观察者模式

import java.util.ArrayList;
import java.util.List;

//抽象通知者
abstract class Subject{
    List<Observer> observers=new ArrayList<>();
    //增加观察者
    protected void attach(Observer observer){
        observers.add(observer);
    }
    //移除观察者
    protected void detach(Observer observer){
        observers.remove(observer);
    }
    //进行通知
    protected void notifying(){
        for (Observer o :observers) {
            o.update();
        }
    }
}
//抽象观察者
abstract class Observer{
    abstract void update();
}

//具体通知者，将有关状态存入具体观察者对象，
// 在具体主题的内部状态改变时，给所有登记过的观察者发出通知。
class ConcreteSubject extends Subject{
    private String subjectState;
    //具体被观察者状态

    public String getSubjectState() {
        return subjectState;
    }

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

//具体观察者
class ConcreteObserver extends Observer{
    private String name;
    private String observerState;
    private ConcreteSubject subject;
    
    public ConcreteObserver(ConcreteSubject subject,String name){
        this.subject=subject;
        this.name=name;
    }
    
    @Override
    void update() {
        observerState=subject.getSubjectState();
        System.out.println("观察者"+name+"的状态："+observerState);
    }
}


public class ObserverPattern {
    public static void main(String[] args) {
        ConcreteSubject s=new ConcreteSubject();
        s.attach(new ConcreteObserver(s,"one"));
        s.attach(new ConcreteObserver(s,"two"));
        s.attach(new ConcreteObserver(s,"three"));
    
        s.setSubjectState("ABC");
        s.notifying();
    }
}