# Collections

Give each collection row its own reactive value, so an edit updates the readers of that row.
Use a lookup for arbitrary requested keys and a selector for selection membership.

- [Projections](projections.fsx): keyed and index rows, factory lifetimes, pending rows and UI lists.
- [Lookups](lookups.fsx): one derived value per requested key.
- [Selectors](selectors.fsx): selection membership that wakes only the two ends of a change.
- [Deep and keyed updates](collection-updates.fsx): immutable updates, focused reads and editable keyed collections.
- [Collection views](collection-views.fsx): filtering, mapping, sorting, grouping and paging.
- [Aggregates](aggregates.fsx): incremental totals, counts and folds over rows.

Start with [Signals](signals.fsx) and [Memos](memos.md). For loading and errors, see
[Async and pending](async-and-pending.md).

Use `createKeyedCollection keyOf` when updates arrive one item at a time. Its `Rows` property
feeds the same views and aggregates as a projection over an immutable collection. Use
`NewKeyReader()` for membership and order, or `NewValueReader()` for settled row changes too;
see [Reading changes](projections.fsx#reading-changes).

For renderers that create one view object per key, projections also offer create-first teardown
ordering and `createProjectionOnce`, which stores a factory result without a row memo. The latter
supports key readers but does not support value readers or collection view operators; see
[Store a factory result once per key](projections.fsx#store-a-factory-result-once-per-key).