java设计模式--观察者模式

观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新

参考连接 https://www.cnblogs.com/luohanguo/p/7825656.html

一.我们先不用java为观察者模式提供的内置支持

1.创建主题接口 

public interface Subject {
    //注册成为观察者
    public void registerObserver(Observer o);
    //移除观察者
    public void removeObserver(Observer o);
    //当主题改变时，通知观察者
    public void notifyObserver();
}

2.创建观察者接口

public interface Observer {
    //所有的观察者都必须实现update方法，以实现观察者接口
    //当msg改变时，主题会把这些值传给观察者
    public void update(String msg);
}

3.创建展示接口

public interface Display {
    //显示接口
    public void display();
}

4.实现主题

public class SendMsg implements Subject {
    private ArrayList observers;
    private String msg;

    public SendMsg() {
        observers = new ArrayList();
    }

    @Override
    public void registerObserver(Observer o) {
        //当注册为观察者时，我们只需要把他加进去
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if (i >= 0) {
            observers.remove(i);
        }
    }

    @Override
    public void notifyObserver() {
        for (int i = 0; i < observers.size(); i++) {
            Observer observer = (Observer) observers.get(i);
            observer.update(msg);
        }
    }

    public void msgChanged(){
        notifyObserver();
    }
    public void setMsg(String msg) {
        this.msg = msg;
        msgChanged();
    }
}

5.实现观察者

public class ReceiveMsg implements Observer,Display{
    private String msg;
    private Subject sendMsg;
    private String receiver;

    public ReceiveMsg(String receiver){
        this.receiver = receiver;
    }
    @Override
    public void display() {
        System.out.print(receiver + "收到消息:"+ msg);
    }

    @Override
    public void update(String msg) {
        this.msg = msg;
        display();
    }
}

6.测试程序

public class Client {
    public static void main(String[] args){
        SendMsg sendMsg = new SendMsg();
        ReceiveMsg receiveMsg = new ReceiveMsg("A");
        ReceiveMsg receiveMsgB = new ReceiveMsg("B");
        sendMsg.registerObserver(receiveMsg);
        sendMsg.registerObserver(receiveMsgB);

        sendMsg.setMsg("周一八点开会");
    }
}

二.利用java内置对象