Update: This reply was initially written based mostly on Version 0.51 of the physics system. The documentation of model 1.0 of the physics system has a chapter on processing collision detection occasions that exhibits an answer making use of a few of the new idioms. That article may be extra useful than this reply.

You can do that with a JobComponentSystem which schedules a job implementing ICollisionEventsJob*. You additionally have to set that job as a dependency for the FinishSimulationEntityCommandBufferSystem utilizing AddJobHandleForProducer. Here is an instance script:

utilizing Unity.Burst;
utilizing Unity.Entities;
utilizing Unity.Jobs;
utilizing Unity.Physics;
utilizing Unity.Physics.Systems;
utilizing UnityEngine;

public class CollisionSystem : JobComponentSystem {

    [BurstCompile]
    personal struct CollisionJob : ICollisionEventsJob {
        public void Execute(CollisionOccasion collisionEvent) {
            Debug.Log($"Collision between entities { collisionEvent.EntityA.Index } and { collisionEvent.EntityB.Index }");
        }
    }

    personal ConstructPhysicsWorld constructPhysicsWorldSystem;
    personal StepPhysicsWorld stepPhysicsWorldSystem;
    personal FinishSimulationEntityCommandBufferSystem commandBufferSystem;

    protected override void OnCreate() {
        base.OnCreate();
        constructPhysicsWorldSystem = World.GetExistingSystem<ConstructPhysicsWorld>();
        stepPhysicsWorldSystem = World.GetExistingSystem<StepPhysicsWorld>();
        commandBufferSystem = World.GetExistingSystem<FinishSimulationEntityCommandBufferSystem>();
    }

    protected override JobHandle OnUpdate(JobHandle inputDeps) {    
        JobHandle jobHandle = new CollisionJob().Schedule(
            stepPhysicsWorldSystem.Simulation, 
            ref constructPhysicsWorldSystem.PhysicsWorld, 
            inputDeps);    
        commandBufferSystem.AddJobHandleForProducer(jobHandle);    
        return jobHandle;    
    }
}

Note that the physics system will often generate a number of collision occasions on what seems to be only a single collision to the participant. There is some debate about whether or not that is a bug or a characteristic.

_{* Yes, I’m conscious how ineffective this documentation article is true now. Hopefully it’ll get extra helpful in future variations of the documentation.}