アーキテクチャ上どのレイヤでバリデーションするのか

import 'dart:math';

abstract class Document {
  void validate();
}

final _humans = <Human>[];

class HumanRepository {
  void save(Human human) {
    human.validate();
    _humans.add(human);
    print('save: $human');
  }

  Human find(int id) {
    return _humans.firstWhere((v) => v.id == id);
  }
}

class Human implements Document {
  final id = Random().nextInt(10000);
  final int hp;

  Human(this.hp) {
    validate();
  }

  @override
  String toString() => 'Human(hp: $hp)';

  @override
  void validate() {
    if (hp < 0) throw ArgumentError('invalid hp: $hp');
  }

  Human attacked(int damage) {
    assert(damage >= 0);
    final nextHp = max(0, hp - damage);
    return Human(nextHp);
  }
}

class Presentation {
  Human create(String hp) {
    final value = int.parse(hp);
    if (value < 0) {
      throw Exception('hp($hp) should be 0 or positive number');
    }
    return BusinessLogic().create(int.parse(hp));
  }

  Human attack(String id, String damage) {
    final damageValue = int.parse(damage);
    if (damageValue <= 0) {
      throw Exception('damage($damage) should be positive number');
    }
    final idValue = int.parse(id);
    return BusinessLogic().attack(idValue, int.parse(damage));
  }
}

class BusinessLogic {
  final repo = HumanRepository();
  Human create(int hp) {
    final human = Human(hp);
    repo.save(human);
    return human;
  }

  Human attack(int id, int damage) {
    final human = repo.find(id);
    return human.attacked(damage);
  }
}

void main(List<String> arguments) {
  final presentation = Presentation();
  final human = presentation.create(arguments[0]);
  final attackedHuman = presentation.attack(human.id.toString(), arguments[1]);
  print(attackedHuman);
}