观察者模式

观察者设计模式UML类图

观察者设计模式java实现

/**
 * WeatherType enumeration.
 */
public enum WeatherType {

  SUNNY, RAINY, WINDY, COLD;

  @Override
  public String toString() {
    return this.name().toLowerCase();
  }
}

/**
 * Observer interface.
 */
public interface WeatherObserver {

  void update(WeatherType currentWeather);

}

/**
 * Orcs.兽人
 */
public class Orcs implements WeatherObserver {

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

  @Override
  public void update(WeatherType currentWeather) {
    switch (currentWeather) {
      case COLD:
        //兽人是冰冷的。
        LOGGER.info("The orcs are freezing cold.");
        break;
      case RAINY:
        //兽人湿淋淋的。
        LOGGER.info("The orcs are dripping wet.");
        break;
      case SUNNY:
        //太阳伤害了兽人的眼睛。
        LOGGER.info("The sun hurts the orcs' eyes.");
        break;
      case WINDY:
        //兽人的气味在风中几乎消失了。
        LOGGER.info("The orc smell almost vanishes in the wind.");
        break;
      default:
        break;
    }
  }
}

/**
 * Hobbits.霍比特人
 */
public class Hobbits implements WeatherObserver {

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

  @Override
  public void update(WeatherType currentWeather) {
    switch (currentWeather) {
      case COLD:
        //霍比特人在寒冷的天气中发抖。
        LOGGER.info("The hobbits are shivering in the cold weather.");
        break;
      case RAINY:
        //霍比特人从雨中寻找掩饰。
        LOGGER.info("The hobbits look for cover from the rain.");
        break;
      case SUNNY:
        //快乐的霍比特人在温暖的阳光下歌唱。
        LOGGER.info("The happy hobbits bade in the warm sun.");
        break;
      case WINDY:
        //霍比特人在大风天紧紧地戴上帽子。
        LOGGER.info("The hobbits hold their hats tightly in the windy weather.");
        break;
      default:
        break;
    }
  }
}

/**
 * Weather can be observed by implementing {@link WeatherObserver} interface and registering as
 * listener.
 */
public class Weather {

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

  private WeatherType currentWeather;
  private List<WeatherObserver> observers;

  public Weather() {
    observers = new ArrayList<>();
    currentWeather = WeatherType.SUNNY;
  }

  public void addObserver(WeatherObserver obs) {
    observers.add(obs);
  }

  public void removeObserver(WeatherObserver obs) {
    observers.remove(obs);
  }

  /**
   * Makes time pass for weather.
   */
  public void timePasses() {
    WeatherType[] enumValues = WeatherType.values();
    currentWeather = enumValues[(currentWeather.ordinal() + 1) % enumValues.length];
    LOGGER.info("The weather changed to {}.", currentWeather);
    notifyObservers();
  }

  private void notifyObservers() {
    for (WeatherObserver obs : observers) {
      obs.update(currentWeather);
    }
  }
}

/**
 * The Observer pattern is a software design pattern in which an object, called the subject,
 * maintains a list of its dependents, called observers, and notifies them automatically of any
 * state changes, usually by calling one of their methods. It is mainly used to implement
 * distributed event handling systems. The Observer pattern is also a key part in the familiar
 * model–view–controller (MVC) architectural pattern. The Observer pattern is implemented in
 * numerous programming libraries and systems, including almost all GUI toolkits.
 *
 * <p>In this example {@link Weather} has a state that can be observed. The {@link Orcs} and {@link
 * Hobbits} register as observers and receive notifications when the {@link Weather} changes.
 观察者模式是一种软件设计模式，在该模式中，称为主题的对象会维护其依赖项的列表（称为观察者），并通常通过调用其方法之一来自动通知状态更改。 它主要用于实现分布式事件处理系统。 观察者模式也是熟悉的模型-视图-控制器（MVC）体系结构模式中的关键部分。 Observer模式在众多编程库和系统中实现，包括几乎所有的GUI工具包。
<p>在此示例中，{@ link Weather}的状态可以观察到。 {@link Orcs}和{@link Hobbits}注册为观察者，并在{@link Weather}更改时接收通知。
 */
public class App {

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

  /**
   * Program entry point.
   *
   * @param args command line args
   */
  public static void main(String[] args) {

    Weather weather = new Weather();
    weather.addObserver(new Orcs());
    weather.addObserver(new Hobbits());

    weather.timePasses();
    weather.timePasses();
    weather.timePasses();
    weather.timePasses();

    // Generic observer inspired by Java Generics and Collection by Naftalin & Wadler
    LOGGER.info("--Running generic version--");
    GWeather genericWeather = new GWeather();
    genericWeather.addObserver(new GOrcs());
    genericWeather.addObserver(new GHobbits());

    genericWeather.timePasses();
    genericWeather.timePasses();
    genericWeather.timePasses();
    genericWeather.timePasses();
  }
}

极客时间

实际应用案例

• EventBus Guava 事件总线