ADR-0008 · accepted

An event is a rare, meaningful fact

The simulation pushes events into an EventBuffer during a step, and the runtime drains it after the frame's steps (and after start and modify) and passes each event to the engine's listeners, such as the UI. An event is a rare, meaningful fact, such as "level solved" or "leveled up"; events never carry per-entity data, and the HUD gets the state through samples (CONTEXT.md, the README's rules of thumb). Each event is a new object made during a step and then a message in the UI, so events stay cheap only while they are rare.

Consequences

• Survivors reports levelUp and runEnded, and push-puzzle reports levelSolved. Hits, shots, kills and pickups are not events: with about 750 enemies they can happen hundreds of times a second.
• A frame with no events allocates nothing for them, since EventBuffer.drain returns a shared empty array.
• A turn-based HUD also goes through a sample, timed by a version (ADR-0005).
• #24 decided how sounds are triggered against this rule: not by events, but by cue counters in the state that the presentation reads (ADR-0014).