外观.md

外观 Facade

重点等级：:star::star::star:

为子系统中的一组接口提供统一的接口。 Facade 定义了一个更高级别的接口，使得子系统更易于使用。

场景

• 你想为一个复杂的子系统提供一个简单的接口。 随着子系统的发展，子系统往往变得更加复杂。 大多数模式在应用时会导致类越来越多和越来越小。 这使得子系统更容易重用和定制，但是它也变得更难用于不需要定制它的客户端。 Facade 可以为子系统提供一个简单的默认视图，这对于大多数客户来说已经足够好了。 只有需要更多定制性的客户才需要看到立面之外的东西
• 客户端和抽象的实现类之间有许多依赖关系。 引入 facade 将子系统与客户端和其他子系统解耦，从而提高子系统的独立性和可移植性
• 你想把子系统分层。 使用 facade 定义每个子系统级别的入口点。 如果子系统是相互依赖的，那么您可以简化它们之间的依赖关系，方法是让它们仅仅通过外观进行相互通信

示例代码

/**
* 矿工挖矿示例
* 矿工抽象类
* 提供矿工日常工作及作息方法
*/
public abstract class DwarvenMineWorker {

  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenMineWorker.class);

  public void goToSleep() {
    LOGGER.info("{} goes to sleep.", name());
  }

  public void wakeUp() {
    LOGGER.info("{} wakes up.", name());
  }

  public void goHome() {
    LOGGER.info("{} goes home.", name());
  }

  public void goToMine() {
    LOGGER.info("{} goes to the mine.", name());
  }

  private void action(Action action) {
    switch (action) {
      case GO_TO_SLEEP:
        goToSleep();
        break;
      case WAKE_UP:
        wakeUp();
        break;
      case GO_HOME:
        goHome();
        break;
      case GO_TO_MINE:
        goToMine();
        break;
      case WORK:
        work();
        break;
      default:
        LOGGER.info("Undefined action");
        break;
    }
  }

  public void action(Action... actions) {
    Arrays.stream(actions).forEach(this::action);
  }

  /**
  * 细分工种矿工，不同的实现方式
  */
  public abstract void work();

  public abstract String name();

  static enum Action {
    GO_TO_SLEEP, WAKE_UP, GO_HOME, GO_TO_MINE, WORK
  }
}

/**
* 隧道挖掘工
*/
public class DwarvenTunnelDigger extends DwarvenMineWorker {

  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenTunnelDigger.class);

  @Override
  public void work() {
    LOGGER.info("{} creates another promising tunnel.", name());
  }

  @Override
  public String name() {
    return "Dwarven tunnel digger";
  }
}
/**
* 黄金挖掘工
*/
public class DwarvenGoldDigger extends DwarvenMineWorker {

  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenGoldDigger.class);

  @Override
  public void work() {
    LOGGER.info("{} digs for gold.", name());
  }

  @Override
  public String name() {
    return "Dwarf gold digger";
  }
}
/**
* 黄金运输工
*/
public class DwarvenCartOperator extends DwarvenMineWorker {

  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenCartOperator.class);

  @Override
  public void work() {
    LOGGER.info("{} moves gold chunks out of the mine.", name());
  }

  @Override
  public String name() {
    return "Dwarf cart operator";
  }
}

/**
* 管理所有矿工的作息
*/
public class DwarvenGoldmineFacade {

  private final List<DwarvenMineWorker> workers;

  public DwarvenGoldmineFacade() {
      workers = List.of(
            new DwarvenGoldDigger(),
            new DwarvenCartOperator(),
            new DwarvenTunnelDigger());
  }

  public void startNewDay() {
    makeActions(workers, DwarvenMineWorker.Action.WAKE_UP, DwarvenMineWorker.Action.GO_TO_MINE);
  }

  public void digOutGold() {
    makeActions(workers, DwarvenMineWorker.Action.WORK);
  }

  public void endDay() {
    makeActions(workers, DwarvenMineWorker.Action.GO_HOME, DwarvenMineWorker.Action.GO_TO_SLEEP);
  }

  private static void makeActions(Collection<DwarvenMineWorker> workers,
      DwarvenMineWorker.Action... actions) {
    workers.forEach(worker -> worker.action(actions));
  }
}

DwarvenGoldmineFacade facade = new DwarvenGoldmineFacade();
facade.startNewDay();
// Dwarf gold digger wakes up.
// Dwarf gold digger goes to the mine.
// Dwarf cart operator wakes up.
// Dwarf cart operator goes to the mine.
// Dwarven tunnel digger wakes up.
// Dwarven tunnel digger goes to the mine.
facade.digOutGold();
// Dwarf gold digger digs for gold.
// Dwarf cart operator moves gold chunks out of the mine.
// Dwarven tunnel digger creates another promising tunnel.
facade.endDay();
// Dwarf gold digger goes home.
// Dwarf gold digger goes to sleep.
// Dwarf cart operator goes home.
// Dwarf cart operator goes to sleep.
// Dwarven tunnel digger goes home.
// Dwarven tunnel digger goes to sleep.