.
• Investigating why a feed doesn't update after a mute/​follow/​bookmark change.
• Deciding whether to extend a ViewModel or write a new filter.
• Understanding the Android ⇄ Desktop sharing boundary for feeds.

Architecture

┌─────────────────────────────────────────────────────────────┐
│ commons/​.../​viewmodels/  (shared, KMP)                      │
│   FeedViewModel  ◄── ListChangeFeedViewModel                │
│                 ◄── ChatroomFeedViewModel                   │
│                 ◄── MarmotGroupFeedViewModel                │
│                                                             │
│   FeedContentState — the flow the UI collects               │
└─────────────────────────────────────────────────────────────┘
              ▲
              │ uses
              │
┌─────────────────────────────────────────────────────────────┐
│ amethyst/​.../​ui/​dal/  (Android; feeds defined per screen)   │
│   FeedFilter<T>   (abstract)                                │
│   AdditiveComplexFeedFilter<T, U>                           │
│   ChangesFlowFilter                                         │
│   FilterByListParams                                        │
│   DefaultFeedOrder                                          │
│                                                             │
│   Plus concrete feeds: HomeFeedFilter, HashtagFeedFilter,   │
│   BookmarkListFeedFilter, NotificationFeedFilter, …         │
└─────────────────────────────────────────────────────────────┘
              ▲
              │ reads
              │
┌─────────────────────────────────────────────────────────────┐
│ model/​LocalCache.kt + Account.<featureFlow>                 │
└─────────────────────────────────────────────────────────────┘

Key Files

Shared (commons)

commons/​src/​commonMain/​kotlin/​com/​vitorpamplona/​amethyst/​commons/​viewmodels/:

• FeedViewModel.kt — abstract class FeedViewModel(localFilter, cacheProvider). Holds a FeedContentState, subscribes to invalidation signals (from Account flows and LocalCacheFlow), re-runs the filter, and emits a new FeedState for the UI.
• ListChangeFeedViewModel.kt — specialization for feeds whose membership changes frequently (e.g. bookmarks).
• ChatroomFeedViewModel.kt — DM thread feed.
• MarmotGroupFeedViewModel.kt — NIP-29 / marmot group feed.
• LiveStreamTopZappersViewModel.kt, SearchBarState.kt, ChatNewMessageState.kt — narrower, non-feed states that share the plumbing.

Android DAL (the filters)

amethyst/​src/​main/​java/​com/​vitorpamplona/​amethyst/​ui/​dal/:

• FeedFilters.kt — abstract class FeedFilter<T>. Has feed(): List<T> (the sync query against LocalCache) and feedKey(): String (identity used to cache).
• AdditiveComplexFeedFilter.kt — abstract class AdditiveComplexFeedFilter<T, U> : FeedFilter<T>(). Adds incremental updates (the "additive" part): when a single new event arrives, the filter can decide whether to graft it onto the existing list without recomputing everything.
• ChangesFlowFilter.kt — wraps a filter with a coarse "Account state changed" signal so the ViewModel knows to re-query.
• FilterByListParams.kt — common parameters (author set, exclude muted, limit, since/​until) shared across many filters.
• DefaultFeedOrder.kt — standard sort (by createdAt desc, plus tiebreakers for stable paging).

Concrete filters (Home, Hashtag, Profile, Bookmark, Notifications, Communities, etc.) live in feature subfolders under amethyst/​.../​ui/​screen/​loggedIn/​*/ — each extends FeedFilter or AdditiveComplexFeedFilter.

Adding a New Feed

1. Define the filter. Extend AdditiveComplexFeedFilter<Note, Set<HexKey>> (or plain FeedFilter<Note> if additivity doesn't matter). Implement:
   • feedKey() — stable identity (e.g. hashtag name, account pubkey).
   • feed() — synchronous scan over LocalCache / Account state producing an ordered list.
   • limit() — pagination hint.
   • If using AdditiveComplexFeedFilter: applyFilter(collection: Set<Note>): Set<Note> and sort(collection: Set<Note>): List<Note>.
2. Pick or write a ViewModel. If the feed's membership shifts often (bookmarks, notifications), extend ListChangeFeedViewModel. Otherwise FeedViewModel.
3. Wire invalidation. The ViewModel must observe the right Account flows + LocalCacheFlow so it re-queries when state changes.
4. Render. In the composable, collect viewModel.feedState.feedContent and render with a LazyColumn { items(..., key = { it.id }) { NoteCompose(it) } }.
5. Subscribe to relays. Most feeds also need a Subscribable to fetch historical events. See the relay-client skill.

Filter Sharing (Android vs Desktop)

• FeedFilter and the concrete filters currently live in amethyst/​.../​ui/​dal/ — Android-only. Desktop has parallel filters in desktopApp/​.../​feeds/.
• ViewModels are in commons/​commonMain/ — shared. That's the boundary: filter is Android (could be extracted), ViewModel is shared.
• When porting a new feed, extract the filter to a KMP-friendly location only if both platforms need it.

Gotchas

• Never scan LocalCache from a composable. Always go through a FeedFilter + FeedViewModel, which does it on a background dispatcher and debounces invalidation.
• feedKey() is used as a cache key. Two different semantic feeds must produce different keys, otherwise their state cross-contaminates.
• Additive updates must stay consistent with the full recompute. If applyFilter accepts a note that feed() wouldn't include, UX drifts.
• Paging isn't free — use limit() and since/​until in FilterByListParams rather than trimming a giant scan.
• Notifications feed is special — it inspects Account.followListFlow and LocalCache deletions to hide muted/​deleted content; always run through FilterByListParams.exclude* paths rather than filtering post-hoc.

References

• references/​feed-filter-composition.md — step-by-step for adding a feed.
• references/​viewmodel-base-classes.md — inheritance graph for the FeedViewModel family.
• Complements: account-state (where the data lives), relay-client (how to subscribe), compose-expert (how to render).