BallMovementSystem.cs

using Tutorials.Kickball.Execute;
using Tutorials.Kickball.Step1;
using Tutorials.Kickball.Step2;
using Unity.Burst;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;

namespace Tutorials.Kickball.Step3
{
    [UpdateBefore(typeof(TransformSystemGroup))]
    public partial struct BallMovementSystem : ISystem
    {
        [BurstCompile]
        public void OnCreate(ref SystemState state)
        {
            state.RequireForUpdate<BallMovement>();
            state.RequireForUpdate<Config>();
        }

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

            // The world or its SystemGroups may sometimes override the current time or delta time,
            // for various reasons, so you should use SystemAPI.Time instead of UnityEngine.Time.
            var dt = SystemAPI.Time.DeltaTime;

            var decayFactor = config.BallVelocityDecay * dt;
            var minDist = config.ObstacleRadius + 0.5f; // the ball radius is 0.5f
            var minDistSQ = minDist * minDist;

            // For every ball entity, we need to read and modify its LocalTransform and Velocity.
            foreach (var (ballTransform, velocity) in
                     SystemAPI.Query<RefRW<LocalTransform>, RefRW<Velocity>>()
                         .WithAll<Ball>()
                         .WithDisabled<Carry>())  // Relevant in Step 5
            {
                // Skip the ball if it isn't moving!
                if (velocity.ValueRO.Value.Equals(float2.zero))
                {
                    continue;
                }

                var magnitude = math.length(velocity.ValueRO.Value);
                var newPosition = ballTransform.ValueRW.Position +
                             new float3(velocity.ValueRO.Value.x, 0, velocity.ValueRO.Value.y) * dt;

                // Check if the ball intersects an obstacle. If so, reflect the ball's velocity vector.
                foreach (var obstacleTransform in
                         SystemAPI.Query<RefRO<LocalTransform>>()
                             .WithAll<Obstacle>())
                {
                    if (math.distancesq(newPosition, obstacleTransform.ValueRO.Position) <= minDistSQ)
                    {
                        newPosition = DeflectBall(ballTransform.ValueRO.Position, obstacleTransform.ValueRO.Position,
                            velocity, magnitude, dt);

                        // As long as the obstacles are spaced apart, it's impossible
                        // for one ball to hit two obstacles in a single frame, so we can
                        // break after detecting collision with one obstacle.
                        break;
                    }
                }

                ballTransform.ValueRW.Position = newPosition;

                // Velocity decay.
                var newMagnitude = math.max(magnitude - decayFactor, 0);
                velocity.ValueRW.Value = math.normalizesafe(velocity.ValueRO.Value) * newMagnitude;
            }
        }

        private float3 DeflectBall(float3 ballPos, float3 obstaclePos, RefRW<Velocity> velocity, float magnitude, float dt)
        {
            var obstacleToBallVector = math.normalize((ballPos - obstaclePos).xz);
            velocity.ValueRW.Value = math.reflect(math.normalize(velocity.ValueRO.Value), obstacleToBallVector) * magnitude;
            return ballPos + new float3(velocity.ValueRO.Value.x, 0, velocity.ValueRO.Value.y) * dt;
        }
    }
}