观察者模式

在java的util包下有Observer接口和Observable类，大致描述了观察者模式的基本要素。为了便于个人理解，在原有基础上做了小调整并增加了测试用例ObserverTest和Observer接口的实现类Observer1。

观察者接口

package org.demo.observer;

public interface Observer {

	void update(Observable observable);
	
}

被观察者对象

package org.demo.observer;

import java.util.Collection;
import java.util.HashSet;

public class Observable {

	private boolean changed = false;
	private Collection<Observer> obs;

	public Observable() {
		obs = new HashSet<Observer>();
	}

	public void addObserver(Observer ob) {
		obs.add(ob);
	}

	public void deleteObserver(Observer ob) {
		obs.remove(ob);
	}

	public void notifyObservers() {
		if (changed) {
			for (Observer ob : obs) {
				ob.update(this);
			}
		}
	}

	public void deleteObservers() {
		obs.removeAll(obs);
	}

	protected void setChanged() {
		changed = true;
	}

	public boolean hasChanged() {
		return changed;
	}

	protected void clearChanged() {
		changed = false;
	}

	public int countObservers() {
		return obs.size();
	}

}

观察者一

package org.demo.observer;

public class Observer1 implements Observer {

	private boolean notified = false;

	public Observer1() {
	}

	@Override
	public void update(Observable observable) {
		notified = true;
	}

	public boolean hasNotified() {
		return notified;
	}

	protected void clearNotified() {
		notified = false;
	}

}

观察者测试用例

package org.demo.observer;

import junit.framework.TestCase;

public class ObserverTest extends TestCase {

	public void testObserver() {
		Observable observable = new Observable();

		assertEquals(observable.countObservers(), 0);
		Observer1 observer1 = new Observer1();
		observable.addObserver(observer1);
		assertEquals(observable.countObservers(), 1);

		observable.setChanged();
		observable.notifyObservers();
		assertTrue(observer1.hasNotified());

		observable.deleteObserver(observer1);
		assertEquals(observable.countObservers(), 0);

		observable.clearChanged();
		observer1.clearNotified();
		observable.setChanged();
		observable.notifyObservers();
		assertFalse(observer1.hasNotified());
	}

}

简单描述下观察者模式的要素：
1、被观察者提供方法注册和注销观察者对象。
2、被观察者发生状态变化后，根据注册的观察者名单逐一通知观察者，使观察者有机会对被观察者的状态变化作出反应。