设计模式9：外观模式

外观模式（Facade DP），提供了一个统一接口，来访问子系统中的一群接口。定义高层接口，让子系统更容易使用。
外观这个名字，可以这么理解：假设你要开机，机箱的外观上只有一个开机按钮，但是你按下去之后，机箱里面有一系列的运作，最后实现开机。也就是说，这个外观让一些操作简单化，有点一键端的感觉。
不过这个说法也有点问题，因为外观模式不会掩盖内部的结构，如果你想要直接调用子系统的底层操作也是可以的。这么理解的话，就好比机箱的侧盖没有关上，你仍然可以通过操作一些电路来实现开机，面板上的开机按钮只是提供了一个简化使用的选项。

代码：
现在有一个矮人金矿系统，里面有各种工人。
矮人金矿工人基类：

/**
 * 
 * DwarvenMineWorker is one of the goldmine subsystems.
 *
 */
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;
    }
  }

  /**
   * Perform actions
   */
  public void action(Action... actions) {
    for (Action action : actions) {
      action(action);
    }
  }

  public abstract void work();

  public abstract String name();

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

/**
 * 
 * DwarvenCartOperator is one of the goldmine subsystems.
 *
 */
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";
  }
}

/**
 * 
 * DwarvenGoldDigger is one of the goldmine subsystems.
 *
 */
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";
  }
}

/**
 * 
 * DwarvenTunnelDigger is one of the goldmine subsystems.
 *
 */
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";
  }
}

矮人金矿外观：

/**
 * 
 * DwarvenGoldmineFacade provides a single interface through which users can operate the subsystems.
 * 
 * This makes the goldmine easier to operate and cuts the dependencies from the goldmine user to the
 * subsystems.
 *
 */
public class DwarvenGoldmineFacade {

  private final List workers;

  /**
   * Constructor
   */
  public DwarvenGoldmineFacade() {
    workers = new ArrayList<>();
    workers.add(new DwarvenGoldDigger());
    workers.add(new DwarvenCartOperator());
    workers.add(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 workers,
      DwarvenMineWorker.Action... actions) {
    for (DwarvenMineWorker worker : workers) {
      worker.action(actions);
    }
  }
}

把子系统的接口统一起来，这样只需要很少的代码就能使金矿运作：

public static void main(String[] args) {
    DwarvenGoldmineFacade facade = new DwarvenGoldmineFacade();
    facade.startNewDay();
    facade.digOutGold();
    facade.endDay();
  }