十二、观察者模式

   观察者模式，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，使他们能够自动更新自己。

  

Subject

public class Subject {
	
	private List<Observer> observers = new ArrayList<Observer>();

	

	public void attach(Observer observer){
		observers.add(observer);
	}

	public void detach(){
		observers.remove(observer);
	}

	public void notify(){
		for(Observer observer : observers){
			observer.update();
		}
	}

}

Observer

public class Observer {
	
	public void update(){

	}

}

ConcreteSubject

public class ConcreteSubject extends Subject {

	private String subjectState;

	public void setSubjectState(String subjectState){
		this.subjectState = subjectState;
	}

	public String getSubjectState(){
		return this.subjectState;
	}

}

ConcreteObserver

public class ConcreteObserver extends Observer {
	
	private String name;
	private String observerState;
	public ConcreteSubject subject;

	public ConcreteObserver(ConcreteSubject subject, String name){
		this.subject = subject;
		this.name = name;
	}	

	public void setSubject(ConcreteSubject subject){
		this.subject = subject;
	}

	public ConcreteSubject getSubject(){
		return this.subject;
	}

	public void update(){
		observerState = m_ConcreteSubject.subjectState;
		System.out.println("观察者"+name+"的新状态是"+observerState);
	}

}

Client

class Client 
{
	public static void main(String[] args) 
	{
		ConcreteSubject s = new ConcreteSubject();
		s.attach(new ConcreteObserver(s,"X"));
		s.attach(new ConcreteObserver(s,"Y"));
		s.attach(new ConcreteObserver(s,"Z"));
		
		s.subjectState = "ABC";
		
		s.notify();
		
	}
}

   当一个对象的改变需要通知其他对象的时候，并且他不知道有多少个对象需要改变的时候，应该考虑使用观察者模式。