Simulation results

After completion of PhysicsSimulationGroup, the simulation step is complete. The results of the simulation are:

• New positions, rotations and velocities of dynamic bodies - stored inside a PhysicsWorld.
• Collision and trigger events - stored internally in Simulation streams.

After PhysicsSimulationGroup finishes, the final system of the PhysicsSystemGroup can update - ExportPhysicsWorld. Its role is copy the updated body positions, rotations and velocities from PhysicsWorld (physics simulation data) to ECS data.

Events

In addition to updated positions, rotations and velocities of bodies, the results of simulation also include events. Events are stored in streams stored inside the Simulation struct. You can access them directly from Simulation by calling (SystemBase|SystemAPI|EntityQuery).GetSingleton<SimulationSingleton>().AsSimulation().(CollisionEvents|TriggerEvents) and iterate through them. The other approach is to use specialised jobs. To schedule these jobs, you will need SimulationSingleton. Get it using (SystemBase|SystemAPI|EntityQuery).GetSingleton<SimulationSingleton>(). (See below)

Note: Events are valid after the PhysicsSimulationGroup has finished, and up until it starts in the next frame. Using them while PhysicsSimulationGroup is updating will lead to undefined behaviour.

Collision events

Collision events are raised from colliders that opted in to this behaviour. For each collision that involves those colliders, a collision event will be raised. To access them directly, see above. To access them through a specialised job, implement and schedule ICollisionEventsJob; see example below. See example below.

Example:

[UpdateInGroup(typeof(FixedStepSimulationSystemGroup))]
[UpdateBefore(typeof(PhysicsSimulationGroup))] // We are updating before `PhysicsSimulationGroup` - this means that we will get the events of the previous frame
public partial struct GetNumCollisionEventsSystem : ISystem
{
    [BurstCompile]
    public partial struct CountNumCollisionEvents : ICollisionEventsJob
    {
        public NativeReference<int> NumCollisionEvents;
        public void Execute(CollisionEvent collisionEvent)
        {
            NumCollisionEvents.Value++;
        }
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        NativeReference<int> numCollisionEvents = new NativeReference<int>(0, Allocator.TempJob);
        
        state.Dependency = new CountNumCollisionEvents
        {
            NumCollisionEvents = numCollisionEvents
        }.Schedule(SystemAPI.GetSingleton<SimulationSingleton>());

        // ...
    }
}

Trigger events

Trigger events are raised from colliders that opted in this behaviour. These colliders will not collide with anything, but will instead raise a trigger event once a collision should have happened. To access them directly, see above. To access them through a specialised job, implement and schedule ITriggerEventsJob. See example below.

Example:

[UpdateInGroup(typeof(FixedStepSimulationSystemGroup))]
[UpdateAfter(typeof(PhysicsSimulationGroup))] // We are updating after `PhysicsSimulationGroup` - this means that we will get the events of the current frame.
public partial struct GetNumTriggerEventsSystem : ISystem
{
    [BurstCompile]
    public partial struct CountNumTriggerEvents : ITriggerEventsJob
    {
        public NativeReference<int> NumTriggerEvents;
        public void Execute(TriggerEvent collisionEvent)
        {
            NumTriggerEvents.Value++;
        }
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        NativeReference<int> numTriggerEvents = new NativeReference<int>(0, Allocator.TempJob);
        
        state.Dependency = new CountNumTriggerEvents
        {
            NumTriggerEvents = numTriggerEvents
        }.Schedule(SystemAPI.GetSingleton<SimulationSingleton>());

        // ...
    }
}