CannonBallSystem.cs

using Tutorials.Tanks.Step4;
using Unity.Burst;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;

namespace Tutorials.Tanks.Step5
{
    [BurstCompile]
    partial struct CannonBallSystem : ISystem
    {
        [BurstCompile]
        public void OnCreate(ref SystemState state)
        {
            state.RequireForUpdate<Execute.CannonBall>();
        }

        [BurstCompile]
        public void OnUpdate(ref SystemState state)
        {
            var ecbSingleton = SystemAPI.GetSingleton<EndSimulationEntityCommandBufferSystem.Singleton>();

            var cannonBallJob = new CannonBallJob
            {
                ECB = ecbSingleton.CreateCommandBuffer(state.WorldUnmanaged),
                DeltaTime = SystemAPI.Time.DeltaTime
            };

            cannonBallJob.Schedule();
        }
    }

    // IJobEntity relies on source generation to implicitly define a query from the signature of the Execute function.
    [BurstCompile]
    public partial struct CannonBallJob : IJobEntity
    {
        public EntityCommandBuffer ECB;
        public float DeltaTime;

        void Execute(Entity entity, ref CannonBall cannonBall, ref LocalTransform transform)
        {
            var gravity = new float3(0.0f, -9.82f, 0.0f);
            var invertY = new float3(1.0f, -1.0f, 1.0f);

            transform.Position += cannonBall.Velocity * DeltaTime;

            // bounce on the ground
            if (transform.Position.y < 0.0f)
            {
                transform.Position *= invertY;
                cannonBall.Velocity *= invertY * 0.8f;
            }

            cannonBall.Velocity += gravity * DeltaTime;

            var speed = math.lengthsq(cannonBall.Velocity);
            if (speed < 0.1f)
            {
                ECB.DestroyEntity(entity);
            }
        }
    }
}