设计模式之观察者模式

概念

观察者模式（Observer）定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使它们能够自动更新自己

优点

• 促进了松耦合，允许主题及其观察者在没有紧密依赖的情况下进行交互，从而简化了维护和可伸缩性
• 允许动态订阅和取消订阅观察者

什么时候使用观察者模式

• 当一个抽象有两个方面时，其中一个依赖于另一个。将这些方面封装在单独的对象中，可以独立地更改和重用它们
• 当对一个对象的更改需要更改其他对象时，而且通常你不知道需要更改多少对象
• 一个对象应该能够通知其他对象，而不需要假设这些对象是谁。换句话说，你不希望这些对象紧密耦合

示例

霍比特人和半兽人(观察者)对天气(主题)的反应

public interface WeatherObserver {

  void update(WeatherType currentWeather);
}

public class Orcs implements WeatherObserver {

  @Override
  public void update(WeatherType currentWeather) {
    LOGGER.info("The orcs are facing " + currentWeather.getDescription() + " weather now");
  }
}

public class Hobbits implements WeatherObserver {

  @Override
  public void update(WeatherType currentWeather) {
    switch (currentWeather) {
      LOGGER.info("The hobbits are facing " + currentWeather.getDescription() + " weather now");
    }
  }
}

public class Weather {

  private WeatherType currentWeather;
  private final 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() {
    var enumValues = WeatherType.values();
    currentWeather = enumValues[(currentWeather.ordinal() + 1) % enumValues.length];
    LOGGER.info("The weather changed to {}.", currentWeather);
    notifyObservers();
  }

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

public static void main(String[] args) {

  var 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 Collections by Naftalin & Wadler
  LOGGER.info("--Running generic version--");
  var genericWeather = new GenWeather();
  genericWeather.addObserver(new GenOrcs());
  genericWeather.addObserver(new GenHobbits());

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

21:28:08.310 [main] INFO com.iluwatar.observer.Weather -- The weather changed to rainy.
21:28:08.312 [main] INFO com.iluwatar.observer.Orcs -- The orcs are facing Rainy weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Hobbits -- The hobbits are facing Rainy weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Weather -- The weather changed to windy.
21:28:08.312 [main] INFO com.iluwatar.observer.Orcs -- The orcs are facing Windy weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Hobbits -- The hobbits are facing Windy weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Weather -- The weather changed to cold.
21:28:08.312 [main] INFO com.iluwatar.observer.Orcs -- The orcs are facing Cold weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Hobbits -- The hobbits are facing Cold weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Weather -- The weather changed to sunny.
21:28:08.312 [main] INFO com.iluwatar.observer.Orcs -- The orcs are facing Sunny weather now
21:28:08.312 [main] INFO com.iluwatar.observer.Hobbits -- The hobbits are facing Sunny weather now
21:28:08.312 [main] INFO com.iluwatar.observer.App -- --Running generic version--
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenWeather -- The weather changed to rainy.
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenOrcs -- The orcs are facing Rainy weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenHobbits -- The hobbits are facing Rainy weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenWeather -- The weather changed to windy.
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenOrcs -- The orcs are facing Windy weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenHobbits -- The hobbits are facing Windy weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenWeather -- The weather changed to cold.
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenOrcs -- The orcs are facing Cold weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenHobbits -- The hobbits are facing Cold weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenWeather -- The weather changed to sunny.
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenOrcs -- The orcs are facing Sunny weather now
21:28:08.313 [main] INFO com.iluwatar.observer.generic.GenHobbits -- The hobbits are facing Sunny weather now

注意事项

• 如果没有正确注销观察者，可能导致内存泄漏
• 未指定通知的顺序，可能导致潜在的意外行为
• 大量观察者可能导致性能问题