Facade

Structural

Gang Of Four
Decoupling

大约 3 分钟

目的

为一个子系统中的一系列接口提供一个统一的接口。外观定义了一个更高级别的接口以便子系统更容易使用。

解释

真实世界的例子

一个金矿是怎么工作的？“嗯，矿工下去然后挖金子！”你说。这是你所相信的因为你在使用一个金矿对外提供的一个简单接口，在内部它要却要做很多事情。这个简单的接口对复杂的子系统来说就是一个外观。

用通俗的话说

外观模式为一个复杂的子系统提供一个简单的接口。

维基百科说

外观是为很大体量的代码（比如类库）提供简单接口的一种对象。

程序示例

使用上面金矿的例子。这里我们有矮人的矿工等级制度。

@Slf4j
public abstract class DwarvenMineWorker {

  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);
  }

  public abstract void work();

  public abstract String name();

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

@Slf4j
public class DwarvenTunnelDigger extends DwarvenMineWorker {

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

  @Override
  public String name() {
    return "Dwarven tunnel digger";
  }
}

@Slf4j
public class DwarvenGoldDigger extends DwarvenMineWorker {

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

  @Override
  public String name() {
    return "Dwarf gold digger";
  }
}

@Slf4j
public class DwarvenCartOperator extends DwarvenMineWorker {

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

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

为了操纵所有这些矿工我们有了这个外观

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));
  }
}

现在来使用外观

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.

类图

适用性

使用外观模式当

Facade

# 目的

# 解释

# 类图

# 适用性

目的

解释

类图

适用性