system-example.cs

// system example

public class SystemManager : MonoBehaviour {

  [SerializeField] List<CoreSystem> _systems;

  // Start is called before the first frame update
  void Start() {
    foreach (var system in _systems) {
      system.Init();
    }
  }

  void FixedUpdate() {
    foreach (var system in _systems) {
      system.OnFixedUpdate();
    }
  }

  // public T? GetSystem<T>() where T : CoreSystem {
  //   return _systems.Find(system => system is T) as T;
  // }
}

[CreateAssetMenu(fileName = "PlayerSystem", menuName = "Systems/PlayerSystem", order = 1)]
public class PlayerSystem : CoreSystem {

  // ...

  List<PlayerController> _players = new List<PlayerController>();

  public override void Init() {
    SpawnPlayer();
  }

  public override void OnFixedUpdate() {
    foreach (var player in _players.ToArray()) {
      player.OnFixedUpdate();
    }
  }

  void SpawnPlayer() {
    // spawn player
    var player = Instantiate(_playerPrefab, _spawnPosition, Quaternion.identity);
    player.Init(_colorSystem.PaddleColor);
    _players.Add(player);
  }
}

public class PlayerController : MonoBehaviour {

  [SerializeField] List<Ability> _abilities;
  // ...

  public void Init(Color paddleColor) {
    _rigidbody = GetComponent<Rigidbody>();
    _renderer.material.SetColor("_Color", paddleColor);
  }

  public void OnFixedUpdate() {
    var movement = _moveSpeed * Time.fixedDeltaTime * _movementInput;
    foreach (var modifier in _abilities.OfType<IMovementModifier>()) {
      movement = modifier.ModifyMovement(movement);
    }
    var nextPosition = transform.position + movement;
    nextPosition.x = Mathf.Clamp(nextPosition.x, -_xMovementRange, _xMovementRange);
    // update movement
    _rigidbody.MovePosition(nextPosition);
  }

  public void OnMove(InputAction.CallbackContext context) {
    var moveInput = context.ReadValue<Vector2>().normalized;
    _movementInput = new Vector3(moveInput.x, 0, 0);
  }

}