Variable: OnCollision2D

const OnCollision2D: Trait<{ onEnter: string; onExit: string; }>

Declarative collision/sensor reaction — the no-code path (B) on top of the Physics2DEvents manager (C). Put this on the SAME entity as a Collider2D (e.g. a Sensor Zone): when that collider begins/ends overlap with another, the physics system dispatches the named UIAction, passing the OTHER entity as ctx.target and { self, other, phase } in ctx.params.

REQUIRES a RigidBody2D on the same entity (or a rigidbody ancestor). A lone Collider2D with no body creates no Rapier collider and fires no events — for a static sensor zone use a RigidBody2D({ bodyType: 'static' }). (The physics system warns once about such an orphan collider.)

Works for both sensors (Collider2D.isSensor) and solid contacts. The dispatch is pipeline-safe (dispatchGameAction — never throws on a missing handler, and is inert unless the sim is running), so an unwired action name is a warning, not a frame-aborting crash. Leave a field empty to react only to the other phase.

For richer reactions (arbitrary game state, filtering, cross-entity logic) skip this trait and subscribe to the physics2DEvents manager directly in code.

Variable: OnCollision2D ​

Variable: OnCollision2D