View

In one line: a query result you keep and refresh, that reports what changed each time.

Where a one-shot query is a snapshot answer, a view (tx.Query<Unit>()…ToView()) is a result set you hold: Refresh(tx) brings it up to date, GetDelta() returns the Added / Removed / Modified entity keys, and ClearDelta() resets for the next cycle. That delta is exactly what a reactive system or a UI needs.

A view built on an indexed WhereField predicate updates incrementally — the engine moves only the entities that crossed the boundary. A view on a free Where is a pull view, recomputed on Refresh. Same delta either way; the difference is cost. A view is also the input a QuerySystem runs over, and what a subscription publishes to clients.

How it relates

• Query — a view is a query made durable-over-time.
• Index — an indexed predicate makes the view incremental.
• System — a QuerySystem takes a view as its input set.
• Subscription — publishes a view's deltas to remote clients.

In the API

• EcsView<T> — Contains / Refresh / GetDelta / ClearDelta, and foreach.

Learn & use

• Narrative: Guide ch.4 §3 — live views
• Feature detail: reactive dispatch & change filters · published views