设计模式之观察者模式 Observer

 

代码实现1

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

public class ObserverA implements Observer{
    
    private int myState;  //myState需要跟目标对象的state保持一致

    @Override
    public void update(Subject subject) {
        myState = ((ConcreteSubject)subject).getState();
    }

    public int getMyState() {
        return myState;
    }

    public void setMyState(int myState) {
        this.myState = myState;
    }
    
}

public class Subject {

    protected List<Observer> list  = new ArrayList<Observer>();
    
    public void registerObserver(Observer obs){
        list.add(obs);
    }
    
    public void removeObserver(Observer obs){
        list.remove(obs);
    }
    
    //通知所有的观察者
    public void notifyAllObservers(){
        for (Observer obs:list) {
            obs.update(this);
        }
    }
    
}

public class ConcreteSubject extends Subject{

    private int state;

    public int getState() {
        return state;
    }

    public void setState(int state) {
        this.state = state;
        //主题对象发生了变化，通知所有的观察者
        this.notifyAllObservers();
    }
    
}

public class Client {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject(); //目标对象
        
        //创建多个观察者
        ObserverA obs1 = new ObserverA();
        ObserverA obs2 = new ObserverA();
        ObserverA obs3 = new ObserverA();
        
        //将这三个观察者添加到subject对象的观察者队伍中
        subject.registerObserver(obs1);
        subject.registerObserver(obs2);
        subject.registerObserver(obs3);
        
        //改变subject的状态
        subject.setState(3000);
        
        //查看观察者的状态是否发生变化
        System.out.println(obs1.getMyState());
        System.out.println(obs2.getMyState());
        System.out.println(obs3.getMyState());
        
    }
}

代码实现2

public class ObserverA implements Observer{

    private int myState;
    
    public int getMyState() {
        return myState;
    }

    public void setMyState(int myState) {
        this.myState = myState;
    }

    @Override
    public void update(Observable o, Object arg) {
        myState = ((ConcreteSubject)o).getState();
    }

}

//目标对象
public class ConcreteSubject extends Observable{

    private int  state;
        
    public int getState() {
        return state;
    }

    public void setState(int state) {
        this.state = state;
    }

    public void set(int s){
        state = s ;   //目标对象的状态发生了变化
        setChanged(); //表示目标对象已经做了更改
        notifyObservers(state);         //通知所有的观察者
    } 
}

public class Client {

    public static void main(String[] args) {
        //创建目标对象Observer
        ConcreteSubject subject = new ConcreteSubject();
        
        //创建观察者
        ObserverA obs1 = new ObserverA();
        ObserverA obs2 = new ObserverA();
        ObserverA obs3 = new ObserverA();
        
        //将观察者添加到目标对象subject的容器中
        subject.addObserver(obs1);
        subject.addObserver(obs2);
        subject.addObserver(obs3);
        
        //改变subject对象的状态
        subject.set(5000);
        
        //查看观察者的状态是否变化
        System.out.println(obs1.getMyState());
        System.out.println(obs2.getMyState());
        System.out.println(obs3.getMyState());
    }
}