JAVA设计模式之观察者模式

观察者模式：定义对象间的一种一对多的依赖关系。当一个对象的状态发生改变的时候，所有依赖于它的对象都得到通知并被自动更新。

简单例子：比如有两个人张三和李四订阅了气象台的天气推送消息。

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

    public void attach(Observer observer)
    {
        observers.add(observer);
        observer.update(this);
    }
    
    public void detach(Observer observer)
    {
        observers.remove(observer);
    }
    
    protected void notifyObservers()
    {
        for(Observer observer: observers)
        {
            observer.update(this);
        }
    }
}

public interface Observer 
{
    public void update(Subject subject);
}

public class ConcreteSubject extends Subject 
{
    private String subjectState;

    public String getSubjectState() 
    {
        return subjectState;
    }

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

import static java.lang.System.out;
public class ConcreteObserver implements Observer
{
    private String observerState;

    public void update(Subject subject) 
    {
        observerState = ((ConcreteSubject)subject).getSubjectState();
    }
    
    public void info()
    {
        out.println("我是：" + observerName);
        out.println("信息：" + observerState);
        out.println();
    }
    
    private String observerName;

    public void setObserverName(String observerName) 
    {
        this.observerName = observerName;
    }
}

public class Client 
{
    public static void main(String[] args) 
    {
        ConcreteSubject weatherSubject = new ConcreteSubject();
        weatherSubject.setSubjectState("天气晴朗，25°");
        
        ConcreteObserver zhangsan = new ConcreteObserver();
        zhangsan.setObserverName("张三");
        
        ConcreteObserver lisi = new ConcreteObserver();
        lisi.setObserverName("李四");
        
        weatherSubject.attach(zhangsan);
        weatherSubject.attach(lisi);
        
        zhangsan.info();
        lisi.info();
        
        weatherSubject.detach(zhangsan);
        weatherSubject.setSubjectState("多云，20°");
        
        zhangsan.info();
        lisi.info();
    }
}

我是：张三
信息：天气晴朗，25°

我是：李四
信息：天气晴朗，25°

我是：张三
信息：天气晴朗，25°

我是：李四
信息：多云，20°