写在前头：希望你了解到的不仅是模式，而学会用OOP的眼光分析问题，解决问题

关键字：一对多，变化通知 、push&pull

描述： 观察者模式、订阅发布模式

原则： 好莱坞原则

示例： 图形编程、股票涨跌、订阅发布

//1. 股票涨跌

//观察者接口
public interface Observer {
	public void update(Stock stock, Object arg);
}

//散户
public class PrivateInvestor implements Observer {
  
	@Override
	public void update(Stock stock, Object arg) {
		System.out.println("PrivateInvestor: Current price:" +stock.getPrice()+", Sell 100!");
		System.out.println("State:"+arg);
	}
}

//投资机构
public class InstitutionInvestor implements Observer {

	@Override
	public void update(Stock stock, Object arg) {
		
		System.out.println("InstitutionInvestor: Current price:" +stock.getPrice()+", Sell 1000!");
		System.out.println("State:"+arg);
	}
}

//股票类
public class Stock {
	//所有关注股票的观察者的集合
	private List<Observer> observers= new ArrayList<Observer>();
	//股票价格
	private double price;
	
	public double getPrice() {
		return this.price;
	}

	public Stock(double price) {
		this.price=price;
	}
	//通知所有观察者
	public void notifyAllObservers() {
		for(Observer observer: observers) {
			observer.update(this,"all ok!");
		}
	}
	
	//新增观察者
	public void addObserver(Observer observer) {
		observers.add(observer);
	}
	//移除观察者
	public void removeObserver(Object observer) {
		observers.remove(observer);
	}
	//改变价格
	public void setPrice(double p) {
		this.price=p;
		
		if(this.price>80 || this.price<10) {
			notifyAllObservers();
		}
	}
	
	//test
	public static void main(String[] args) {
		Stock stock=new Stock(10);
		
		stock.addObserver(new PrivateInvestor());
		stock.addObserver(new InstitutionInvestor());
		
		stock.setPrice(100);
		stock.setPrice(4);	
	}
}

//2. 订阅发布

//主体类
public interface Subject {
	
	public void update(Object arg);
	public void addObserver(Subject s);
}

public class User implements Subject {
	
	private List<Subject> observers= new ArrayList<>();
	private String name="none";
	private String info="Hello World!";
	
	//构造函数
	public User(String name) {
		this.name=name;
	}
	public User(Subject s, String name) {
		s.addObserver(this);
		this.name=name;
	}
	
	@Override
	public void addObserver(Subject s) {
		observers.add(s);
	}

	
	public String getInfo() {
		return info;
	}
	public void setInfo(String info) {
		this.info = info;
		notifyAllObservers();
	}
	
	@Override
	public void update(Object arg) {
		System.out.println(arg+" to "+this.name);
	}
	
	
	public void notifyAllObservers() {
		
		for(Subject o: observers) {
			o.update(this.name+": "+this.getInfo());
		}
		
	}
	
	public static void main(String[] args) {
		User user1=new User("User1");
		
		//user2 观察 user1
		User user2=new User(user1, "User2");
		user1.setInfo("I have updated!");
		
		//user2添加观察者user3, user3 观察user2
		user2.addObserver(new User("User3"));
		user2.setInfo("I got mistake!");
		
		for(int i=0; i<10; i++) {
			user1.addObserver(new User("user0"+i));
		}
		
		user1.setInfo("I have the second time updated!");
	}

}