Update: This answer was originally written based on Version 0.51 of the physics system. The documentation of version 1.0 of the physics system has a chapter on processing collision detection eventsa chapter on processing collision detection events that shows a solution applying some of the new idioms. That article might be more helpful than this answer.
You can do this with a JobComponentSystem which schedules a job implementing ICollisionEventsJob*. You also need to set that job as a dependency for the EndSimulationEntityCommandBufferSystem using AddJobHandleForProducer. Here is an example script:
using Unity.Burst;
using Unity.Entities;
using Unity.Jobs;
using Unity.Physics;
using Unity.Physics.Systems;
using UnityEngine;
public class CollisionSystem : JobComponentSystem {
[BurstCompile]
private struct CollisionJob : ICollisionEventsJob {
public void Execute(CollisionEvent collisionEvent) {
Debug.Log($"Collision between entities { collisionEvent.EntityA.Index } and { collisionEvent.EntityB.Index }");
}
}
private BuildPhysicsWorld buildPhysicsWorldSystem;
private StepPhysicsWorld stepPhysicsWorldSystem;
private EndSimulationEntityCommandBufferSystem commandBufferSystem;
protected override void OnCreate() {
base.OnCreate();
buildPhysicsWorldSystem = World.GetExistingSystem<BuildPhysicsWorld>();
stepPhysicsWorldSystem = World.GetExistingSystem<StepPhysicsWorld>();
commandBufferSystem = World.GetExistingSystem<EndSimulationEntityCommandBufferSystem>();
}
protected override JobHandle OnUpdate(JobHandle inputDeps) {
JobHandle jobHandle = new CollisionJob().Schedule(
stepPhysicsWorldSystem.Simulation,
ref buildPhysicsWorldSystem.PhysicsWorld,
inputDeps);
commandBufferSystem.AddJobHandleForProducer(jobHandle);
return jobHandle;
}
}
Note that the physics system will usually generate multiple collision events on what appears to be just a single collision to the player. There is some debate about whether that's a bug or a feature.
* Yes, I am aware how useless this documentation article is right now. Hopefully it will get more useful in future versions of the documentation.