设计模式 | 观察者模式

OO原则

• 封装变化
• 多用组合，少用继承
• 针对接口编程，不针对实现编程
• 为对象之间的松耦合设计努力

观察者模式

• 对象之间定义一对多的依赖，当一个subject对象改变状态时，依赖它的对象会收到通知，并自动更新
• 应用：JavaBeans / RMI / GUI
• 实现：java.util.Observable的问题在于 subject不是接口，而是抽象类
• 细节：push/pull两种方式，push更友好；多个观察者不应该依赖特定的通知次序
• JDK自带的观察者observable并不灵活，可以自己实现

自己实现最简单的观察者

public interface Subject {
    void registerObserver(Observer o);
    void removeObserver(Observer o);
    void notifyObservers();
}

public interface Observer {
    void update(float temp, float humidity, float pressure);
}

public class SubjectIml implements Subject {
    private List<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;

    public SubjectIml() {
        observers = new ArrayList<Observer>();
    }

    public void registerObserver(Observer o) {
        observers.add(o);
    }

    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if (i >= 0) {
            observers.remove(o);
        }
    }

    public void notifyObservers() {
        for (Observer o : observers) {
            o.update(temperature, humidity, pressure);
        }
    }

    public void measurementsChanged() {
        notifyObservers();
    }

    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementsChanged();
    }
}

缺点：

• 每次变更，直接push给观察者，没有在合适的时候推送
• 缺少pull的方式
• 可能有线程安全问题

JDK自带的观察者

• notifyObservers方法：根据changed决定是否通知

  public void notifyObservers(Object arg) {
  /*
  * a temporary array buffer, used as a snapshot of the state of
  * current Observers.
  */
  Object[] arrLocal;

    synchronized (this) {
        /* We don't want the Observer doing callbacks into
         * arbitrary code while holding its own Monitor.
         * The code where we extract each Observable from
         * the Vector and store the state of the Observer
         * needs synchronization, but notifying observers
         * does not (should not).  The worst result of any
         * potential race-condition here is that:
         * 1) a newly-added Observer will miss a
         *   notification in progress
         * 2) a recently unregistered Observer will be
         *   wrongly notified when it doesn't care
         */
        if (!changed)
            return;
        arrLocal = obs.toArray();
        clearChanged();
    }
  
    for (int i = arrLocal.length-1; i>=0; i--)
        ((Observer)arrLocal[i]).update(this, arg);
  

  }

• pull 方式：null代表没有更新，观察者需要调用主题对象的get()方法

  public void notifyObservers() {
  notifyObservers(null);
  }

• 缺点：不是接口，是一个class